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I rebuilt my SX-28a about 10 years ago including pulling the RF deck out etc. I have never experienced a hum with the bass switch in either position.
On Sat, Feb 15, 2025, 10:15?PM don Root via <drootofallevil= [email protected]> wrote: It seems that the way to look at this switch is that in one position the plate load is provided by the choke while in the other the load is switched to R37, and as an aside the choke gets shorted ? ??Any votes on this? ?just for fun try the thumbs ?voting machine, I won¡¯t look. ?
--
don??? va3drl
|
Tom, as you are able to measure it, what is the ripple across C48 and across C44 when the bass switch is ¡°IN¡± ? What is not normal is to have more ¡°hum¡± when the bass switch is ¡°OUT¡± because the first triode section of the 6SC7 will have less gain in low frequencies (CH2 reactance will drop as the frequency lowers) and also at high frequencies (C43 reactance will drop). The CH2 ¨C C43 should be broadly resonant around 1kHz or so¡ But if there is a 120Hz component present across C44, it will be coupled to the V14 grid thru C45 and also to V13, inverted in phase due to the see-saw phase splitter circuit driving the second 6SC7 grid (pin 3). ? The whole situation reminds me of a restoration report about a SX-28A that I read sometime ago, in which the C48 was found almost ¡°open¡± (less than 1?F in value). ? ? ? ?
? 73, Jacques, VE2JFE in Montreal
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Do you mean the 6SA7 or 6SC7? The 6SC7 is the dual triode that acts
as the first audio and phase splitter. Pulling that leaves the 6V6's
active. CH-2, the bass boost resonator, connects from the grid of V-14.
one of the output tubes, and one plate of the 6SC7, the side that's used
for the audio pre-amp. Just lift one side of the choke. I don't think
its the choke. See what the Bass switch is actually doing. According to
the diagram its shorting the choke when in the IN position, Since it is
resonating at the grid I think it should be the other way. In any case
disconnecting it should prove one way or the other. If it turns out its
boosting the bass in the OUT position it would explain why the hum gets
greater, simply because the gain goes way up, but not the source of the
hum. Also, does the volume control have any effect on the hum? If so
what effect? Also note the phase splitter grid comes from the junction
of the two 6V6 grid resistors at R-39. At this point it is getting the
unbalanced audio from the output of the two halves of the 6SC7 and
generates a balancing signal. This is the out of phase signal that dries
the other 6V6. It is also what puts the effect of the tone control and
bass boost on both sides. I have forgotten the name of this type of
phase splitter but its very common. Also, look at C-44 for ripple. There
should be very little. If the boost circuit works as I think any ripple
or even a heater to cathode leak in the 6SC7 will be magnified by the
resonant boost in the grid of the first audio and in the phase splitter.
A couple of minutes with the scope should tell you. BTW, did you
change the 6SC7, if its got a leaky heater it could be the cause of the
hum. More poking is needed.
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On 2/15/2025 2:33 PM, thoyer via groups.io wrote: ¡°When it comes to the switch area, that schematic is spinning my head¡±
Glad it is not just me¡¡ ?
I have not had time to get back to this, maybe tonight.
I believe it is in the 6V6 area because when I pull the 6SA7 the symptom
remains. Maybe the choke has some leakage to ground? I was going to pull
it last night and check it on my Sencore inductance tester. Got side
tracked and didn¡¯t get there. I¡¯ll try tonight.
Tom
W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Tom When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level?
The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking?
Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
I unscrewed CH2 from the chassis and let it float, no change. ?
From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum ? Ok, tried two different 6V6¡¯s, no change. ? On the plates of the 6V6¡¯s there is 6.8Vrms of 120hz ripple. If I remove the 6v6¡¯s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6¡¯s back in (Russian tubes BTW), switching the bass ¡°IN¡±, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o¡¯scope. ? I did some poking through my ¡°inventory¡± of parts and do not have a suitable choke to swap in for CH2 ¨C still thinking there may be some leakage going on there¡¡? ? Fun stuff huh? ? Tom W3TA ? ? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? ? My two cents: If the 120Hz ¡°hum¡± is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
--
don??? va3drl
|
I am curious about that switch. I would interpret IN as meaning the
bass was boosted but the switch shows it the other way. Also the
frequency response chart shows the bass boosted with switch "IN".
The TONE control is a simple RC high end roll off so should not
affect the hum. I have at least two SX-28 handbooks, one the military
handbook, all are the same.
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On 2/15/2025 6:10 PM, thoyer via groups.io wrote: Richard,
When the bass switch is ¡°in¡± it boosts the bass. Going by the schematic
it shows that it shorts the choke / cap. Audibly it definitely boosts
the bass, I need to confirm the wiring of the switch vs the schematic.
Wouldn¡¯t be the first error I¡¯ve found.
Yes, the tone adj and the bass boost are independent.
You suggested shorting the choke ¨C but isn¡¯t that effectively what the
switch is doing?
And I agree, I had most of this radio torn apart so it is a good
possibility there is a miswire somewhere in that circuit ¨C wouldn¡¯t be
the first time¡¡¡..
Thanks,
Tom
W3TA
*From:*[email protected] <[email protected]> *On
Behalf Of *Richard Knoppow via groups.io
*Sent:* Saturday, February 15, 2025 7:15 PM
*To:* [email protected]
*Subject:* Re: [HallicraftersRadios] SX-28A Hum
I have not read all the posts on this thread so may be repeating
something. Tom, I do not have an SX-28, In normal operation what does
the bass switch do? Does the bass increase or decrease when the switch
is in the IN position? The handbook is confusing. The schematic suggests
the BASS switch boosts the bass by adding a resonant choke on the output
of the first audio amplifier. The schematic shows the switch cutting off
the choke in ON, that looks backward to me. The additional TONE control
is just a conventional high roll off. It appears that both of these tone
controls are independent, is that correct.
It seems to me the bass boost should INCREASE hum.
The plate transformer of the output stage connects to the input of
the B+ filter. This is not raw AC since there is a fairly large cap
across it. The balanced circuit should remove any residual hum. The
rest of the amplifier is fed by filtered B+ and seems unlikely to have
significant hum on it. Try shorting out the tone control choke, if its
getting induced hum that should eliminate it.
Since you practically rebuilt the receiver perhaps some wiring
error was made or some new part is actually bad. Worth going over again.
It is always frustrating to me to try to trouble shoot problems
where I can't just jump in an make measurements.
On 2/13/2025 4:21 PM, thoyer via groups.io wrote:
I just finished pretty much a complete overhaul of a nice condition
(physically) SX-28A. Replaced just about every resistor and all caps.
Radio is working well except for a 120hz hum. When I switch the Bass
in,
the hum goes away.
Yes, there is ripple on the plates of the 6V6¡¯s, about 6vrms, but that
is there independent of the bass switch position. The output side of
the
HV filter is clean, no ripple. I paralleled another 47uf 450v cap
across
the choke input cap and there was no difference.
I double checked the wiring against the schematic along with component
values. Is the schematic correct in this area? I found once schematic
error maybe there is another?
Thoughts?
Tom
W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Richard, we agree about the switch position labels on the schematic being reversed. ?
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From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:00 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? I am curious about that switch. I would interpret IN as meaning the
bass was boosted but the switch shows it the other way. Also the
frequency response chart shows the bass boosted with switch "IN". +++++++++ -- don??? va3drl
|
The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line.
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On 2/15/2025 7:15 PM, don Root wrote: It seems that the way to look at this switch is that in one position the
plate load is provided by the choke while in the other the load is
switched to R37, and as an aside the choke gets shorted ? Any votes on
this? ?just for fun try the thumbs ?voting machine, I won¡¯t look.
**
--
don??? va3drl
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
6SC7, sorry. ? Thanks for the input, yes more poking is needed. Hopefully later tody. ? Tom ?
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From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 12:08 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Do you mean the 6SA7 or 6SC7? The 6SC7 is the dual triode that acts
as the first audio and phase splitter. Pulling that leaves the 6V6's
active. CH-2, the bass boost resonator, connects from the grid of V-14.
one of the output tubes, and one plate of the 6SC7, the side that's used
for the audio pre-amp. Just lift one side of the choke. I don't think
its the choke. See what the Bass switch is actually doing. According to
the diagram its shorting the choke when in the IN position, Since it is
resonating at the grid I think it should be the other way. In any case
disconnecting it should prove one way or the other. If it turns out its
boosting the bass in the OUT position it would explain why the hum gets
greater, simply because the gain goes way up, but not the source of the
hum. Also, does the volume control have any effect on the hum? If so
what effect? Also note the phase splitter grid comes from the junction
of the two 6V6 grid resistors at R-39. At this point it is getting the
unbalanced audio from the output of the two halves of the 6SC7 and
generates a balancing signal. This is the out of phase signal that dries
the other 6V6. It is also what puts the effect of the tone control and
bass boost on both sides. I have forgotten the name of this type of
phase splitter but its very common. Also, look at C-44 for ripple. There
should be very little. If the boost circuit works as I think any ripple
or even a heater to cathode leak in the 6SC7 will be magnified by the
resonant boost in the grid of the first audio and in the phase splitter.
A couple of minutes with the scope should tell you. BTW, did you
change the 6SC7, if its got a leaky heater it could be the cause of the
hum. More poking is needed.
On 2/15/2025 2:33 PM, thoyer via groups.io wrote: ¡°When it comes to the switch area, that schematic is spinning my head¡±
Glad it is not just me¡¡ ?
I have not had time to get back to this, maybe tonight.
I believe it is in the 6V6 area because when I pull the 6SA7 the symptom
remains. Maybe the choke has some leakage to ground? I was going to pull
it last night and check it on my Sencore inductance tester. Got side
tracked and didn¡¯t get there. I¡¯ll try tonight.
Tom
W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom ?
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From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, February 16, 2025 3:26 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level? The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking? Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux. Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote: I unscrewed CH2 from the chassis and let it float, no change. ? From: HallicraftersRadios@groupsio <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum ? Ok, tried two different 6V6¡¯s, no change. ? On the plates of the 6V6¡¯s there is 6.8Vrms of 120hz ripple. If I remove the 6v6¡¯s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6¡¯s back in (Russian tubes BTW), switching the bass ¡°IN¡±, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o¡¯scope. ? I did some poking through my ¡°inventory¡± of parts and do not have a suitable choke to swap in for CH2 ¨C still thinking there may be some leakage going on there¡¡? ? Fun stuff huh? ? Tom W3TA ? ? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? ? My two cents: If the 120Hz ¡°hum¡± is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
--
don??? va3drl
|
Tom OK, does R35 have any affect on the hum?
Can you scope the terminals of CH2 when the Bass switch is in both positions?? You most likely will have to do either a differential measurement using channel B minus channel A because of the B+ or you can disconnect one end of R37 to remove B+ from CH2.? You might also adjust R35 to see if it has any affect on the o'scope display.? Testing so far seems to indicate that CH2 is the source of the hum in the audio amplifier.??
Have you reset the mounting screws for both CH1, CH2 and the power transformer?? Somewhere there is a path for magnetic flux to affect the audio amplifier.? 120 cycle hum seems to indicate that the source is after the rectifier but the only center tap secondary winding in the power transformer is in the high voltage winding so I also suspect the power transformer.
The ripple you see on the plates of the 6V6 appear to be what is expected.? 7 volts of ripple amounts to 2.5% of the B+ which indicates adequate power supply filtering so the filter cap C49 is doing it's job. Sticky wicket! Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom ?
From: [email protected] <[email protected]> On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, February 16, 2025 3:26 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum ? When R33, the audio gain control, is set for minimum volume, do you still hear the 120 cycle hum regardless of S10, the Bass IN / OUT switch position?? If you still hear hum then the source of the hum is not before the volume control.? The hum must be coupled into the audio amplifier after the volume control.? Does R35, the tone control, have any affect on the 120 cycle hum level? The SC-28 manual does not show the physical locations of chokes CH1 and CH2.? Are they located next to each other?? Is it possible to swap the mounting ends of one of the chokes to see if it affects the hum level?? Could it be possible that they are now physically configured as hum aiding instead of hum bucking? Finally, do all the audio "ground" connections to the chassis use screws and washers or are they soldered?? Soldered is the least problematic, screws more so due to corrosion which may not be obvious to the casual observer.? Soldering would require a 100 watt American Beauty or similar soldering iron which are, unfortunately, not as common as they once were.? Nokorode is OK to use, it is not an acid flux. Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Saturday, February 15, 2025 at 07:54:33 PM CST, thoyer via groups.io <thoyer1@...> wrote: I unscrewed CH2 from the chassis and let it float, no change. ? From: HallicraftersRadios@groupsio <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum ? Ok, tried two different 6V6¡¯s, no change. ? On the plates of the 6V6¡¯s there is 6.8Vrms of 120hz ripple. If I remove the 6v6¡¯s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6¡¯s back in (Russian tubes BTW), switching the bass ¡°IN¡±, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o¡¯scope. ? I did some poking through my ¡°inventory¡± of parts and do not have a suitable choke to swap in for CH2 ¨C still thinking there may be some leakage going on there¡¡? ? Fun stuff huh? ? Tom W3TA ? ? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? ? My two cents: If the 120Hz ¡°hum¡± is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
--
don??? va3drl
|
Richard ,agree again Re ¡°I am pretty sure the markings on the schematic are reversed.???? ??Agree again
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+.¡±??? perhaps if the choke resistance is high? ¡in the ?Halli ¡°OUT¡± position, at mid frequencies, won¡¯t the choke resistance determine the gain? ? ? After a gazillion hours, here is ?how Don sees the base boost switch. ?.. comments? Image here
End image and all
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From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:31 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line. ? -- don??? va3drl
|
Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ? ? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other. CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
Don, Richard: I do not agree. CH2 is listed as a 4Hy choke in the parts list, and the C43 across is a 5100pF (0.0051?F). Both form a parallel resonant circuit centered at 1114 Hz. So when they are both ¡°in circuit¡± loading the plate of V12, a maximum stage gain will be developed at this frequency and almost nothing way below or way over that. Not sure HOW that can be considered a ¡°bass boost¡±. For me, the maximum bass response is developed when the V12 plate is loaded only with the R37 resistor. On the IN position of the switch (according to the schematic). ? 73, Jacques, VE2JFE in Montreal ? ? Richard ,agree again Re ¡°I am pretty sure the markings on the schematic are reversed.???? ??Agree again
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+.¡±??? perhaps if the choke resistance is high? ¡in the ?Halli ¡°OUT¡± position, at mid frequencies, won¡¯t the choke resistance determine the gain? ? ? After a gazillion hours, here is ?how Don sees the base boost switch. ?.. comments? Image here
End image and all
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From: [email protected] [mailto:[email protected]] On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 5:31 AM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? The B+ goes to the plate of the first audio through R-37, when the
Bass switch is in the IN position (as shown on the schematic) the choke
is bypassed, the B+ to the plate through R-37, when the switch is in the
OUT position, as shown on the diagram R-37 is bypassed and the B+ now
from R-38 goes to the choke and thence to the plate. So, in one position
the plate load is R-37 and in the other position its the choke and
condenser. I am pretty sure the markings on the schematic are reversed.
It seems to me the Bass switch also changes the gain of the stage by
increasing the B+. If I am seeing it right either R-37 or the resonant
choke is put out of the circuit by the Bass switch. It is 2:30AM and I
have no business being on line. ?
--
don??? va3drl
|
Jacques that¡¯s not fair, you are doing your own arithmetic.? And much of what you say must be right, but nobody mentioned what C42 is doing. In my quick re evaluation after peaking at the manual, without the choke and its cap the output is not what I would call normal/ flat, instead C-42 is pulling down the response[plate AC volts] as frequency goes up. I suppose that can be base boost, but usually base boost does not mean ??kill the mids and really kin the highs. I will leave it to you to propose exactly how the response works when the out works, I¡¯m gonna cheat and look in the book. I had to draw stuff, maybe this will help some others doodle around.. ? Image
End image
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From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 3:31 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum
Importance: High? Don, Richard: I do not agree. CH2 is listed as a 4Hy choke in the parts list, and the C43 across is a 5100pF (0.0051?F). Both form a parallel resonant circuit centered at 1114 Hz. So when they are both ¡°in circuit¡± loading the plate of V12, a maximum stage gain will be developed at this frequency and almost nothing way below or way over that. Not sure HOW that can be considered a ¡°bass boost¡±. For me, the maximum bass response is developed when the V12 plate is loaded only with the R37 resistor. On the IN position of the switch (according to the schematic). ? 73, Jacques, VE2JFE in Montreal -- don??? va3drl
|
Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ?
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From: [email protected] <[email protected]> On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 3:17 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
Try another tube. Maybe heater to cathode leakage
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-------- Original message -------- From: "thoyer via groups.io" <thoyer1@...> Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum
Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
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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
____________________________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
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From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 6:32 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Try another tube. Maybe heater to cathode leakage -------- Original message -------- Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
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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.
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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
|
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
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
|
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. When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out. 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). 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.
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.
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. 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 substituting
it.
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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
|
Jacques_VE2JFE
Jim Whartenby
don Root
Mike Langner