Re: Interesting observation concerning crystals...
What I run into more than anything else are drifted crystals, which can play havoc in circuits such as band-switched heterodyne oscillators and seriously affect dial accuracy.? I haven't tried the method you mentioned for trying to revive a dead crystal, but will give it a shot sometime!
?
73 - Steve, KW4H
|
Re: Interesting observation concerning crystals...
If I understand this the crystals are physically present but have
not been working. Yes, it is possible that hitting them with drive from
a signal generator may have awakened them. I would suggest that you try
baking all the "dead" crystals. Someone will correct me but I think the
temperature is around 350F for a few hours. This will sometimes wake up
dead crystals and also move drifted crystals back to their original
frequencies. Reports are that it works much of the time. The reason is
not certain but it seems to clean deposits off the surface or, perhaps
renew aged plating or soldering. Whatever is the explanation it seems to
work. It is possible that driving the crystals hard with an external
signal may accomplish a similar effect.
toggle quoted message
Show quoted text
On 3/11/2025 10:31 PM, Justin Bowser - KI5GKD wrote: My sickness concerning old vintage radios had morphed into revisiting
the CB radios I messed with when I was a young 'un.? I have noticed that
several have had one or more crystals out, which in a synthesized radio
one crystal is likely to take out 4 or more channels.? During my beating
on these old relics I decided to put a DDS signal generator across the
crystal, dial in it's frequency, and verify the "lost" channels were now
there and usable.? All worked as I expected except when I unhooked the
signal generator the channel was still functional!? Sometimes it would
stop working when I switched back and forth to a channel not on the
crystal, sometimes not.? The latest I tried was a Realistic TRC-47 which
is a 23 ch SSB rig and it had one crystal "dead."? I hooked up the SG
and brought the group of channels back to life and unhooked the SG.? All
channels were still alive and remained that way for the rest of the
day!? I'll see tomorrow if they still work when I turn it on.
Can someone explain this behavior?? It's almost like "goosing" the tired
crystal "repaired" it, or something.? After I get finished playing with
these old CBs I'm going to pull a Heathkit receiver off the shelf that
has a couple bad crystals and see if it will wake them up.
73,
--
Justin B.
KI5GKD
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Interesting observation concerning crystals...
My sickness concerning old vintage radios had morphed into revisiting the CB radios I messed with when I was a young 'un.? I have noticed that several have had one or more crystals out, which in a synthesized radio one crystal is likely to take out 4 or more channels.? During my beating on these old relics I decided to put a DDS signal generator across the crystal, dial in it's frequency, and verify the "lost" channels were now there and usable.? All worked as I expected except when I unhooked the signal generator the channel was still functional!? Sometimes it would stop working when I switched back and forth to a channel not on the crystal, sometimes not.? The latest I tried was a Realistic TRC-47 which is a 23 ch SSB rig and it had one crystal "dead."? I hooked up the SG and brought the group of channels back to life and unhooked the SG.? All channels were still alive and remained that way for the rest of the day!? I'll see tomorrow if they still work when I turn it on.
?
Can someone explain this behavior?? It's almost like "goosing" the tired crystal "repaired" it, or something.? After I get finished playing with these old CBs I'm going to pull a Heathkit receiver off the shelf that has a couple bad crystals and see if it will wake them up.
?
73,
--
Justin B.
KI5GKD
|
Re: Fw: Re: [HallicraftersRadios] Off-The-Wall Question
Got the manual today, very nice work.? This is the first I had ordered from you and most likely won't be the last. :-)
?
73,
--
Justin B.
KI5GKD
|
The only component not replaced is a large-ish mica cap buried under the LO coil assembly. I will get to that in a day or two after gently lifting the coil assembly.
?
Tom
On 03/10/2025 2:23 PM PDT Jim Whartenby via groups.io <old_radio@...> wrote:
?
?
Tom
The two original paper caps in the LO compartment are the ones that you say are still there and need to be replaced?
?
What C# are they?
Jim
?
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
?
?
?
?
Don, here's a photo of the underside in its as-found state. Not too bad but not something a sane person would plug in and turn on without at least replacing the electrolytics. Obviously it had seen some repairs over the years. I have since replaced most resistors and capacitors and replaced the old 2-wire power cord with a modern 3-wire.
?
Tom
?
On 03/08/2025 5:06 PM PST don Root <drootofallevil@...> wrote:
?
?
Tom, did you mention what state it was in when you got it.. as to the components around the 6K8 . original looking? ¡ or otherwise? ??Any mucking by somebody could have moved a connection¡. and¡so!
?
--
don??? va3drl
?
|
Tom The two original paper caps in the LO compartment are the ones that you say are still there and need to be replaced?
What C# are they? Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Don, here's a photo of the underside in its as-found state. Not too bad but not something a sane person would plug in and turn on without at least replacing the electrolytics. Obviously it had seen some repairs over the years. I have since replaced most resistors and capacitors and replaced the old 2-wire power cord with a modern 3-wire.
?
Tom
?
On 03/08/2025 5:06 PM PST don Root <drootofallevil@...> wrote:
?
?
Tom, did you mention what state it was in when you got it.. as to the components around the 6K8 . original looking? ¡ or otherwise? ??Any mucking by somebody could have moved a connection¡. and¡so!
?
--
don??? va3drl
|
Re: Classic Exchange - let's get these old radios on the air!
I got on the air for CX Phone this evening and received a couple interesting questions.? One was wondering why I was CQing on such an odd frequency such as 7205.525 kHz and another asking if it was possible that my transmit frequency might be changing as much as 100 Hz.? Fun questions!
There's another 45 minutes left for CX today and it'll resume on Tuesday.? CX for CW will be on Mar 30 and April 1.? Details at
If you use the spotting feature, drag the scroll bar at the bottom to the right to enter your callsign or posting won't work.
Cheers
Halden VE7UTS
|
Jim & Tom? The Hexode signal grid has a max? or min? voltage of -3? presumably to the cathode [depends who you read] , but my conclusion from a Sylvania ??blurb? is that it can go as positive as -3 volts, and as negative as -30 where there is little transconductance, and so is often connected to the AVC, but apparently not in the S-20R. but that does not help the situation. As Jim says, on band 1 ?there is no ground, and I say there is no control of THAT grid voltage by ground, AVC or other, so it floats. On the other bands the bottoms of ?3,2,1 can be grounded [all depending] providing a DC ground ?but it goes thru the coil{s} so is strange to me. But band 1 is a real mystery to me too. ? ? Tom, I suggested a test for the bandswitch area? was it not practicle? ? ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Sunday, March 9, 2025 2:46 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice? That is interesting.? The tube manual indicates that -3 volts should be there.? Have you replaced C5 and C8?? Minor leakage in either could cancel the expected negative bias on G3 of the 6K8.? From the QST article, I expect that there are unintended interactions when the expected voltages are out of spec. In looking at the schematic for the umpteenth time, I see how the upper band tank coils are returned to ground through switch 7D rotor that shorts the bottom of the band coils to ground.? Not sure how L13 is grounded when the band switch is in the BCB setting.? If you measure the DC resistance from the grid cap to ground you should measure R12 (33 ohms +/- 20%) plus the coil resistance for each band switch setting.? Is this the case??? Measuring from the switch side of R12, you should measure just the tank coil resistance to ground.? That resistance should increase as the band switch connects to the next lower tank coil.? If you don't see the coil resistance then perhaps the oscillator stator sections of either C1 or C2 is grounded? Likewise, the switch side of C37 should measure the LO coil resistance to ground for each band switch setting.? Like the above, each lower band coil resistance will be higher.? Is this the case? Remote troubleshooting is a s-l-o-w process, Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy On Sunday, March 9, 2025 at 11:47:14 AM CDT, Allthumbs via groups.io <btse1@...> wrote: Thank you, Jim. The 6K8 control grid (grid cap) measures 0.00 vdc, pretty much as expected. -- don??? va3drl
|
Re: Fw: Re: [HallicraftersRadios] Off-The-Wall Question
Pete - That was me!? I just noticed that I accidentally ordered
two copies but only need one...
73
Justin B. - KI5GKD
On 3/8/2025 1:05 PM, Peter A Markavage
via groups.io wrote:
toggle quoted message
Show quoted text
?
Justin:
?
I didn't recognize your name in my initial response. I
indicated I just finished a manual for a HE-43C. When I was
putting the mailing label on the envelope, I realized it was for
you. So it's on the way. Ah; the perils of aging.
?
Pete, wa2cwa
?
----- Forwarded Message -----
Date:
Sat, 8 Mar 2025 12:49:44 -0500
Subject: Re: [HallicraftersRadios] Off-The-Wall
Question
?
Lafayette HE-43C is not rare. It is identical to the HE-20C
(which we sold 1000's of these sets - HE-20, 20A, 20B, 20C(relay
and electronic switching versions), 43C) except the 43C works on
110V AC and 6 volts DC. The 20C works on 110V AC and 12 volts
DC. Other then that, they are identical. I just finished a
HE-43C manual for a customer. Both the HE-20C and 43C were made
by United Scientific Laboratories (USL) in Long Island City, NY.
A tunable receive and a crystal controlled transmitter was
quite common with a number of manufacturers in the early 60's. A
Spot switch obviously was common on these transceivers.
?
Pete, wa2cwa
?
?
On Sat, 08 Mar 2025 08:46:48 -0800 "Justin Bowser - KI5GKD"
< justin.bowser@...>
writes:
Pete - I'll give that a try and let you know.? I also got
another rare/unusual bird coming, an HE-43C.? I've never seen
a CB where you transmit with crystal and receive by VFO.? What
I plan doing is to put in a circuit that replaces the crystals
with an Arduino controlled 5351 module.
?
73,
Justin B.
KI5GKD
_._,_._,
--
Justin B.
KI5GKD
|
Tom That is interesting.? The tube manual indicates that -3 volts should be there.? Have you replaced C5 and C8?? Minor leakage in either could cancel the expected negative bias on G3 of the 6K8.? From the QST article, I expect that there are unintended interactions when the expected voltages are out of spec.
In looking at the schematic for the umpteenth time, I see how the upper band tank coils are returned to ground through switch 7D rotor that shorts the bottom of the band coils to ground.? Not sure how L13 is grounded when the band switch is in the BCB setting.? If you measure the DC resistance from the grid cap to ground you should measure R12 (33 ohms +/- 20%) plus the coil resistance for each band switch setting.? Is this the case???
Measuring from the switch side of R12, you should measure just the tank coil resistance to ground.? That resistance should increase as the band switch connects to the next lower tank coil.? If you don't see the coil resistance then perhaps the oscillator stator sections of either C1 or C2 is grounded?
Likewise, the switch side of C37 should measure the LO coil resistance to ground for each band switch setting.? Like the above, each lower band coil resistance will be higher.? Is this the case? Remote troubleshooting is a s-l-o-w process, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Thank you, Jim. The 6K8 control grid (grid cap) measures 0.00 vdc, pretty much as expected.
?
Tom
On 03/08/2025 3:26 PM PST Jim Whartenby via groups.io <old_radio@...> wrote:
?
?
Tom
The following is excerpted from QST, February, 1941
?
"The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen.
These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip...
The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio."
?
The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode.
Just a thought,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
?
?
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty¡ªthe LO is not oscillating.
?
I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I¡¯m posting the readings in the chart below. FYI¡ªI took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.
?
?
If it helps, here's the mixer section:
If you¡¯re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I¡¯m posting will point to a solution that would mean I won¡¯t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)
?
Any thoughts? Thank you,
?
Tom
|
Don, here's a photo of the underside in its as-found state. Not too bad but not something a sane person would plug in and turn on without at least replacing the electrolytics. Obviously it had seen some repairs over the years. I have since replaced most resistors and capacitors and replaced the old 2-wire power cord with a modern 3-wire.
?
Tom
?
On 03/08/2025 5:06 PM PST don Root <drootofallevil@...> wrote:
?
?
Tom, did you mention what state it was in when you got it.. as to the components around the 6K8 . original looking? ¡ or otherwise? ??Any mucking by somebody could have moved a connection¡. and¡so!
?
--
don??? va3drl
|
Thank you, Jim. The 6K8 control grid (grid cap) measures 0.00 vdc, pretty much as expected.
?
Tom
On 03/08/2025 3:26 PM PST Jim Whartenby via groups.io <old_radio@...> wrote:
?
?
Tom
The following is excerpted from QST, February, 1941
?
"The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen.
These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip...
The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio."
?
The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode.
Just a thought,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
?
?
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty¡ªthe LO is not oscillating.
?
I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I¡¯m posting the readings in the chart below. FYI¡ªI took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.
?
?
If it helps, here's the mixer section:
If you¡¯re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I¡¯m posting will point to a solution that would mean I won¡¯t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)
?
Any thoughts? Thank you,
?
Tom
|
Re: For Sale - Hallicrafters R-274/FRR with speaker
No need for a deposit.? Here is my email address where I can send you my addesss: dwight_talley@...?
Sorry for the late reply.?
?
?
Dwight
|
Tom, did you mention what state it was in when you got it.. as to the components around the 6K8 . original looking? ¡ or otherwise? ??Any mucking by somebody could have moved a connection¡. and¡so! ? -- don??? va3drl
|
Jim, interesting stuff? but I wonder, after your bold part You seem to have forgotten to emphasize ?¡°but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained.¡±??? From my readings, ?in 1939/40, the 6K8 was the preferred mixer for all wave radios, not that it matters to Tom¡¯s obstinant oscillator ?problem. Presently , I wonder what controls the hexode signal grid DC voltage, as using my eyes, it is not connected to ground, AVC, or anything. {a Riders book shows it to be ?connected to avc} ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Saturday, March 8, 2025 6:27 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-20R mixer cct. #photo-notice? The following is excerpted from QST, February, 1941 "The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen. These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip... The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio." The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode. -- don??? va3drl
|
Thank you, Jim. I will measure it and report. In the meantime, I just got back from a big swap meet and need to dig out from all my new treasures. ?
?
Tom
On 03/08/2025 3:26 PM PST Jim Whartenby via groups.io <old_radio@...> wrote:
?
?
Tom
The following is excerpted from QST, February, 1941
?
"The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen.
These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip...
The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio."
?
The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode.
Just a thought,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
?
?
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty¡ªthe LO is not oscillating.
?
I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I¡¯m posting the readings in the chart below. FYI¡ªI took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.
?
?
If it helps, here's the mixer section:
If you¡¯re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I¡¯m posting will point to a solution that would mean I won¡¯t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)
?
Any thoughts? Thank you,
?
Tom
|
Tom The following is excerpted from QST, February, 1941
"The high oscillator transconductances of the 6K8 and 6SA7 make them particularly suited for all-around usage. They oscillate strongly at high frequencies where L/C ratios are unfavorable. The 6A8 construction is not satisfactory for amateur usage because of instability in the oscillator. The oscillator electrode is a pair of rods located in the tube between the No. 1 grid and the screen. These side rods collect electrons from the cathode stream and the electrode current is controlled by the No. 1 grid. Unfortunately, changes in signal-grid or screen voltage also change the anode current. This conductance between signal grid and oscillator causes instability with variation in a.v.c. voltage. Fluctuations in screen voltage due to supply regulation also change the frequency ... skip... The 6K8 has been used extensively by the amateur and also the commercial manufacturer principally because it gives fair stability, and design problems are usually simple. The tuned-grid oscillator shown in Fig. 5 gives very little trouble and is easy to build.?The oscillator frequency is not independent of screen and a.v.c. voltages, but in most designs the frequency shift caused by one is offset by the other so that good stability is obtained. The 6K8 has an effect known as space-charge coupling which is experienced at high frequencies. This effect is as follows: The oscillator voltage on the No. 1 grid causes a fluctuation in the number of electrons in the region of the signal grid. The electron density changes at the oscillator frequency and as a result a displacement current flows into the signal grid. At high frequencies where the signal grid and oscillator frequencies are quite close, the impedance of the signal grid circuit at the oscillator frequency is quite high and as a result the displacement current produces an a.o. voltage across the signal grid circuit. This voltage, when smaller than the bias, reduces the gain of the tube slightly. Under extreme conditions it overrides the bias and causes rectification in the signal-grid circuit, causing a serious loss in gain. The coupling can be neutralized by a small capacitance - approximately 2 or 3 ??f d. - between oscillator and signal grids. Commercial practice is to use a condenser (known as a "gimmick") made by wrapping two pieces of wire together to give the desired capacitance. Neutralizing the space charge increases the gain and image ratio."
The bold print above is my doing.? Perhaps a little attention should be given to other tube sections.? ?In your tube pin measurements, shown below, you do not measure the grid cap voltage.? If C5 is leaking then the 6K8 control grid will be positive which will have an affect on the triode oscillator section of 6K8 because of the common cathode. Just a thought, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Newish member of this group, first time posting. I have an S-20R in need of repair. I am not a trained technician and am not particularly good at interpreting symptoms. However, I do have the chassis working just as it should from the grid of the first i.f. through the audio output. But the mixer stage is faulty¡ªthe LO is not oscillating.
?
I have a voltage chart for an S-20R that was made up by a member of the Antique Radio Forum. The tube pin voltage readings on my chassis agree almost entirely with his chart, except for the 6K8 mixer. I¡¯m posting the readings in the chart below. FYI¡ªI took these readings with the chassis powered via a bucking transformer so that it runs on 117 vac. On the first line are his readings and on the second line, in red, are my own. FWIW, my voltage readings were made with a DMM, not a VTVM.
?
?
If it helps, here's the mixer section:
If you¡¯re familiar with these sets, you might recall that the underside of the 6K8 socket is a bit difficult to access because the oscillator coil assembly partially covers it. Maybe the symptoms I¡¯m posting will point to a solution that would mean I won¡¯t face the daunting task of partially disconnecting and ?lifting the whole coil assembly.? :-)
?
Any thoughts? Thank you,
?
Tom
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Fw: Re: [HallicraftersRadios] Off-The-Wall Question
?
Justin:
?
I didn't recognize your name in my initial response. I indicated I just
finished a manual for a HE-43C. When I was putting the mailing label on the
envelope, I realized it was for you. So it's on the way. Ah; the perils of
aging.
?
Pete, wa2cwa
?
----- Forwarded Message -----
Date: Sat, 8 Mar 2025 12:49:44
-0500
Subject: Re: [HallicraftersRadios] Off-The-Wall Question
?
Lafayette HE-43C is not rare. It is identical to the HE-20C (which we sold
1000's of these sets - HE-20, 20A, 20B, 20C(relay and electronic switching
versions), 43C) except the 43C works on 110V AC and 6 volts DC. The 20C works on
110V AC and 12 volts DC. Other then that, they are identical. I just finished a
HE-43C manual for a customer. Both the HE-20C and 43C were made by United
Scientific Laboratories (USL) in Long Island City, NY.
A tunable receive and a crystal controlled transmitter was quite common
with a number of manufacturers in the early 60's. A Spot switch obviously was
common on these transceivers.
?
Pete, wa2cwa
?
?
On Sat, 08 Mar 2025 08:46:48 -0800 "Justin Bowser - KI5GKD" < justin.bowser@...>
writes:
Pete - I'll give that a try and let you know.? I also got another
rare/unusual bird coming, an HE-43C.? I've never seen a CB where you
transmit with crystal and receive by VFO.? What I plan doing is to put in
a circuit that replaces the crystals with an Arduino controlled 5351
module.
?
73,
Justin B.
KI5GKD
_._,_._,
|
Re: Off-The-Wall Question
?
For what its worth - the Lafayette mechnical filter Part Number in the
HB-600 is 1253-24.
?
Pete, wa2cwa
?
On Sat, 8 Mar 2025 17:35:22 GMT "k2orsfan via groups.io" < k4pf@...>
writes:
Hi,
re: your Lafayette HB-600 CB
The Kokusai mechanical
filter might be repairable.
A common failure mode is that the foam plastic
originally used
within the filter case to dampen mechanical shock
has
turned to gunk over the years.
Here is a repair article:
I'm
guessing that the filter is not used during transmit.
For an A.M. - only CB
it is more likely to have a high level A.M.
modulated class C final. So,
your lack of modulation
may be due to another problem in the audio
chain.
73,
Ed Knobloch K4PF
_._,_._,_
|
Re: Off-The-Wall Question
?
Lafayette HE-43C is not rare. It is identical to the HE-20C (which we sold
1000's of these sets - HE-20, 20A, 20B, 20C(relay and electronic switching
versions), 43C) except the 43C works on 110V AC and 6 volts DC. The 20C works on
110V AC and 12 volts DC. Other then that, they are identical. I just finished a
HE-43C manual for a customer. Both the HE-20C and 43C were made by United
Scientific Laboratories (USL) in Long Island City, NY.
A tunable receive and a crystal controlled transmitter was quite common
with a number of manufacturers in the early 60's. A Spot switch obviously was
common on these transceivers.
?
The HB-600 had one major service bulletin. There were excessive failures in
the 2N2949 RF Power Amplifier so a board and circuitry with a different RF Power
Amplifier transistor was designed and circuit diagram and install instructions
were provided.
?
For a short period, The HB-600 was replaced by a HB-600A which had the
revised RF Power Amplifier already installed. There were also some other circuit
changes but don't recall what they were.
?
?
On Sat, 08 Mar 2025 08:46:48 -0800 "Justin Bowser - KI5GKD" < justin.bowser@...>
writes:
?
Pete - I'll give that a try and let you know.? I also got another
rare/unusual bird coming, an HE-43C.? I've never seen a CB where you
transmit with crystal and receive by VFO.? What I plan doing is to put in
a circuit that replaces the crystals with an Arduino controlled 5351
module.
?
73,
--
Justin B.
KI5GKD
?
|
Allthumbs
Jim Whartenby
don Root
Peter A Markavage