Uniden PC122XL No Transmit

[Tim Backstrom]

Hello all! This is my first post here. Thanks for letting me join your community.

Referring back to this post:
https://www.worldwidedx.com/threads/uniden-pc122xl-no-transmit.32955/

I found doing a Google search on my issue.

My Uniden PC122XL, that about a month and a half back I did a complete electrolytic capacitor swap on, and was working fine afterwards up until this past weekend, stopped transmitting but receive is still working great. mechanic's post in the above thread said to measure the voltage on the shorting bar in the left rear of the radio and it should be about 13.8 volts. I'm getting 5.93v unkeyed and 5.82v keyed on the bar. Could this be the AM Regulator ( 2SA1012 ) Switch Kit says to change out? Thanks for any help on this.

EDIT: By what this page says: http://www.cbtricks.com/handyandy/PC-122/Yellow.htm it looks like the voltages I read are correct. I need to see if I get 12v on SSB.

 

[guitar_199]

Hey Tim! Also check that 8 V TX in the lower right hand corner. That will only show power when you key up.

What do you have at your disposal for test equipment?

 

[Tim Backstrom]

Hi guitar_199, I have a Dosy triple CB meter and a digital multimeter. Thanks, I'll check that voltage also. I'm thinking it's probably the driver and/or final that died.

 

[nomadradio]

If you have nearly six Volts on the shorting bar, that modulator transistor won't be the cause of the trouble.

If the shorting bar showed zero when you key the mike, that would point to it.

First thing to find out is whether or not a transmit signal is being generated in the first place. If you have a second radio you can put on the same channel as the PC122, plug a coax jumper into it and screw the shell back over the plug on the far end. This exposes the center pin as a half-inch long "sniffing" antenna.

You should hear a carrier when you key the PC122. And you should hear a sideband signal transmitting in either USB or LSB in the monitor radio.

If you don't hear anything at first, holding the sniffing antenna near the final, driver and the mixer chip upstream from them. If you hear nothing this way, don't go unsoldering the final and driver just yet.

And if you DO hear the PC122 transmitting, it's time to check the driver and/or final transistor.

73

 

[dxing.bruce]

Hi: Tim.

Not sure if you fixed your PC-122xl yet.

The 2SA-1012 is the am regulator & it blows in all these radios with the upd2824pll.
The realistic TRC465, TRC453, PC122xl.

I've fixed many of these radio and it's a common fault.
I replace the 2SA-1012 with a Motorola 2N6124 & non have ever blow again.
You have to keep these radios at 4w on AM & 12w on SSB, more puts too much
strain on the am regulator.

The 2N6124 is pin for pin identical so just remove the 2SA-1012 & replace with
the 2N6124.

Another common issue with these radio is the Audio chip runs hot as Uniden uses one of the legs on the audio chip as a voltage source & it over heats the chip so they burn out in these rigs.

Third is there is an IC chip in the center of the pc board it's a AN612 I have replaced several of these, not because of a chip problem but because the 47mf 10v electrolytic capacitor in front of it shorts out & destroys this chip.
I replace these capacitors with one of 25v rating to avoid this issue.
Usually if this chip fails due to the cap shorting out, it blows a chuck out of the chip. Same thing happens to the audio chip when it fails.


Other than these issues I would say replace all the large electrolytic capacitors in the rear of the radio with new ones of a higher voltage rating. IE: is it's a 16v cap, replace with a 25v one.

These are good little radios, Uniden just did some stupid things like cheap out on circuit design to save a few cents on each radio.

They should have used a better / higher rated AM regulator & not piggy back off the audio chip as a voltage source & added a separate voltage regulator for their needs.


I hope this help.

Bruce.

Toronto. Canada.

 

[Kazimierz]

Hello everyone

I am looking for a clear diagram for CB radio Realistic TRC-453
Regards
Kazimierz

 

[NZ8N]

http://www.cbradioinfo.com/cbtricksoldsite/radios/index.htm