Hello, I'm new here with an old radio. I am troubleshooting a 60-year-old Knight-Kit C-540 6 tube transceiver. It receives 23 channels, transmits on 1 (just have 1 crystal.) The radio now receives and transmits reasonably well, though it has seemed weak for many years, but I still have one persistent current problem: after transmitting, the receiver frequency suddenly shifts many channel positions on the dial. It does not depend on the length of transmission or speed or amount of button pushing.
This is not normal warm-up drift. The receiver can remain stable for quite a while, but pressing the microphone button and transmitting will cause the received channel position to jump (up or down on the dial.) The new position can then remain stable until the next Tx.
When properly adjusted, the channels can line up quite well. The problem is that transmitting causes an abrupt change in receiver frequency. I have not noticed any voltage changes happening at the time of the jump. It could be a brief event. The tuning capacitor & coils are not moving but do need to be physically moved to re-align the channels with the dial after a jump.
Some history that may be relevant:
The radio has worked as it should whenever used all these years until I turned it on in 2020 when the transmit-induced frequency shift along with a lot of static and popping suddenly appeared. It had not been mishandled in any way. I've replaced many old components on and off over the past 6 years and removed some of the noise. Some went away by itself. Tx and Rx remain weak.
The frequency change after TX is sudden rather than a gradual thermal drift. I don't see anything wrong with the mic contacts or wiring.
Multiple tube pin voltages are not correct, some very much so, but must be close enough because it mostly works.
I remember the audio output modulator,12AQ5, running very hot back in the '60's and it had to be replaced once long ago, it got weak. The current 12AQ5 tube seems weak now simply because it is not the obviously hottest tube in the set. It seems to be drawing far too little current. The audio preamp, 6AN8, and receiver oscillator mixer, 6CL8, seem to be drawing less current than expected also.
I also remember some capacitors (C-7 near the transmitter oscillator amplifier) causing much noise when objects were brought close to them. They are not doing that now.
A few years ago the static noise had an up and down pattern to it that happened by itself. This year the static was mostly uniform, you could say static, punctuated by loud popping sounds. Replacing many original filter caps got rid of a little static.
The bad static I had since 2020 went away recently after the radio was on for several hours. It wasn't anything I did, as far as I know. I wasn't even working on it at the time. Other types of noise have come and gone by themselves too.
I have another CB available as a signal source and normally transmit into a 50-ohm dummy load so I can use its leakage as a relatively weak reference signal.
At the moment the old radio actually sounds quite good and has relatively little unusual noise. The remaining major problem is this receiver-frequency jump caused by transmitting.
I would particularly appreciate opinions on whether this sounds more like:
>a receiver-oscillator component changing value when TX/RX voltages switch
>an intermittent contact in the TX/RX switching, (doubtful IMO)
>oscillator pulling through shared circuitry,
>a temperature-sensitive capacitor around L-4, (doubtful IMO)
>a problem with the L-4 coil or its slug, (doubtful IMO)
>weak tube(s),
>or something else typical of old tube CB sets.
I have the full original schematic,which can show any changes I made, and can post voltage measurements or close-up photographs of the transmitter/receiver oscillator areas (L-4/V-2) if useful. What I don't have is troubleshooting experience or any diagnostic equipment. I have 2 tranceivers and a multimeter.
Included is a schematic showing components I've replaced. I hope you can read it.
Thanks for your help. Jim55
This is not normal warm-up drift. The receiver can remain stable for quite a while, but pressing the microphone button and transmitting will cause the received channel position to jump (up or down on the dial.) The new position can then remain stable until the next Tx.
When properly adjusted, the channels can line up quite well. The problem is that transmitting causes an abrupt change in receiver frequency. I have not noticed any voltage changes happening at the time of the jump. It could be a brief event. The tuning capacitor & coils are not moving but do need to be physically moved to re-align the channels with the dial after a jump.
Some history that may be relevant:
The radio has worked as it should whenever used all these years until I turned it on in 2020 when the transmit-induced frequency shift along with a lot of static and popping suddenly appeared. It had not been mishandled in any way. I've replaced many old components on and off over the past 6 years and removed some of the noise. Some went away by itself. Tx and Rx remain weak.
The frequency change after TX is sudden rather than a gradual thermal drift. I don't see anything wrong with the mic contacts or wiring.
Multiple tube pin voltages are not correct, some very much so, but must be close enough because it mostly works.
I remember the audio output modulator,12AQ5, running very hot back in the '60's and it had to be replaced once long ago, it got weak. The current 12AQ5 tube seems weak now simply because it is not the obviously hottest tube in the set. It seems to be drawing far too little current. The audio preamp, 6AN8, and receiver oscillator mixer, 6CL8, seem to be drawing less current than expected also.
I also remember some capacitors (C-7 near the transmitter oscillator amplifier) causing much noise when objects were brought close to them. They are not doing that now.
A few years ago the static noise had an up and down pattern to it that happened by itself. This year the static was mostly uniform, you could say static, punctuated by loud popping sounds. Replacing many original filter caps got rid of a little static.
The bad static I had since 2020 went away recently after the radio was on for several hours. It wasn't anything I did, as far as I know. I wasn't even working on it at the time. Other types of noise have come and gone by themselves too.
I have another CB available as a signal source and normally transmit into a 50-ohm dummy load so I can use its leakage as a relatively weak reference signal.
At the moment the old radio actually sounds quite good and has relatively little unusual noise. The remaining major problem is this receiver-frequency jump caused by transmitting.
I would particularly appreciate opinions on whether this sounds more like:
>a receiver-oscillator component changing value when TX/RX voltages switch
>an intermittent contact in the TX/RX switching, (doubtful IMO)
>oscillator pulling through shared circuitry,
>a temperature-sensitive capacitor around L-4, (doubtful IMO)
>a problem with the L-4 coil or its slug, (doubtful IMO)
>weak tube(s),
>or something else typical of old tube CB sets.
I have the full original schematic,which can show any changes I made, and can post voltage measurements or close-up photographs of the transmitter/receiver oscillator areas (L-4/V-2) if useful. What I don't have is troubleshooting experience or any diagnostic equipment. I have 2 tranceivers and a multimeter.
Included is a schematic showing components I've replaced. I hope you can read it.
Thanks for your help. Jim55
