Years ago, a PA system amplifier that had been donated to us for hamfest use had an annoying hum when we checked it out before its first hamfest use.
Nothing I tried would ¡°fix¡± the hum.
Until ---
(Trumpets, please)
I thought about a possible ground loop within the amplifier¡¯s chassis.
Bingo!
There was a ¡°single point¡± bare ground buss wire that ran from one end of the chassis to the other.
But --
(More trumpets) --
It, the ¡°single point ground,¡± was connected to the chassis at both ends.
I snipped one end of the buss wire, hanging it in the air.
No more hum!
I wonder how many hundreds of these PA amplifiers were out in the field, merrily humming away!
All for the lack of snipping one wire!
Yes, ground loops creating hum within chassis are very real!
Mike/
K5MGR
_________________________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 4:08 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Jacques, ? I ¡°may¡± guilty of not returning caps to the same ground as original¡¡¡¡ I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? ? Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
don??? va3drl
|
I recapped my Swan 270B about 21 years ago and its been my most modern
HF rig since. When I use a nice old set of headphones, there is no hum.
But a modern set of high fidelity phones introduces enough hum to
notice but not to impair operation.
So I just use the older phones and assume that it might be possible to
find the source of the hum, but maybe not.
73,
Maynard
W6PAP
toggle quoted message
Show quoted text
On 2/18/25 14:03, Jim Whartenby via groups.io wrote: Tom and Jacques
You are overlooking an alternative, that nothing is actually wrong.
AFAIK. the radio works well but there is a noticeable hum.? Hum is
subjective, if you have listened to solid state audio most of your life
you expect to hear no background audio.? If you have listened to tube
audio all of your life, you expect some hum.
The question about what type of speaker is used with the SX-28 has been
asked twice now but there has been no answer.? Newer speakers have a
very wide dynamic range, both in frequency and volume.? Hum in tube
powered radios is common, actually it is part of the charm.? If an older
midrange speaker is connected to the SX-28, what then?
The Bass IN/OUT thing could be attributed to the change in gain of the
1st audio amp.? Figure 11, the audio filter curve, shows that the gain
is 100+X higher when the bass switch is in the IN position at 60
cycles.? I would not say that the hum is gone in the OUT position, but I
would say that it is not easily heard in the speaker.? If you put your
ear next to the speaker, is there a weak hum present?
If the control grids of both 6V6 tubes are grounded, then no hum is
heard.? What about grounding the two control grids of the 6SC7?? With
this done, the two 6V6 tubes should have proper self bias and the 6V6
control grids should not have any audio input applied.? In this
condition the quietest 6V6 tubes can be selected assuming that several
are available to pick from.
It just occurred to me that if C49 is defective, or not actually 30uF,
then the plates of the 6V6 would have a high ripple AC on them.? Do you
have another 30 uF 450 volt electrolytic to temporarily bridge C49 with?
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
On Tuesday, February 18, 2025 at 01:44:16 PM CST, Jacques_VE2JFE via
groups.io <jacques.f@...> wrote:
*Let¡¯s resume the case?:*
Tom pulled the 6SC7 and the ¡°hum¡± remained.
He shorted the two 6V6 grids to GND and the hum disappeared.
No hum when the ¡°bass¡± switch is in the IN position (both plate loads of
the 6SC7 equal and CH2 + C43 shorted).
Hum present when the ¡°bass¡± sw in OUT position.
The ¡°hum¡± is at 60Hz¡
Which suspects remains ??
_Does the C44 is ¡°grounded¡± at the same place than the filaments of the
6V6s and this connection is resistive ??_
_Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum._
_When it is not: hum !_
Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and
that the C43 is practically an open circuit at 60 Hz.
*Could it be that the 60Hz injection point is the C44 grounding lug ?*
My two SX-28As are too far to be quickly reached.
But If I manage to dig the ¡°never touched¡± one out of the storage during
the next weekend, I can check how it is wired.
*Keep thinking¡.*
And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is
ENABLED (literally, ¡°bass¡± is in).
When out: bass is out (DISABLED).
CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz.
The obvious intent is to ¡°tailor¡± an audio bandpass response for voice
frequencies (300Hz ¨C 3kHz)
*73, Jacques, VE2JFE in Montreal***
Gents, dunno about all that, but why the no hum in the In position? And
when base is boosted?
don??? va3drl
|
Jim ? Re:: ¡°The Bass IN/OUT thing could be attributed to the change in gain of the 1st audio amp.? Figure 11, the audio filter curve, shows that the gain is 100+X higher when the bass switch is in the IN position at 60 cycles.? I would not say that the hum is gone in the OUT position, but I would say that it is not easily heard in the speaker.?¡° ?? But ?post #1 says ?¡°When I switch the Bass in, the hum goes away.¡± , from witch I gather there IS a HUM problem in ¡°OUT¡± where the 60Hz gain is lower ,, ???? I was doing mental cartwheels about this and my head is still sore. ? ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Tuesday, February 18, 2025 5:03 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? You are overlooking an alternative, that nothing is actually wrong.? AFAIK. the radio works well but there is a noticeable hum.? Hum is subjective, if you have listened to solid state audio most of your life you expect to hear no background audio.? If you have listened to tube audio all of your life, you expect some hum.? -- don??? va3drl
|
Jacques, ? I ¡°may¡± guilty of not returning caps to the same ground as original¡¡¡¡ I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Jacques_VE2JFE via groups.io
Sent: Tuesday, February 18, 2025 2:44 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
don??? va3drl
|
Ok, made a couple videos, excuse the camera work and narrating¡¡¡. ?
?
?
? Tom W3TA ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Monday, February 17, 2025 8:59 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Time to re-evaluate where I¡¯m at and what I¡¯ve tried, yes the hum is still there¡¡. ? Tried three different 6SC7¡¯s no noticeable difference between each one. ? Confirmed the wiring of the switch agrees with the schematic. I have no reason to suspect that the wiring was changed by a PO at the switch ¨C looks original to me. ? Speaking of that switch, I found by poking around with my magic wooden stick that pressing on the switch terminals would result in noise / popping sounds ¨C could this be the ah ha moment? Scrounged my switch stock and found one that looked the same. Checked the switch action with a DVM, all good Installed switch ¨C same results¡¡¡ Checked original switch with DVM and it definitely has issues. Must have dirty contacts inside as one direction is a solid 0.3 ohms and the other would vary from 10 ohms to over 100 by wiggling the terminal. ? I also confirmed that the hum is 60hz, not 120hz. I see 120hz on the audio xfmr primary side to gnd but see 60hz on the 500 ohm output (with the scope). ? I checked my wiring ¨C all good. ? Now here is where things get interesting. I removed the choke (measured 4.3hy on my Sencore tester) and installed the one I found last night using 8¡± or so clip leads so I could lay the choke on the bench. Seemed like the hum decreased, thought another ah ha moment. Swapped the original choke back in using the same clip leads, same result hum still present but seemingly lower in amplitude. Interesting¡¡.. ? Put original choke back into the radio, hum is easily 2x louder which is what I remembered before removing the choke. So that makes me wonder if the choke is really the culprit. ? I need to take some baseline measurements with the choke mounted, unmounted and then with the extension leads. ? Then I¡¯m going to wire in the other choke where the original one went and compare measurements. ? Right now, my brain is not up to the task so it¡¯ll have to wait for tomorrow. Been up since before 5AM and going all day. Once tiredness sets in (usually around 8:30, 9pm) I avoid playing around high voltages¡¡¡¡ ? Thanks for all of the discussion, I find it educational. ? Tom W3TA ? ? Last night I was able to find another 4hy choke. I knew I had one around just couldn't put my finger on it. It was salvaged from an old R390A AF module.?Dawned on me last night at 8:30 to go check the shed. Bundled up headed out into the 50mph wind storm we were having (dog didn't even want to follow me out). Found the choke, swapped it into the radio - hum still there.......... That tube is looking more and more suspect............ On Monday, February 17, 2025 at 07:56:32 AM EST, thoyer via groups.io <thoyer1@...> wrote: One other tid bit, whenever I switch between in/out there is a loud thump in the speaker - annoying to say the least. On Monday, February 17, 2025 at 02:11:24 AM EST, Jim Whartenby via groups.io <old_radio@...> wrote: So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position? I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch. It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies. Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Sunday, February 16, 2025 at 08:01:10 PM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote: The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
I don¡¯t know Jim, I have several other radios (too many actually¡) of this vintage that don¡¯t have this hum. ? Yes I neglected to address the speaker, sorry. My bench setup uses an older Hammarlund 8 ohm speaker that I have a 600 to 8 ohm xfmr inline. To rule that out I took an R42 speaker from another radio setup and connected it to the 500 ohm output ¨C same result. ? I did bridge an additional 47uf cap across C49, no difference. All three electrolytics have been restuffed as part of my overhaul. ? I took some videos and am working on getting links to them so we can all ¡°hear¡± the hum¡¡¡.. ? Tom W3TA ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Tuesday, February 18, 2025 5:03 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? You are overlooking an alternative, that nothing is actually wrong.? AFAIK. the radio works well but there is a noticeable hum.? Hum is subjective, if you have listened to solid state audio most of your life you expect to hear no background audio.? If you have listened to tube audio all of your life, you expect some hum.? The question about what type of speaker is used with the SX-28 has been asked twice now but there has been no answer.? Newer speakers have a very wide dynamic range, both in frequency and volume.? Hum in tube powered radios is common, actually it is part of the charm.? If an older midrange speaker is connected to the SX-28, what then? The Bass IN/OUT thing could be attributed to the change in gain of the 1st audio amp.? Figure 11, the audio filter curve, shows that the gain is 100+X higher when the bass switch is in the IN position at 60 cycles.? I would not say that the hum is gone in the OUT position, but I would say that it is not easily heard in the speaker.? If you put your ear next to the speaker, is there a weak hum present? If the control grids of both 6V6 tubes are grounded, then no hum is heard.? What about grounding the two control grids of the 6SC7?? With this done, the two 6V6 tubes should have proper self bias and the 6V6 control grids should not have any audio input applied.? In this condition the quietest 6V6 tubes can be selected assuming that several are available to pick from. It just occurred to me that if C49 is defective, or not actually 30uF, then the plates of the 6V6 would have a high ripple AC on them? Do you have another 30 uF 450 volt electrolytic to temporarily bridge C49 with? Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Tuesday, February 18, 2025 at 01:44:16 PM CST, Jacques_VE2JFE via groups.io <jacques.f@...> wrote: Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
don??? va3drl
|
Jacques, I was going thru your recent post and started turning into a pretzel;? ?adding and then removing a resistor. See my ¡°side-by-each¡± . ????180 ¡°M?¡± but no squiggle ?
?
toggle quoted message
Show quoted text
|
Tom and Jacques You are overlooking an alternative, that nothing is actually wrong.? AFAIK. the radio works well but there is a noticeable hum.? Hum is subjective, if you have listened to solid state audio most of your life you expect to hear no background audio.? If you have listened to tube audio all of your life, you expect some hum.?
The question about what type of speaker is used with the SX-28 has been asked twice now but there has been no answer.? Newer speakers have a very wide dynamic range, both in frequency and volume.? Hum in tube powered radios is common, actually it is part of the charm.? If an older midrange speaker is connected to the SX-28, what then?
The Bass IN/OUT thing could be attributed to the change in gain of the 1st audio amp.? Figure 11, the audio filter curve, shows that the gain is 100+X higher when the bass switch is in the IN position at 60 cycles.? I would not say that the hum is gone in the OUT position, but I would say that it is not easily heard in the speaker.? If you put your ear next to the speaker, is there a weak hum present?
If the control grids of both 6V6 tubes are grounded, then no hum is heard.? What about grounding the two control grids of the 6SC7?? With this done, the two 6V6 tubes should have proper self bias and the 6V6 control grids should not have any audio input applied.? In this condition the quietest 6V6 tubes can be selected assuming that several are available to pick from.
It just occurred to me that if C49 is defective, or not actually 30uF, then the plates of the 6V6 would have a high ripple AC on them.? Do you have another 30 uF 450 volt electrolytic to temporarily bridge C49 with? Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
don??? va3drl
|
Let¡¯s resume the case?: Tom pulled the 6SC7 and the ¡°hum¡± remained. He shorted the two 6V6 grids to GND and the hum disappeared. No hum when the ¡°bass¡± switch is in the IN position (both plate loads of the 6SC7 equal and CH2 + C43 shorted). Hum present when the ¡°bass¡± sw in OUT position. The ¡°hum¡± is at 60Hz¡ ? Which suspects remains ?? Does the C44 is ¡°grounded¡± at the same place than the filaments of the 6V6s and this connection is resistive ?? Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum. When it is not: hum ! Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and that the C43 is practically an open circuit at 60 Hz. Could it be that the 60Hz injection point is the C44 grounding lug ? ? My two SX-28As are too far to be quickly reached. But If I manage to dig the ¡°never touched¡± one out of the storage during the next weekend, I can check how it is wired. Keep thinking¡. ? And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is ENABLED (literally, ¡°bass¡± is in). When out: bass is out (DISABLED). CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to ¡°tailor¡± an audio bandpass response for voice frequencies (300Hz ¨C 3kHz) ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ??
don??? va3drl
|
Gents, dunno about all that, but why the no hum in the In position? And when base is boosted? ?? ? -- don??? va3drl
|
Richard I was a bit clumsily in my previous attempt so I will try again.??
All aluminum electrolytic capacitors are self resonate.? ?There is a frequency where the capacitor becomes an inductor.? C44 is such a capacitor.? The graph below is for a modern aluminum electrolytic capacitor in a round can with terminals on top.? The type of capacitor in question, found in the audio section of the SX-28 I believe, are twist lock.? In my search, I could not find any data on the self resonate frequency of this type capacitor package but I suspect that it is well within the midrange audio frequencies.? This self resonate frequency was not considered important in the late 1930's.? The sole electrolytic capacitor function at that time was as a power supply filter.??
I also suspect that electrolytic capacitors with low working voltages perform adequately at audio frequencies but those with high working voltages, not so much.? Their construction to withstand the high voltage made the internal parasitic components much larger and the self resonate frequency much lower.? C44 is rated at 300 volts and must filter audio frequencies to ground.? You contend that C44 is filtering the DC from the power supply on the other end of R38 which is 47k ohms.? That seems unlikely.? I contend that the job of C44 is removing midrange audio and it is doing this job poorly, even back in the day.
None of this means much now since it does not seem to be the fault in this SX-28 thread.? All of my observations were based on the reported 120 cycle hum.? It now seems that it is really 60 cycle hum.? I suspect that the SX-28 speaker is a modern type that has a frequency range that easily extends below 60 cycles.? The loud speakers back in the day were not so efficient.? The improvement noticed when the bass boost is out is the result of the lower voltage amplification in this position.? When the bass boost is in, the triode has higher gain so the hum is more noticeable.
The 60 cycle hum may be an artifact from the heater voltage or the relatively low value filter capacitors.? Increasing the filter capacitor value may reduce the hum but doing so will stress the power rectifier tube.? Large value filter capacitors demand a higher charging current which the 5Z3 cannot long survive.? This will also stress the power transformer because of I^2R losses.? If the source of the hum is the heater voltage, perhaps the three audio tubes can be selected for low hum? Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
I suppose I have to reply to each comment. The plate load is the
impedance in the plate circuit between supply voltage and plate, that
develops the signal output. In the half of the 6SC7 that the bass switch
is in it is either the resonant choke or R-37 depending on the position
of the switch. R-38 is not part of the plate load because there is no AC
on it. It is bypassed to ground via C-44. C-44 is a 10uF electrolytic
but is probably effective over the entire audio range. If electrolytic
caps were as bad as you indicate no bypass cap could work. The voltage
at the junction of C-44 and R-38 is well filtered DC.
If you look up the circuit for a "floating paraphase" phase splitter
you will find exactly what is in the SX-28.
The term "load" may be confusing, I mean the impedance in the plate
circuit. In this circuit it is coupled to the grid of one of the 6V6
tubes via a capacitor C-45 which is connected to the grid resistor of
one 6V6 R-41. The actual load on the 6SC7 is the combination of the
plate load and the following grid load.
I still want to know what is actually in these receivers. The
presentation of the bass tone switch is exactly the same in both the
SX-28 and SX-28A and in the military manuals. I have seen cases of other
errors carried over from generation to generation of instruction manuals
or even text books but a real answer would be to look at an actual
receiver and see how its wired. From the curves in the books the
labeling means that the boost is ON when the switch is IN.
To clarify a previous remark, at the time lots of bass was
considered desirable for entertainment audio. Not necessarily good bass,
just lots of it. This was how juke boxes were designed. Probably also
intended for use with the phonograph input.
This still does not address the hum problem but I strongly suspect
that the increased low end gain is exaggerating a hum problem in the tube.
On 2/17/2025 7:56 AM, Jim Whartenby via groups.io wrote: Richard
Comments inserted in the text below.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
On Monday, February 17, 2025 at 03:09:24 AM CST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37.
No, the plate load for the 1st triode section is R37 and R38.? The
voltage gain of the 1st audio stage is approximately the plate load
resistance divided by the cathode resistance or 147k / 1k? or 147 when
SW10 is in the IN position. All of those bypass capacitors that are
sprinkled throughout the audio amplifier cause a fast roll off of the
midrange audio so that the bass frequencies are emphasized because of
the higher gain.
When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out.
What about R38?? In the out position, the voltage gain of the 1st triode
is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction
when SW10 is in the OUT position so Figure 11 actually agrees with the
schematic.
Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz).
Again, what about R38?? It is still in series with the parallel
combination of CH2 and C43.? In the OUT position, the frequency response
of the 1st Audio is now relatively flat so there is no base boost
compared to what you see in the IN position curve.
I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies.
You are considering the electrolytic capacitors to be ideal.? The
components in the late 1930s and early 1940s were far from that.? The
ESL and ESR were not controlled back then.? Modern components are much
closer to the ideal but still aluminum electrolytic capacitors start to
become inductive at midrange audio frequencies.? This is why they are
not recommended as coupling & bypass capacitors for high end audio.
They cause distortion.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
This begs the question, what is the speaker that is used with this
SX-28?? Could it simply be the lower gain of the 1st audio amplifier
when SW10 is in the OUT position that causes the normal 120 cycle hum to
disappear into the mud?? If so, then there is actually no problem in the
SX-28 audio amplifier.? We are chasing our tail.
While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A.
If you consider that BASS really means BASS BOOST then what is happening
will make more sense.
Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substitutingit.
On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote:
So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass
(boost)
IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 &
R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are
believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the
contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Yes, wiring is as shown on schematic
Yes bass "in" boost the bass response.
Please confirm: 1, the wiring is actually as shown on the diagram.
2, the bass boost is present with the switch in the IN position.
If so I am thoroughly puzzled.
If the hum is 60Hz I wonder if filament current is getting in
somehow. One side of the filament returns through chassis ground. I have
had a cases where a poor ground in the filament return caused 60 Hz hum.
Don't know why the Bass switch would change it.
On 2/17/2025 5:59 PM, thoyer via groups.io wrote: Time to re-evaluate where I¡¯m at and what I¡¯ve tried, yes the hum is
still there¡¡.
Tried three different 6SC7¡¯s no noticeable difference between each one.
Confirmed the wiring of the switch agrees with the schematic. I have no
reason to suspect that the wiring was changed by a PO at the switch ¨C
looks original to me.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
|
|
Please confirm: 1, the wiring is actually as shown on the diagram.
2, the bass boost is present with the switch in the IN position.
If so I am thoroughly puzzled.
If the hum is 60Hz I wonder if filament current is getting in
somehow. One side of the filament returns through chassis ground. I have
had a cases where a poor ground in the filament return caused 60 Hz hum.
Don't know why the Bass switch would change it.
toggle quoted message
Show quoted text
On 2/17/2025 5:59 PM, thoyer via groups.io wrote: Time to re-evaluate where I¡¯m at and what I¡¯ve tried, yes the hum is
still there¡¡.
Tried three different 6SC7¡¯s no noticeable difference between each one.
Confirmed the wiring of the switch agrees with the schematic. I have no
reason to suspect that the wiring was changed by a PO at the switch ¨C
looks original to me.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Time to re-evaluate where I¡¯m at and what I¡¯ve tried, yes the hum is still there¡¡. ? Tried three different 6SC7¡¯s no noticeable difference between each one. ? Confirmed the wiring of the switch agrees with the schematic. I have no reason to suspect that the wiring was changed by a PO at the switch ¨C looks original to me. ? Speaking of that switch, I found by poking around with my magic wooden stick that pressing on the switch terminals would result in noise / popping sounds ¨C could this be the ah ha moment? Scrounged my switch stock and found one that looked the same. Checked the switch action with a DVM, all good. Installed switch ¨C same results¡¡¡ Checked original switch with DVM and it definitely has issues. Must have dirty contacts inside as one direction is a solid 0.3 ohms and the other would vary from 10 ohms to over 100 by wiggling the terminal. ? I also confirmed that the hum is 60hz, not 120hz. I see 120hz on the audio xfmr primary side to gnd but see 60hz on the 500 ohm output (with the scope). ? I checked my wiring ¨C all good. ? Now here is where things get interesting. I removed the choke (measured 4.3hy on my Sencore tester) and installed the one I found last night using 8¡± or so clip leads so I could lay the choke on the bench. Seemed like the hum decreased, thought another ah ha moment. Swapped the original choke back in using the same clip leads, same result hum still present but seemingly lower in amplitude. Interesting¡¡.. ? Put original choke back into the radio, hum is easily 2x louder which is what I remembered before removing the choke. So that makes me wonder if the choke is really the culprit. ? I need to take some baseline measurements with the choke mounted, unmounted and then with the extension leads. ? Then I¡¯m going to wire in the other choke where the original one went and compare measurements. ? Right now, my brain is not up to the task so it¡¯ll have to wait for tomorrow. Been up since before 5AM and going all day. Once tiredness sets in (usually around 8:30, 9pm) I avoid playing around high voltages¡¡¡¡ ? Thanks for all of the discussion, I find it educational. ? Tom W3TA ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Monday, February 17, 2025 8:02 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Last night I was able to find another 4hy choke. I knew I had one around just couldn't put my finger on it. It was salvaged from an old R390A AF module.?Dawned on me last night at 8:30 to go check the shed. Bundled up headed out into the 50mph wind storm we were having (dog didn't even want to follow me out). Found the choke, swapped it into the radio - hum still there.......... That tube is looking more and more suspect............ On Monday, February 17, 2025 at 07:56:32 AM EST, thoyer via groups.io <thoyer1@...> wrote: One other tid bit, whenever I switch between in/out there is a loud thump in the speaker - annoying to say the least. On Monday, February 17, 2025 at 02:11:24 AM EST, Jim Whartenby via groups.io <old_radio@...> wrote: So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position? I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch. It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies. Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Sunday, February 16, 2025 at 08:01:10 PM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote: The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Richard and Jim, ? I got caught on the top paragraphs, and did not go thru it all.. but¡!!! As I have seen it, at 10mfd, C44 is hopefully ?holding the ?¡°reduced B+¡± steady for both parts of the 6SC7, including the voltage to all the ¡°stuff¡± going to the Right plate. If we don¡¯t have a consensus there, It is hard discuss any of the filtering business. If that C44 ?point is not a fixed DC voltage, the left tube will go wonky too. ? ? As Jacques mentioned, the switch ¡°IN¡± position does boost the base at the expense of all else; Early on, I did not think about C42 associated with the tone control [ya, I assumed..], but it becomes obvious that this cap likes to gobble up the mids and highs, leaving a dominant base response. ? The OP [Jim] said originally ?¡°Radio is working well except for a 120hz hum. ??When I switch the Bass in, the hum goes away.¡± So to me hum is there in the ¡°OUT¡± position and so the hum would seem to be related to the choke area stuff and its wiring. ?There is no obvious ground to open in the area, but Nearby B+ will easily couple to any open connections. ??Here would be my FIRST look, with switch wiring first. ? Presumably there is a normal radio signal with hum on top of it ?? and not just hum?? ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Monday, February 17, 2025 10:56 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Comments inserted in the text below. On Monday, February 17, 2025 at 03:09:24 AM CST, Richard Knoppow via groups.io <1oldlens1@...> wrote: Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37. No, the plate load for the 1st triode section is R37 and R38.? The voltage gain of the 1st audio stage is approximately the plate load resistance divided by the cathode resistance or 147k / 1k? or 147 when SW10 is in the IN position.??All of those bypass capacitors that are sprinkled throughout the audio amplifier cause a fast roll off of the midrange audio so that the bass frequencies are emphasized because of the higher gain. When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out. What about R38?? In the out position, the voltage gain of the 1st triode is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction when SW10 is ??++++++++++++++++++++++++++++ ???????????? ++++++++++ -- don??? va3drl
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Emanuele, very good that you did all this, you can let your brain cool off now nor a bit. First, do you have any idea if this is an early run, or a later one?. See what is in red first! Richard mentioned long ago about messing with the guts of variable cap drive parts, and putting things back together but the cap did not fully open the plates, since a pin put in incorrectly stopped it. I know we asked you about unmeshing etc but Just maybe. ? ? Seems to me this confirms the mixer tanks in question ?have the same sort of response and problem.. for better or worse. ? It seems again, the top scamming? of tuning is very bad, but the bottom 500 kc is quite close ¡. Yet it is the bottom [say 600 kc area that] has bad reception. Is this how you see it?? what could explain this? so you think signals pass thru the RF tanks but not the mixer tanks?? ? Re ?¡°Setup is the same as ANT but the injection point is V1pin8 and the probing point is V2pin8. The signal in band 1 is very low (effect of lack of coupling between V1 and v2). I had to rise the TG level to -5dBm and the ref level to -50dBm to see something.¡± So ?our ongoing questions about how do V1 plate signals get to the Tanks? resurfaces with ¡°no magic coupling¡± It would be good if the band would be 500-1000KHz and the alignment frequency 900KHz.... but it is not! Re ?¡°Changing figures,?????????? ?the same happens on band 2,?? ?3 and 4 ?(the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band). But the tracking between tanks appears satisfactory.¡± ? ??????????? This make sense for band 2, but I thought Band 3,4 were ?aligning and receiving well before ??? For what it is worth I understand a tuning problem at the high end for both band 1,2 tanks and? a loss of V1,v2 coupling for 1,2. ?¡ all confirming past suspicions. Because the tuning [peaking] at the bottom is in the ball park, L and Max total capacitance seem to be about ,, right however the top ?peaks, but is at much lower frequency than required. Presume for the moment that your poor reception at 600Kc is to ?V1-V2 coupling, and NOT 600Kc tracking, that leaves an apparent too much capacitance in the trimmers, OR in some hidden spot. After all your work, you/we are back to square one, [149 posts now?] ?we gotta beat the impossible! Part of my reasoning above[about the 600kc ] is that the main tuning cap swamps out the small caps so it and L3 set that frequency, so they should be OK.{always a should be} .but at the top end there must be too much capacitance [too much minimum capacitance? somewhere in the circuit] Is it possible that those trimmers are really strange? Perhaps it would be easy to lift one end just to test? ? ??¡°¡±Make absolutely sure that the bandspread cap is set at minimum capacity (minimum mesh) when doing this alignment. What you describe is exactly what happens with too much capacitance in the circuit. Also make sure that the main tuning capacitor is at (or extremely close to minimum capacitance) when the dial is set at the top of the range.????? Rodger WQ9E¡±¡± ? Here is a strange story.. ¡°oh it is just like the? last stuff¡±¡. So no need to look at drawings ?just run wires to the same spots as before, its ALL the same¡±? well almost the same.? OR maybe ¡°I cant see anything different in the drawing .. so do it the same¡±? that could be coils too. I have seen worse than that. ? Well I went to minimum Capacity, to negative capacity; maybe it is contagious? ????
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Emanuele Girlando via groups.io
Sent: Monday, February 17, 2025 4:01 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!? Setup is the same as ANT but the injection point is V1pin8 and the probing point is V2pin8. The signal in band 1 is very low (effect of lack of coupling between V1 and v2). I had to rise the TG level to -5dBm and the ref level to -50dBm to see something. Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity) The peak is a little below 500KHz (in fact not visible - measured to be 480KHz)) - L6 has no core to tune; C15 has no visible effects. Turning the main dial toward higher frequencies, I see the peak moving right. Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C15 fully close (max capacitance) The peak stops at 900KHz. Something very similar to the ANT tank happens: I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 900KHz (!!) without even reaching the alignment frequency of 1400KHz. Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C15 fully open (min capacitance) Rises to 1MHz turning C15 to min capacity. C15 has 100KHz range capability. ? It would be good if the band would be 500-1000KHz and the alignment frequency 900KHz.... but it is not! Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band). But the tracking between tanks appears satisfactory. Here is a retina flash of the findings: Aaah! my brain is exhausted. -- don??? va3drl
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Greetings to the group:
?
?? I see I will have to go back and duplicate Emanuele's measurement setup exactly; I can do that but it will take a while.
?
? In the mean time, I have taken some more pictures with my network analyzer.?? Someone was wondering about how the input looked on the lowest two bands when configured as an unbalanced input.?? I can confirm that the input looks very strange.?? On the upper two bands, it looks pretty reasonable and the best match point moves around with the main tuning dial.
?
Here are the screen captures:
?
?
S-85 band 2 input
?
?
S-85 Band 3 Input
?
?
S-85 Band 4 Input
?
If you are wondering what happened to band 1, it is so far from 50 ohms as to be effectively not there; the plot doesn't tell you much.
?
Here are the responses taken from the mixer grid with the tube removed; they follow the main tuning dial reasonably accurately.
?
?
Band 1 high, mixer grid.
?
?
Band 1 low mixer grid
?
?
Band 2 High Mixer grid
?
?
Band 2 Low Mixer Grid
?
?
Band 3 High Mixer Grid
?
?
Band 3 low Mixer Grid
?
?
Band 4 High mixer grid
?
?
Band 4 low mixer grid.
?
73,
--
Jim T.
KB6GM
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Sweep of MIX tanks:
Setup is the same as ANT but the injection point is V1pin8 and the probing point is V2pin8.
The signal in band 1 is very low (effect of lack of coupling between V1 and v2). I had to rise the TG level to -5dBm and the ref level to -50dBm to see something.
?
Results:
Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity)
The peak is a little below 500KHz (in fact not visible - measured to be 480KHz)) - L6 has no core to tune; C15 has no visible effects.
Turning the main dial toward higher frequencies, I see the peak moving right.
Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C15 fully close (max capacitance)
The peak stops at 900KHz.
Something very similar to the ANT tank happens: I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 900KHz (!!) without even reaching the alignment frequency of 1400KHz.
Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C15 fully open (min capacitance)
Rises to 1MHz turning C15 to min capacity. C15 has 100KHz range capability.
?
?
It would be good if the band would be 500-1000KHz and the alignment frequency 900KHz.... but it is not!
Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band). But the tracking between tanks appears satisfactory.
?
Here is a retina flash of the findings:
?
Aaah! my brain is exhausted.
?
--
Emanuele (IU1KNR).
|
I suppose I have to reply to each comment. The plate load is the
impedance in the plate circuit between supply voltage and plate, that
develops the signal output. In the half of the 6SC7 that the bass switch
is in it is either the resonant choke or R-37 depending on the position
of the switch. R-38 is not part of the plate load because there is no AC
on it. It is bypassed to ground via C-44. C-44 is a 10uF electrolytic
but is probably effective over the entire audio range. If electrolytic
caps were as bad as you indicate no bypass cap could work. The voltage
at the junction of C-44 and R-38 is well filtered DC.
If you look up the circuit for a "floating paraphase" phase splitter
you will find exactly what is in the SX-28.
The term "load" may be confusing, I mean the impedance in the plate
circuit. In this circuit it is coupled to the grid of one of the 6V6
tubes via a capacitor C-45 which is connected to the grid resistor of
one 6V6 R-41. The actual load on the 6SC7 is the combination of the
plate load and the following grid load.
I still want to know what is actually in these receivers. The
presentation of the bass tone switch is exactly the same in both the
SX-28 and SX-28A and in the military manuals. I have seen cases of other
errors carried over from generation to generation of instruction manuals
or even text books but a real answer would be to look at an actual
receiver and see how its wired. From the curves in the books the
labeling means that the boost is ON when the switch is IN.
To clarify a previous remark, at the time lots of bass was
considered desirable for entertainment audio. Not necessarily good bass,
just lots of it. This was how juke boxes were designed. Probably also
intended for use with the phonograph input.
This still does not address the hum problem but I strongly suspect
that the increased low end gain is exaggerating a hum problem in the tube.
toggle quoted message
Show quoted text
On 2/17/2025 7:56 AM, Jim Whartenby via groups.io wrote: Richard
Comments inserted in the text below.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
On Monday, February 17, 2025 at 03:09:24 AM CST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37.
No, the plate load for the 1st triode section is R37 and R38.? The
voltage gain of the 1st audio stage is approximately the plate load
resistance divided by the cathode resistance or 147k / 1k? or 147 when
SW10 is in the IN position. All of those bypass capacitors that are
sprinkled throughout the audio amplifier cause a fast roll off of the
midrange audio so that the bass frequencies are emphasized because of
the higher gain.
When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out.
What about R38?? In the out position, the voltage gain of the 1st triode
is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction
when SW10 is in the OUT position so Figure 11 actually agrees with the
schematic.
Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz).
Again, what about R38?? It is still in series with the parallel
combination of CH2 and C43.? In the OUT position, the frequency response
of the 1st Audio is now relatively flat so there is no base boost
compared to what you see in the IN position curve.
I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies.
You are considering the electrolytic capacitors to be ideal.? The
components in the late 1930s and early 1940s were far from that.? The
ESL and ESR were not controlled back then.? Modern components are much
closer to the ideal but still aluminum electrolytic capacitors start to
become inductive at midrange audio frequencies.? This is why they are
not recommended as coupling & bypass capacitors for high end audio.
They cause distortion.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
This begs the question, what is the speaker that is used with this
SX-28?? Could it simply be the lower gain of the 1st audio amplifier
when SW10 is in the OUT position that causes the normal 120 cycle hum to
disappear into the mud?? If so, then there is actually no problem in the
SX-28 audio amplifier.? We are chasing our tail.
While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A.
If you consider that BASS really means BASS BOOST then what is happening
will make more sense.
Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substitutingit.
On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote:
So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass
(boost)
IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 &
R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are
believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the
contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
_._,_._,_
------------------------------------------------------------------------
Groups.io Links:
You receive all messages sent to this group.
View/Reply Online (#31789) </g/HallicraftersRadios/
message/31789> | Reply to Group <mailto:[email protected]?
subject=Re:%20Re%3A%20%5BHallicraftersRadios%5D%20SX-28A%20Hum> | Mute
This Topic </mt/111174219/518617> | New Topic <https://
groups.io/g/HallicraftersRadios/post>
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Mike Langner
don Root
Jim Whartenby
Jacques_VE2JFE
Emanuele Girlando