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Radio Make and Model: Realistic TRC-450 AM/SSB CB
PCB Number:
PLL Number: MB8734
Test gear available: limited availability of test equipment at present.
Symptoms: As soon as I turn on the radio in CB mode with switch in INT SP (internal speaker) position, I get a howling noise from the radio. If the mic isn't plugged in, I don't get anything unusual. And, I don't get that noise when in SSB mode or in PA mode.

What is working:

What has been tried: I tried another mic with same results.

Did it suddenly happen ?: I haven't had the radio out of the original box in many, many years and it wasn't used much even before that.

Anything else that may be of help?: This radio has not been modified; still OEM condition. I have no intentions of modifying this radio. I have the original Owner's Manual with the schematic included.

I do not know this actual radio, but I suspect that the fault is caused by dried up capacitors in the audio stages. When the microphone is unplugged it cuts out the speaker so the noise will stop.

Sometimes you can spot the bad caps visually as they will sometimes dome up or leak a little, however, some caps can also be bad and not leak or dome up.

I would replace all electrolytic capacitors near the audio amp in the radio.

Look on the main board at the electrolytics. If they are rated at 10V, mark its top with a red marker pen. Check all of them.
When you are done, replace ALL of the marked caps. And make sure you get all of them...

Doing this will cure 95% of these types of fault from this chassis (Uniden MB8719/34) and prevent a lot of future failures too.

When I get one of these in for repair, I will not do anything else without doing this first. If the owner does not agree with this (i.e. trying to get the repair as cheap as possible) then they are welcome to take it elsewhere

This eliminates the dickhead customer "you fixed this only 3 weeks ago and now its doing that so it must be your fault and I want it fixed for free" type issues nicely...

Thanks Guys for your recommendations, which seems to be a common theme, I'll let you know the results. This repair is for myself, so I'll have no one but myself to blame if it doesn't work.