PC-122 Troubleshooting

[WECSOG]

I have a Uniden PC-122 that I have had since it was new, about 1993. It is unmodified. I dug it out recently and powered it up. Channel display and everything else seems to be working perfectly, but it neither receives nor transmits in any mode. Tried an external speaker; still no band noise. The PA function does work, hence the modulator works. I opened it and looked at the board, didn't see anything amiss. None of the electrolytics are swollen or anything like that.
I've reached out to Handy Andy because I've seen references to information he had on cbtricks, but don't have access to the actual files. I know there are other folks on here with in-depth info about this rig, so I figured I would put it out here as well.

Anyone have any insight as to common problems or what is the next step for troubleshooting? Any help will be much appreciated.

 

[sp5it]

First I would check if VCO is working.
MIke

 

[WECSOG]

First I would check if VCO is working.
MIke

Thanks, Mike. Good idea.

 

[Handy Andy]

Well if you have a variable power supply with overload protection, just hook up the power cord, mic, screw on a simple dummy load on the antenna jack - then turn down the voltage setting on the variable power supply to about 9~10 volts if possible, and let it "sit there" and power up to slowly let those caps work out their differences...

Keep ANL / NB off...

Turn (or keep) your RF gain up - so you'd hear any hiss...

Watch the power supply to see if it overloads or engages short circuit protection - if none - proceed, turn on the radio ... and just let it cook for about an hour in this setting;

Turn off, with a moment then turn on radio - listen for a Thunk or if any , or a squeal...

Mode AM,
ANL/NB off -
Volume 1/4 up CB ./ PA on CB.
Squelch fully off...

This might help recharge and rebuilt the dielectric in the caps without too much stress.

The Power supply needs to have short circuit protection to allow you to keep the caps from exploding as you do this...

After about an hour, turn off the radio and then the power supply, if the supply has a meter, look at it - see how long it takes for the main power feed lines to discharge it's cap and the radios caps...

If it dumps or drops right out - then you have a NECESSARY re-cap to do - start planning and buying for the caps in the radio are definitely dried out. ESPECIALLY if it SQUEALS on power up...bootstraps are bad dried shorted...

Else next step is to allow the radio to see 12~13.8V DC for a while...

You may also get noise, like a thunk now when you power up - keep squelch off - but it's a good sign you're getting a radio back.

It's a drying out process, if you know it's stock OEM - then just don't go buy up stuff just to tear out parts we can possibly save and keep you from having more trouble...

Because you're gonna' need to look at the VCO to see if it will even oscillate...

 

[Robalo]

C101 - takes out AN612. Kills 1/2 the IF for anything AM or SSB...

Good call! right ANDY

 

[Handy Andy]

Here's your VCO...



Locate Pin 16 of your PLL chip... See above - is a power pin. Lock is - Pin 15 too, it uses a DETENT pin off of the Channel Selector to VALIDATE pins as well as use a 1uF Cap - polarized, 50V - so you know... it uses a 10K pull up resistor and this chip only uses 5 Volts. Pin 16 is power good, Pin 15 is LOCK . So don't add nothing too big ok? Pin 16 uses 15 and vice a versa...



Do not let all this information intimidate or scare you - it's here for you as a reference - take your time...




Your PLL Lock pin, is 15 - goes low if things are wrong. Prevents you from keying up the TX.

Pin 16 is an ENABLE pin, tells the PLL things are set and power good...

IF something is wrong on 15 or 16 - it won't fire - if 16 is not working - 15 doesn't either...

 

[WECSOG]

Thanks! I've actually had it powered up for a couple hours now. The caps don't seem to be doing anything untoward, externally at least.
I slowly adjusted L13 slug ccw, which made speaker hiss increase.

 

[Handy Andy]

Ok, now that you hear a hiss that's a good sign...

But I don't think that just turning slugs until it works, is really the approach we should take - you need to turn the RIGHT SLUG - Just the RIGHT AMOUNT to restore function - if that even needed to be done...

You have scope on hand?

I just wanted to step back a moment and describe something I used to help me keep radios from blowing up or dying on the workbench because of a short in soldering or other issues - including bad caps.

I made my own power supply short circuit protection unit - not as fancy as those others with immediate protection - nor was that it's purpose - it was used to help me determine if I should dive back into a radio because I caused a short in my work, or there was a short in it and I didn't want it blowing up the more expensive parts in my bigger power supplies.

I just used an older 2.5Amp Radio Shack Regulated power supply - those tiny ones - worked great for just letting a radio idle and power up and also you can throw a simple wedge base bulb and socket in line with the + power cord and use either a 194 wedge (2W) lamp, or even a 921 Wedge (21W) to help determine if a radio has a dead short in the power feed socket - like a blown diode - that if it was you have to fix the blown shorted diode and anything else past it.

The bulbs filament will act as a "shock absorber" and lite up - indicating a fault in the system and drain off or absorb the current flowing thru the bulb, into this badly shorted circuit... it would tell me right away that I had to fix a short in the power supply connector.

If it lit and started to dim out quickly - it simply meant older parts needed to be looked at...

When it lit brightly then dimmed but not fully off - I had something more exotic to work on inside...

So now back to your issues...

You'll need to locate some of those Tank Coils by the VCO / PLL - L13 is one and I hope you still remembered where it was set - because I think you should really move it back there...that hiss just may indicate you're "Air noise" from the radio's own internal birdies...

Find and locate L38, and L13...Only these two - for now...

L38? Insert your tool and turn it inwards, or Clockwise, - count your turns - then reset it back - and"marker" the line the slug makes along it's slot for memory...

Do the Same for L13...marker afterwards to make sure you have memory...

Count not just the number of turns to bottom out these slugs - but use it as a way to restore them to OEM position then "tweak" to see if you can get your Oscillation.

Whoops or Oops? When and - if you go too far - you can then use this procedure to restore-reset the slug to OEM position.

Because you may not get a "lock" - that means then, that the other aspect of old caps and the bane of it...you can swap old C63 and C64 - with new ones and see if it restores the loop...

Again, old caps so be patient with the Patient...

 

[WECSOG]

Ok. Yeah, I'm familiar with the light bulb current limiter/indicator.
The reason I adjusted L13 is I was reading the alignment procedure in Secret CB 24, which calls for adjusting L13 for 4.5 volts at TP2, AM rx, channel 40. Unfortunately I'm getting 7.35 V. If I've correctly identified TP2 through the wax.

I do have a scope, but I'm just using my Fluke 75 atm.

 

[WECSOG]

I've dug out enough wax to ascertain that I was indeed on TP2. I also returned L13 to where it started.

 

[Handy Andy]

Then with it that high - yes you're not in lock.

But you also need to maximize L38 (which you may need that scope to do this) At TP4 - and that goes back to the age of the Electrolytic Caps C63 and C64 - they are in the graphics in my above posts. Too weak and they may not accept anything from L38...

You may not ever get a lock on that TP2 if C63 and C64 have drifted off...

Then TP2 on Channel 40 - your voltage you wanted...thru L13

Those two should get you rolling...

 

[WECSOG]

Ok. Thanks for your help so far!

 

[WECSOG]

I had to order some more caps. I have lots of caps, but couldn't find any 2.2s. So I ordered some, and they should be here Thursday.

 

[Handy Andy]

Well do your caps look like this?

Again from the Wiki...​

Then yes, you have some work ahead of you...

 

[WECSOG]

Nope, not even swelled. I have actually watched one explode before though, an experience I would prefer not to repeat!