UHF PA Board

[fchrist16]

Guys,

The PA Board on two of my UHF radios doesn't want to push power out. When I key up the radio drops from 13.84 (+/- a few microvolts) to 13.82 (+/- a few microvolts).

Any ideas on what cables to check? I have checked all cables from the PA to the Pre-Filter board, etc and all are secure. What should I be looking for now?

Regards,

-Frank C.

[AEC]

We will need some cclarification here...
What brand/make radio are you referring to, and how new/old is it as well.
This will help determine the type of PA used, be it transistor, or module.

For starters, check the current draw, not just voltage drop, this will tell you if the proper amount of current is being drawn, or more than specified.
Too much current, you'll fry the PA in short order.
Are you able to receive signals in-band well, or is the receiver deaf as well, or sounding off frequency?
What test equipment do you have at your disposal?
Check interconnecting cables, coaxial jumpers (if any), as well as any socketed devices that may be inside, under shields.
There may be issues with a specific make/model that you might be using, so all pertinent information regarding this will be most helpful from the start.

[fchrist16]

I'm usually good about this and totally spaced!

Radios: Motorola Spectra D44's in UHF R3 with ALL cap's replaced.

Test Equip: A voltmeter and the amp guage on my Pyramid PS-21KX

The one UHF that does work, when keyed drops from 13.84 (+/- a few microvolts) to 12.76 (+/- a few microvolts). So I know the issue lies within the PA Board not "kicking on".

The radios were completely cleaned and re-assembled and all connectors are tight (at least to my hands) but something is still missing.

Regards,

-Frank C.

[AEC]

Sounds like a dash mount, and the PA is under the cover, underneath.
Common problem was the PA transistor/s overheated due to lengthy keydown times, which caused solder migration on the collector, and base, sometimes shorting both to ground.
Second issue from this initial problem was the traces under the base and collector lifting from the board(same heat issue).
Look for diodes across the transistor(poor man's feedback circuit), and ohm them to make sure they are not shorted.
Many times, there are none, but there's always a time when they show up unexpectadly.
Ohm out anything in the D.C supply, or bias within the PA, if you can't draw at least 10 amps on TX, check the transistor itself, both input and output for RF, note the dB difference between the two if possible.
Always check current draw, it's one of the best indicators of trouble.
Without seeing it in person, it's difficult to say this or that is bad, or this is shorted, Etc..

[fchrist16]

Yes all of them are dash mounts.

I'm not 100% familiar with my Voltmeter so I'm not sure how I would ohm things out with it.

You give me locations but I don't know where half the stuff is on the PA Board. All I really know is how to test for lack of power draw and getting other things fixed up.

Regards,

-Frank C.

[fchrist16]

Anyone?

I got a wattmeter and will hook that up to tell me what's going on, but I assume it'll be the same. micro-watts of power.

Regards,

-Frank C.

[com501]

Find an Elmer in your area.

[fchrist16]

I have but its hard to meet up with him.

Regards,

-Frank C.