Uniden pc122xl weird issues.

[Uncle Ronnie 336]

Hi all I am trying to figure out an issue with this radio in which the resistor that feeds the bias voltage to the driver transistor gets wicked hot. I replaced it with what the schematic told me. This radio has intermittent transmit. Does this mean the driver transistor is bad?

 

[Handy Andy]

Hmmm...

Sorry for the delay...

Was busy with year-end yard work that has to be done before the end of the year, before the snow flies...

Ok, you're having a "bias resistor" heating up on a PC - 122 - these radios are pretty small, compact and in light of how other radios are designed, extremely simple to work on and use - making these a popular radio for back when things were much simpler than today...

That also means that when one part starts to fail, it can take out others because of the same reason - LACK of support parts to reduce the chance for failure...

Heating up means there is a fail in there now, we have to sort this out...

IT may indicate a Bias diode is shot, onto a Blown Driver (Base shorted)

OR this may also mean an assembly issue, where due to the AGE of the radio, the heat sink insulation to mount them may show it's age and needs to be "refreshed" - by removing them , unsoldering - cleaning them up with alcohol and remounting and soldering them back in.

Ok, let's get you started, you may want to try to get there and look at this link...

http://www.cbtricks.com/radios/uniden/index.htm

The above is for UNIDEN - in general, for the work you're going to be performing you should know and be able to locate the different "ways" and values they used to in the "driver and final" and schematics for these radios.

If you can read a schematic, you're well on your way to fixing this problem and preventing it from occurring again.

The Driver uses a 2166 - NPN RF Power transistor - same type they've used in some other radios at the UNIDEN link above - so at least if you can not find a source for the 2166, you can cannibalize one from another radio that used one for, RF power OUTPUT (Yes they are strong enough to be used as FINALS in some AM-Only Radios) - although sounds "Hideous" and mean.

But to do such a thing, may need to be done. For many radios that got abused by their former owners may become the "organ donor" to preserve the life of it's brethren - so you can use a dead radio for, parts to keep the PC-122 operational.

As said before, sounds hideous, but once a radios' circuit board has been cracked in two because of an accident or a careless operator, the parts may not connect per the board, but doesn't always mean the radio cannot provide parts to revive other patients on your bench.

You can also find me at this link...
http://www.cbtricks.com/handyandy/

Where I spend a little more time on some details for the PC-122. The *.HTML they are written in - although I'd love to update those pages, the site that hosts it, has done some changes which no longer allows me the luxury of adding more tips and tricks I've learned along the way since those pages have been posted. Let alone update the html to a more modern look.

SIGH...

Anyways, let's get back to the Driver/Final and overheat issue.

​

Ok, you should have something like this in the back...

​

I'm going to stop here and see where you're radios pieces wind up laying...

What part(s) are heating up?

 

[Uncle Ronnie 336]

The resistor that you put a red arrow to is the one that gets hot. also I noticed the bias pot was in a completely different position than the other two pc122s I have on the shelf to get repaired as well as the TRC 465 that I have operational currently. I think there was a little golden screwdriver action going on in here. I replaced that 12 ohm going to the bias but it is still getting hot and the transmit is intermittent like it works sometimes and doesn't at others

 

[Uncle Ronnie 336]

Btw, the uniden pro 510 XL usually has a 2166 final. I have gotten them to peak around 30 before. For a throwaway radio with the power and Darlington NPC swing mod they can really scream if you don't need to parts pig them.

 

[Handy Andy]

OK, follow thru on those other checks, for much of this points back to the Bias circuit for the Driver being the main focus.

Since these radios were made up until mid-90's that leaves 20 + Years of age related problems on top of the Bias resistor.

In my experience, the Driver needs to be unsoldered and removed from that back panel - have its' "seat" and mounting hardware cleaned up then a new heat sink compound applied - potentially a new insulator wafer has to be used (Recommended) and the part remounted.

It can save you a headache later.

The issue of Intermittent transmit may deal with the age of the capacitors on the main board as being so old, they affect the ability to keep the VCO and local Oscillator locked and working together. The RF sensitive areas can quit or their oscillation "die" because the voltages to them are weak or unsteady due to the filter caps like the electrolytic - are too old to work well

• - it happens because if a radio is not used for a while the compound used in those filter caps contains sulfur - which after time - being electrolytic, turns into sulfuric acid - adding, accelerating their demise.

The other effects are their lag time is greater, meaning they take longer times to recover their charge, which can pull down the voltages-power- the circuit requires on the line they're supposed to filter - which adds to your TX fail problem - it's reciprocal, it affects the Receive too.

So you have some work ahead of you.

Attached, Use what you can, we'll go on from there...

 

[Uncle Ronnie 336]

Without having had the time yet to poke around, what do you think the chances of the golden screwdriver adjustment on the bias pot cooking the diode connected to it?

 

[Handy Andy]

Very high...as in - it can take the diode too far, blown shorted, then took the pot with it.

But that would cause an OPEN (Pop the pots substrate like a fuse) - which would not heat up the Resistor, for in the circuit, the diode is the LAST thing - there is the Resistor - then the POT to feed them both. IF you're looking backwards from the Diode and Driver perspective.

So the "Short" or heating up issue causing the Resistor to heat up, is AFTER the pot and the Resistor, towards the Driver - so the MV-1Y may be shot.

In these older radios it is usually safer to remove the DRIVER, then check for the "short" at the PCB board - repair it - then reinstall the Driver with fresh goo and pad hardware