52 year old tube CB transceivre has ~50VAC chassis to gnd. Can't locate reason. Also noise from internet router.

KeithWalker

Joined Jul 10, 2017
1,677
P.S. Can you explain what you meant by 'Whatever is causing the interference is probably breaking through the IF amplifier.'? Do you mean a similar frequency may be breaking into the IF amp from the outside? Or interfering with the IF amp from the inside because of the similar freq?
If there is a component in the radiated interference that has a frequency close to the transceiver IF frequency or a harmonic of it, It may be picked up by the IF amplifier and mixed with the IF signal at the detector to produce audible interference.
You mentioned that an AM receiver also picked up the interference, so that is a possibility.
Keith
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
I can't find a way to record the noise in mp3 format.
Poking around the radio with my meter the only steady frequency readings I get are 60 Hz on the chassis, even from the speaker playing the noise. This is not the noise freq. The noise pulses are slower and dirty with static.
I tried reconnecting the 0.12 uF, 600 V paper cap but it's leaky. The inductive pickup detects a lot of noise from it. I put in 0.4 uF as ceramics. They are quieter than the one they replace but they are noisier than the electrolytics in the power supply. The whole circuit is quieter than it was before I swapped out the old caps. I have several 0.1 uF caps rated from 5KV to 10KV. They are non polarized but are supposed to be for DC use . I don't want to damage them by putting them in an AC part of the power supply even at a fraction of their rated voltage. I'm trying to find a place to get some new caps. The one remaining store in the area after Radio Shack left is also no longer selling many parts.

I looked for leakage between windings in the power transformer but found none. It is very quiet electrically.

Thanks to all for your help.
 
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Thread Starter

Darkstar

Joined Sep 3, 2010
177
Update...I found the source of the noise. It was a coax coming from the back of the router to a small TV. The TV was completely powered down but when the coax was moved it certain positions, the noise on the CB became very loud. The coax and TV were apparently acting like an antenna for this router signal. I wonder why it was transmitting? An ATT tech told me the routers communicate via wifi even if it is disabled through the computer and the computer is off. He told me how to turn off the router wifi within the router itself, which I did. It had no effect on these transmissions & noise. Like turning the router off, as soon as I unplugged the coax the noise went away while the router remained on.

Someone had mentioned above that I should continue to use the 0.12 uF non-polar paper cap to smooth ripple from the power supply. I'd said ceramic caps were working. Without the noise I could hear the difference. After searching and not finding a replacement cap with similar uF, voltage, size, and non-polarity, I went back to the ceramics. I found that using seven 0.1 uF ceramics in parallel gave a similar sound to the closest cap I had available, a 0.5 uF, 2500 V non polar cap which was way to big to fit into the underside of the CB. The ceramics easily fit.
 
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