Uniden PC78LTW No receive audio or signal when on CB setting

[SkyhawkOH]

Hi, This is my first time posting. I bought a used Uniden PC78LTW. Here are the issues and what I've done so far.
When the radio has sat for awhile, no power connected. I connect the power, turn it on, with the switch set to CB, i hear receive audio for like 2 seconds then it goes away. After that, there is no cb receive audio or signal showing on the meter. I can key the mic and there is output and audio being transmitted as I can hear myself on another CB. I key the other CB and cannot hear myself on the Uniden, nor do I get any receive meter movement on the Uniden. I plugged in an external speaker, still no cb audio.. I plugged in the ext speaker into the PA and it works loud. With the internal speaker, I tried WX and it comes through loud and clear. The volume control is also working on weather. I turned off the Dynamic squelch, no effect. I sprayed all the controls with contact cleaner including the CB, WX, PA switch. No difference. I pressed down my finger across the circuit board. When I got to the audio chip, i heard the hum noise from my finger touching. There were no intermittent circuit traces that popped on when pressing down on the board. I srayed and wiggled all the plug in connectors, no difference. The radio looks unmodified. I need some direction on this. Is it a bad electrolytic or ceramic cap somewhere? Any help appreciated. Thanks. SkyHawk Cleveland, OH

 

[midnight special]

did ya tighten the circuit board screws? sometimes that is the culprit,,

 

[SkyhawkOH]

I'll try it after work....

 

[SkyhawkOH]

Tightened board screws, they were all loose, but did not fix the problem, thanks

 

[guitar_199]

I'm going to jump in and say....

1) if your finger touching the audio chip got a hum/buzz out of the speaker... AND.... you transmit and modulation works then that suggests that all of your audio section is working...and would seem that the transmit is working.

2) if transmit is working... that suggests that your PLL and the transmit oscillator are working.

3) the fact that your meter does not show any received signal (no movement for and incoming signal)... that suggests something from the RF amp through the mixers and IF stages to the detector is not working.

You have any kind of test equipment? Something in the way of a scope or signal tracer would be handy...

 

[SkyhawkOH]

I have an Osilloscope , a DDS Signal generator and vtvm, but I'll tell you, I'm a novice at electronics...learning as I go... I may have to give it to a more experienced tech. I wish i had a photo or the color coded wires as to how they are connected to the CB/PA/WX switch. I'm wondering if someone added a jumper or something. I didn't know you can blast the WX out the PA....

 

[Handy Andy]

To go back to your first post...

You have the Radio in CB mode, but after about 2 minutes, it "dies" no audio.

But your PA works, WX does too - but only thru PA.

Ok...You don't get RX thru the internal speaker when it dies, but do you get it thru PA then? If not, then the answer is in the RX itself.

You lose your PLL lock for RX or the RX side dies because the RX voltage to power your receive - doesn't, or is not - getting into the RX strip - or you have a problem (possibly) with the SQ control too - an open line in the Squelch circuit can do this - but you WOULD NOT lose your RX RF meter. So something in the PLL losing lock - the RX strip itself or the RX / TX switching is causing this.

Start by reheating some soldering joints close to where the RX comes in - may be a broken solder joint - examine caps for leaking bulging or age related foggy aluminum casing cover on that cap. Ok, it's an older radio so even bad soldering is possible - at least you'd know it was soldered as your working your way thru the RX side of this thing. Caps though, can affect the AGC action by sucking power out of the Power lines to the RX side.