Uniden PC122XL - No Transmit

[Pwing]

Hi, this is my first post, and I'm thankful that such a great community exists for the HAM and CB radio hobbies!

I purchased a very good condition PC122XL recently, made in 1992 in the Philippines. The seller said it was working fine, and when I received it, I noted that it was very compact and in great external condition.

The first thing I did was ignorantly hook it up to my bench PSU at 14.4VDC and spend about 1-2 minutes keying the transmitter to see what the power draw was. There was no antenna or dummy load attached to the PL-259 connector in the rear. I now know this was a bad idea.

I also fabricated my own VHF cables for the test setup and accidentally made a shorted cable (short was in the connector). I used this cable intermittently for 1-2 minutes to key the radio into a dummy load with a Power/Mod/SWR meter; this caused the transmitter to output into a short circuit before I identified the problem and fixed the cable.

TLDR: I ignorantly transmitted first into an open circuit, and then into a short circuit! I'm relatively certain that is why the transmit function no longer seems to work properly..

The test equipment I have:

• 50 ohm dummy load
• Power/Mod/SWR Meter
• Speakers connected to the different speaker jacks
• Variable DC Bench power supply
• Working Cobra 148 GTL
• Multimeters
• Soldering Iron

After taking a closer look:

• Unit powers on at 14.4VDC with resting power consumption of 3.4 W.
• When I turn the volume up and the squelch down, noise comes through the internal speaker.
• When I dead key the mic in AM mode:
  • power draw is ~30 W
  • SWR of 1
  • Modulation jumps slightly but stays at 0%
  • steady output of 6.5 W.
  • I can hear a gentle noise floor on the internal speaker.
• When I dead key the mic in USB or LSB mode:
  • power drawn is ~8 W
  • SWR of 1
  • Mod 0%
  • output power of 0 W.
  • No noise floor on the internal speaker.
• When I key the mic and speak or whistle in all 3 transmit modes:
  • no modulation on the meter
  • no mic audio on the speaker
  • no change to the power readings
• When I key the mic and whistle in PA mode with a speaker connected to the PA jack the PA output switches on and I hear some AM crosstalk, but no mic audio.
• When I key the mic and whistle in AM/USB/LSB while listening to the external speaker output, I hear the output switch on but no mic audio.

After this I opened up the radio and briefly inspected the trace side of the board, finding no obvious bad solder joints. On the other side I found no bulging electrolytic caps.

• The shorting bar at TP6 TP7 and TP8 on the schematic has 0 VDC on it at all times, even when keying in AM and SSB mode. The voltage does spike up to about 1.5V and then tracks down to 0V over about 3 seconds when I first touch the multimeter lead to it.
• I unsoldered the AM regulator TR544 (2SA-1012) and it still checks fine for a PNP with 0.65V drop from Collector to Base and Emitter to Base, and no drop for EC, CE, BE and BC.

I was hoping the AM regulator was the problem, but it seems it might be fine, and now I am not sure what to try next.

My Google searches:

PC122 Schematic - I found the schematic and board layouts​

PC122 no transmit​

PC122 no voltage on shorting bar​

PC122 AM regulator​

PC122 no modulation​

PC122 Handy Andy​

The following pages have been useful getting me this far:

Uniden PC122XL No Transmit - Tim Backstrom 2019 - https://www.worldwidedx.com/threads/uniden-pc122xl-no-transmit.244942/​

Uniden PC122XL no transmit - Seawolf18 2009 - https://www.worldwidedx.com/threads/uniden-pc122xl-no-transmit.32955/​

Uniden PC122, low AM modulation - kp3ft 2021 - https://www.worldwidedx.com/threads/uniden-pc122-low-am-modulation.261212/​

Uniden PC-122, ATTN: Handy Andy - ExitThirteen - 2018 - https://www.worldwidedx.com/threads/uniden-pc-122-attn-handy-andy.235723/​

PC-122XL in HD - Handy Andy - 2008 - https://www.angelfire.com/ultra/lsdash/Radio/pc122mods.pdf​

This is my first time trying to work on a radio of any kind. I have an EE degree, which helps me somewhat, but there is a lot to this that is unfamiliar.

I found that a major past resource on the PC122 was Handy Andy's documentation on CBtricks.com, which unfortunately lost its hosting online sometime in the last few years. I could only find the one small pdf listed above on angelfire.com from that lost treasure trove. @Handy Andy could you take a look at my situation and provide any tips?

I have attached the Schematic and PCB layouts for convenience.

 

[SuperLid]

I would check the final PA transistor. The conditions, that it was subjected to, could have caused it to fail. It should be a 2SC1969 or 2SC2312. Another thing I would do first, is see if you have any exciter power, by using a second receiver or spectrum analyzer. If you see signal fro the exciter, your final is most likely bad.

 

[Pwing]

Thank you for your reply @SuperLid , I should be able to find the PA transistor, but I am not familiar with what an Exciter does in a transmitter or where to look for it; I will do some research on that, and if you have any tips please let me know.

 

[Pwing]

Update: The below thread suggests that a diode test of 2SA1012 may not reveal if it is indeed not performing correctly as AM regulator:

Superstar 360FM - UK Vintage Radio Repair and Restoration Discussion Forum

Superstar 360FM Vintage Amateur and Military Radio

www.vintage-radio.net

I have some TIP42C's coming in the mail that I can try.

 

[SuperLid]

Update: The below thread suggests that a diode test of 2SA1012 may not reveal if it is indeed not performing correctly as AM regulator:

Superstar 360FM - UK Vintage Radio Repair and Restoration Discussion Forum

Superstar 360FM Vintage Amateur and Military Radio

www.vintage-radio.net

I have some TIP42C's coming in the mail that I can try.

If the radio does not TX in SSB, look past the AM regulator. The TX achieves RF power in stages. The final, is just that, the final stage. If the stages before the final are operating, you can see a small signal from the exciter on a spec an or using a second RX.

The final is TR538. The pinout is BCE. Lift B and C, diode test BC and BE junctions. Then diode test CE junction. There will be leakage at CE, but if it is shorted, it is bad. Same for BC/BE, but no leakage on those two.

 

[brandon7861]

Pull the shorting bar and see if you get voltage on the feed side contact (TP8) in SSB TX, that will tell us if the voltage is shorting in the finals or if the problem is before that.

 

[Pwing]

So I pulled the shorting bar and did voltage measurements at TP8 keyed and unkeyed in SSB and AM.

With the reference as the negative power supply lead, we have 7 ish volts on TP8 during AM and 14.4 volts in SSB (keyed and unkeyed were the same).

With the reference as the chassis, the voltage of TP8 was always zero.

These two different reference options make it confusing for me; do my voltage measurements show a nominal condition?

 

[SuperLid]

So I pulled the shorting bar and did voltage measurements at TP8 keyed and unkeyed in SSB and AM.

With the reference as the negative power supply lead, we have 7 ish volts on TP8 during AM and 14.4 volts in SSB (keyed and unkeyed were the same).

With the reference as the chassis, the voltage of TP8 was always zero.

These two different reference options make it confusing for me; do my voltage measurements show a nominal condition?

Check the final junctions. Look for a BC short.

 

[Cable Guy]

With the reference as the chassis, the voltage of TP8 was always zero.

Yeah, chassis ground is not used for these measurements, board ground is your Huckleberry. Board grounds are capacitively "floating" above the chassis ground, I hope I said that right. I only clip to the chassis when working with the output of the pa stage, everything else gets clipped to the neg post, tuning can, transformer case or xtal. Good luck.

 

[Pwing]

I lifted all pins on the final (TR538) and the driver (TR539) and found they both tested fine.

TR538:

• BE and BC had 0.64V drop.
• EB, CB and CE were Over Limit.

TR539:

• BE and BC had 0.66V drop.
• EB, CB and CE were Over Limit.

I made a sniffer cable for my Cobra radio, turned off noise reduction, cranked up RF gain and volume and waved it through the components of the PC122 while I was keying the transmitter and talking into the mic. I tried to get the end of the sniffer right up to the components, all over the board. No voice audio sounds, just static and mild clicks and pops when I keyed the mic.

 

[brandon7861]

When you initially measured the voltage on the shorting bar, was that with the chassis as reference? Establishing that the chassis is floating with respect to the negative lead, I must wonder if in fact there was no voltage on the shorting bar when installed.

Verify there is no voltage on the installed shorting bar with negative as reference.

I will re-read above, maybe I missed it.

 

[brandon7861]

Yea, recheck shorting bar voltage. I don't think there is anything wrong with the finals.

You initially report a steady output of 6.5w in AM, although with no modulation. Thats RF, thats good. 0w in SSB with no modulation makes sense too. I think the issue is in the audio chain, not the RF chain. I need to think a little more.

Edit: yea, that comment about no voltage on the shorting bar had me thinking either dead regulator or shorted final, and that is such a common thing that it made me completely forget you did have RF coming out. And that 3 seconds to settle to zero volts makes sense with the caps to the chassis.

 

[brandon7861]

Just thinking out loud, forgive the consecutive posting.

Given there is no modulation in SSB, I think we can look past the entire regulator circuit too (as it would have only impacted AM). Since SSB is also dead, I think the audio is disappearing further back.

edit: Superlid caught that one before i did, #5

 

[brandon7861]

Sometimes, simple things can get overlooked, like a broken wire in the mic connector. Perhaps reading other posts led my brain to exclude that possibility, thinking it was already checked or another mic tried. Looking back, don't see that it was. Perhaps all that testing did that mic audio wire in.

Edit: I'm going to bet that little 122 took the abuse just fine and that fear of the worst led us down the wrong path.

Edit again: Your schematic is the 122, your thread title is the 122xl. The schematic from CB tricks on the 122xl is much clearer.

https://www.worldwidedx.com/cbt/radios/uniden/pc_122xl/graphics/uniden_pc122xl_sch.pdf

 

[Pwing]

I checked the mic which has a 4 pin connector and it was not wired for the correct pinout. Rewired the mic. While I had it apart I noticed it was an electret mic and didn't know if the PC122 takes a dynamic or electret mic. Tried the rewired electret and still no audio or modulation.

After reading that the PC122 takes a dynamic mic, i temporarily rewired my 5 pin Cobra dynamic mic to the PC122 pinout using the electret mic cut apart as an interface, and then it worked beautifully.

Still 6.5 watts AM but now getting good modulation.
And getting somewhere between 15 and 20 watts on SSB.

Getting 60-80% modulation with speech.

Whew! what a simple solution in the end; I guess the seller just threw a random 4 pin mic with this radio when he sold it, as they obviously were using a different mic before selling it.

Thanks so very much for the help everyone; I've learned several good lessons here and gotten my introduction to poking around inside radios!

Now that it works I guess I'll have to find some other excuse to keep heading out to the garage..