> I'm slowly going through all my repeater stuff here and have
> a problem
> with the 100 watt UHF PA PL19d424895g32. No output. I checked the
> exciter into another identical PA and it works fine. Drive is
> a little
> low, 100mw, but it drives the other identical amp just fine.
> The bad PA
> draws 5 amps when keyed, the power control pot varies this
> current from
> 0-5 amps. No output out of the PA.
If it's drawing about 5 amps max with the power adjust cranked all the way
up, my two best-guesses are as follows, both centering around a problem in
the PA driver (40 watt) stage:
1. The driver (40 watt) transistor has developed a cold solder joint at the
collector tab. Try heating it up with a high-powered gun to get the solder
to reflow. If it starts working, that's your problem. But you're not done
yet. You need to diligently remove all of the old solder, including
burned/hardened residue on the PCB trace and underside of the tab, before
laying down new solder. Even if you do this well, there's still a good
chance that the plating on the tab is ruined and the problem will return in
the future.
2. The driver transistor is bad.
There are three less-likely causes:
3. The pre-driver (20 watt) device has solder joint problem per #1 above.
I don't think this is likely; if it were bad, you wouldn't be able to draw 5
amps.
4. There is a "tab" that connects the output of the driver board to the
input of the PA board. Check for broken/cold solder joints there too, and
follow the same repair methods as above.
5. You have four blown finals. Not very likely.
To help narrow it a bit further, key down for about a minute with the power
control turned down to avoid further damage; run it at maybe 1/3 (i.e. about
two amps' draw). Unkey, then feel the tops of the pre-driver and driver
transistors. If the pre-driver is warm but the driver is cold, #1 or #2 are
good possibilities. If the both are cold, #3 is a good bet. If both are
warm, #4 is likely, or #5 as a real longshot.
--- Jeff WN3A
