Old CB radio with tubes, Any Tx causes Rx freq drift up and down. Help me solve this sudden new problem.

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Out of the blue this new problem popped up just after I solved the previous noise problem which was due to 1) an intermittent contact between C-12 and a neighboring connection on L-4 and 2) a bad connection on V-4, hidden 53 yrs, that had been good until just now. The mica caps in the tuning circuit around L-4 in attached schematic were sensitive to anything coming close, conductive or not. This had never before been a problem but lately it could be one day then go away the next day. The mica caps C-9, 11, 12, 13, 14 were not sensitive to heat or cold. L-4 was the only component sensitive to cold. Now it changes with temperature and any time I key the mic. The tuning slug is not moving by itself. Once warmed up it will be stable all day, unless I key the mic. I tried swapping out C-11 and C-12 but there was no change so mica caps were reinstalled. In the previous 53 years there has never been any freq drift, now just a brief transmit can cause a shift in freq up or down of over 23 channels which means every channel on the dial can be shifted way off the dial.

I haven't found any loose connections and I've taken multiple voltage readings in Rx and Tx mode but there don't seem to be any significant variations. The only things that seem capable of causing this are caps and maybe a tube going bad. Rx does seem weak. Last week I heard many people, today nothing, but it does receive signals from other transmitters I have. I guess their signals are strong enough. Anybody have a clue?
 

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debe

Joined Sep 21, 2010
1,251
If L4 is heat sensitive, then it may well be faulty. I have had that type of fault with a coil in a TV back in the day of valves. The only fix then was replace the coil.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
If L4 is heat sensitive, then it may well be faulty. I have had that type of fault with a coil in a TV back in the day of valves. The only fix then was replace the coil.
There is a little drift as the unit warms up. Blowing on it with a heat gun had no effect. Hitting it with freeze spray had a big effect. I don't understand what is making it, or something else in the Rx circuit, be affected by the Tx being used.

The Rx was stable yesterday, day and night, just a little drift. No multiple jumps per second like I was getting a few days ago. It cooled over night and was still good when I turned it on this morning.

Then I used the Tx and all 23 channels shifted off the dial.
 

LowQCab

Joined Nov 6, 2012
776
I would make a "guesstimation" that there is nothing wrong with any of your Tuned Coils,
they are generally only moderately sensitive to Humidity/Corrosion, and not much else besides physical damage.
On the other hand ..........
ALL Capacitors, some more than others, deteriorate with age/heat, ALL OF THEM MUST BE REPLACED.
Modern Capacitors are WAY more stable and will last much longer than ANY Capacitor made before the '60's.

A similar, but not quite as common problem exists with Resistors.
Some of them are reasonably good,
and some of them won't last 10 years without ........
Value Drift / Noise / Intermittent Connections / changes due to intermittent increases in Current / Heat / etc.,
and they probably started life with a 20% Tolerance anyway, which is just not acceptable.
Therefore, replace all the Resistors too.

Next, these aging parts, especially the Capacitors, start to throw-off the Bias Currents in the Tubes because
of developing DC Current Leakage.
This can put tremendous strain on the Tubes and wear them out pre-maturely.
So, just off-hand, don't bother to "test", just replace all the Tubes on general principles, they wear-out.
Replace the Tubes AFTER you replace all the Caps and Resistors.

There's one more bonus that comes with doing a re-build like this .......
Quite often, with equipment this old, previous repairs have been made, sometimes by an IDIOT.
So you will be re-Soldering every single connection in the Radio,
insuring along the way that every connection is properly solid and routed correctly.
And you may uncover a cheezy previous repair attempt.

Such is the price you pay for the "Cool-Factor", and Nostalgia, of using a Tube-Powered piece of equipment.
Don't get me wrong, I have a soft-spot for old Tube Equipment too.
But sorry, even cheap Chinese Junk will ultimately perform better, and probably last longer too.

Bonus .........
After you replace all the questionable parts with far superior modern equivalents,
and then do a proper "Alignment",
you will probably have a decent Radio that will last for generations.

Have you ever watched "Mr. Carlson's Lab" on YouTube ??
https://www.youtube.com/channel/UCU9SoQxJewrWb_3GxeteQPA
If you like Tubes, you'll love it.
.
.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
I would make a "guesstimation" that there is nothing wrong with any of your Tuned Coils,
they are generally only moderately sensitive to Humidity/Corrosion, and not much else besides physical damage.
On the other hand ..........
ALL Capacitors, some more than others, deteriorate with age/heat, ALL OF THEM MUST BE REPLACED.
Modern Capacitors are WAY more stable and will last much longer than ANY Capacitor made before the '60's.

A similar, but not quite as common problem exists with Resistors.
Some of them are reasonably good,
and some of them won't last 10 years without ........
Value Drift / Noise / Intermittent Connections / changes due to intermittent increases in Current / Heat / etc.,
and they probably started life with a 20% Tolerance anyway, which is just not acceptable.
Therefore, replace all the Resistors too.

Next, these aging parts, especially the Capacitors, start to throw-off the Bias Currents in the Tubes because
of developing DC Current Leakage.
This can put tremendous strain on the Tubes and wear them out pre-maturely.
So, just off-hand, don't bother to "test", just replace all the Tubes on general principles, they wear-out.
Replace the Tubes AFTER you replace all the Caps and Resistors.

There's one more bonus that comes with doing a re-build like this .......
Quite often, with equipment this old, previous repairs have been made, sometimes by an IDIOT.
So you will be re-Soldering every single connection in the Radio,
insuring along the way that every connection is properly solid and routed correctly.
And you may uncover a cheezy previous repair attempt.

Such is the price you pay for the "Cool-Factor", and Nostalgia, of using a Tube-Powered piece of equipment.
Don't get me wrong, I have a soft-spot for old Tube Equipment too.
But sorry, even cheap Chinese Junk will ultimately perform better, and probably last longer too.

Bonus .........
After you replace all the questionable parts with far superior modern equivalents,
and then do a proper "Alignment",
you will probably have a decent Radio that will last for generations.

Have you ever watched "Mr. Carlson's Lab" on YouTube ??
https://www.youtube.com/channel/UCU9SoQxJewrWb_3GxeteQPA
If you like Tubes, you'll love it.
.
.
Thanks for your answer. I agree, the coils and most other things look almost brand new. The tuning slug can't be moving when there is nothing touching the radio. The caps in the area are mica so should be good. I don't know what can happen when a tube starts to go bad. I watch Mr. Carlson's Lab too, it's good.

I'm the original owner of this radio. I built it in July, 1967 It was a gift for getting on the honor roll in high school. Aside from replacing a bad 12AQ5 tube (which always ran very hot) I've never had to do repairs before or had any problem with it until this year. Speaking of the 12AQ5, In July this year it had its usual blue glow on one side. In late November I noticed the glow was gone. I haven't found out why. This link should get you to a photo of it: https://forum.allaboutcircuits.com/attachments/light-and-dark-photo-1-jpg.224335/
The 2 poor connections I recently found stayed hidden this long because they weren't bothered until I began poking around recently. With age and wire oxidation the contacts became intermittent. Those have been the only source of noise I've been able to find in my radio. Back in high school we were taught to wrap wires well, then solder. So this chassis, with no PC boards and all point-to-point wiring is full of many tight components. For some reason it's acting in ways it never did before. It will be a big job to rebuild it from scratch, and expensive if I break any hard to replace parts. It would be easier to find a shop and pay them to fix it but this is just a small hobby. That's why I've been trying to narrow down the location of a part that would have suddenly gone bad about 6 days ago for no reason. I have no fancy test equipment so that makes it hard. I could make the rebuild look neater, but this is original. It even has the original Knight Kit tubes, my notes in the instruction book and the color coded wires. That's why the schematic has colored wires. It still transmits and receives and if I could stop the drift it would be fine. I tried different caps around L-4 but they act like the originals. Changing parts would be easier if there was ANY store that sold discrete components anywhere these days. To change one cap it means paying delivery fees and waiting a week just to find out I have to try a different one. I could have finished this long ago if Radio Shack was still around to I could drop in any time and pickup parts. Ordering online takes a lot longer is more expensive. It's even been hard finding soldering irons for solder joints this size. Small tipped irons can't melt this much solder. I've begun making my own small tipped irons from large high wattage irons. I hope the one I have lasts. It too was hard to find.

As long as the radio is mostly working, I'm going to think about putting an antenna in the attic. Either the mobile antenna on a large groundplane or the inverted V which worked better but is more directional. I may have more room above my garage but getting the coax to my unit will be tricky.
Thanks for your help.
 
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