Uniden PC122, low AM modulation

[Uncle Ronnie 336]

yeah, i never had one that was this restricted. usually can pull it back to about 2W and it screams

 

[Handy Andy]

In something like this, I'd start checking the Voltage at the Jumper board.

The best way to tune this is to pre-set the AM power to 5.6 Volts - SOLID.

Don't peak anything at this time, just observe.

Look at wattage and the Bias. Keep Bias mA to 50mA on Driver and Final.

Look on foil side for rework - correct and clean, reheat cold joints as needed Reflow the leads with a touch of rosin core solder to let the flux in it reflow around the leads.

Using No Mod, check jumper voltage across it to ground - then modulate. Does the voltage drop as well as the RF swing in any simple RF meter?

Don't kill it by removing the limiter - at least not yet.

Then don't use too much mod, just low - level types of sound to see if a Positive swing can be made. IF you see forward swing - then the issue is from limiter area - if' you can't get any forward swing at 5.6V - then how far do you turn down the voltage before the swing is not backwards - NOR forwards - but steady?

Once at that voltage - then I'd look at peaking - retune the strip, can you see an increase in power - then recheck - if the thing re-creates the original condition - then something is going on in the limiter section.

This may be an over peaked radio - so be careful with too much power trying to get swing - these are unobtainium right now.

This thread has got three different variants of the same radio - might want to do voltage checks around the Pre-driver - as this thing ages, even the DISC caps can fail - turn into plain old dirt sandwich.

 

[skylarrz]

Keep me posted if - in case, something develops...

Just some random going thru old electronics-i repair pretty much any newer electronics and i found one of these (or similar at least) from 1988. I know its either junk-or someone would prolly kill for it and its parts. Best i can tell it works fine, looks undamaged,
Just wondering, something from 1988 like this that i can barely find info on could be invaluable to someone in need... Just doing my part to help others out here. Shoot me an email at skylarr12227@gmail.com if its not junk-if it is dw about it and ill toss it in a week or so.

 

[Uncle Ronnie 336]

Just some random going thru old electronics-i repair pretty much any newer electronics and i found one of these (or similar at least) from 1988. I know its either junk-or someone would prolly kill for it and its parts. Best i can tell it works fine, looks undamaged,
Just wondering, something from 1988 like this that i can barely find info on could be invaluable to someone in need... Just doing my part to help others out here. Shoot me an email at skylarr12227@gmail.com if its not junk-if it is dw about it and ill toss it in a week or so.

well, does it fire up?

 

[doffo]

To help with Bias...

My main beef has been R172 not having a way of dividing down the 8 volt TX current power load - for when it's driven hard, the Bias itself can climb - and can blow the Bias diode (MV-1Y) by doing so - from too much Bias applied, and SSB signal driven hard during a QSO with the ALC turned nearly off - not effective - against strong output from the TX strip.

If you have to do it (run it) that way...

Might as well install a divider to offset some of this voltage....got a 180 ohm resistor? Install it across R172's leg to the Driver just past VR9 (not L32) to Foil Ground

• - Not chassis (Case) ground - Board POWER ground.

Follow this as an example
View attachment 43310
Whether it's a Cobra SSB or Uniden SSB radio
Realistic or otherwise...
Locate VR9 and R172 install a 180 ohm 1/4W
resistor from the junction of the VR9 output to R172 and Foil Ground underneath VR9
The graphic on the left is for Cobra SSB radios that have Driver VR pot located just before the Bias Diode.

Is it necessary to re-adjust Bias?
You should verify that there is 50mA OR LESS at the Drivers TP jumper,
then you're set. Reduces the chance for a catastrophic Driver failure.


Kinda' (Or Very) IMPORTANT!
View attachment 43314
Locate TRC-453 - R 195 (2.2 ohm) resistor
See how it is installed ...
- L32 on one leg to take place of L32 as two parts.
You may want to install resistor there at L32
keeping L32 on one leg reinstalled to
LOWER Bias Drive and to keep
BIAS and RF power away from each other.
​

This can help go a long way is preserving the Driver from self-destruct caused by Bias 8 volts not having anywhere to go except to SUM into the RF arriving from the Predriver. Too high of voltage - either by bias set for too much current or from excessive drive, the current can rectify and damage the Driver permanently.

Thank you very much for that information. I went ahead and did that to my wife's good working Uniden 122. Added the 2.2 ohm resistor in series with L32, then did the 180 ohm resistor on VR9 to the circuit board ground.

Do these problems exist with the older radios like the TRC-451 and the Cobra 146 GTL? From what I gathered the TRC-465, 453, and the Uniden 122 are basically a "shrunk" version of the older boards.

Double checked the BIAS settings of the driver and final after soldering in the resistors and things checked out ok.