Uniden PC122XL - No Transmit

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:

The following pages have been useful getting me this far:

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.

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.

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


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

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?

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.

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..