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:
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.
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
- 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.
- 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.
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.