Graciously provided by WB5WPA (thanks guy!)......
I've got a couple of things that I did to a UHF GE Mastr Pro
xmtr strip this year to get it back on the air. I also think that
these same problems I found have affected other GE UHF
Mastr Pro strips I've had in the past - and I'm sure that others
have had these problems too.
I've got these listed as 'symptom' and 'cure'.
Symptom #1: Transmitter's output power starts 'going away' and the
striip starts to output garbage on the band.
The cure below worked to 'clean up' the strip and also brought
the power back up. I found in this strip that one of the fingers was
not even touching the tube - and all had become dark-colored and
tarnished (as they are made of silver)!
1) Make sure the silver "fingers" that contact the PA tube are
very clean. In fact, use use a silver polish (I used "Wright's Silver
Cream" bought at the local grocery store) followed by wiping with
a wet rag.
2) Make sure the silver "fingers" contact the PA tube. Bend then
to make contact. Practice a few times putting the tube in - I also
use a swab of WD-40 lubricant on the tube pins to make sure
they slide in easy. I also swab the ring on the tube that the fingers
Symptom #2: UHF Transmitter strip dies - over time. Looking with the
service panel it seems like all readings past position "D" are weak
and don't really 'peak' when tuning the various C's and L's.
I have had around three of four UHF transmitter strips in repeater
service that died in-service over the last 10 years. I think I may
have found the problem with them - there are some caps that go
The one I just fixed died about four months after I cleaned up the
PA tube fingers (as per symptom #1 above) and after I had recently
done some continuous-duty tests on this transmitter.I'm sure this
*really* heated up these caps. I had never done continuous tests
on this particular strip before - it's almost like these caps will 'run'
for just so-many hours before they need to be changed! Like tubes!
Another sign that these this is the same fail I saw: Tuning C115 fully
closed *seems* to help. Max Power out was about 15 Watts whereas
normally with 450 volt B+ it would do over 50 Watts (when it was
good). C115 *was* only about 2/3 to maybe 3/4 meshed before - but
it took fully meshing it to work with the bad caps described below.
1) Replace C107, a 12 pF capacitor on V102 (the IPA stage) pin 10
to the gnd on pin 8.
2) and C111, a 220 pF capacitor from V102 pin 3 to L103/L102.
*Mine* were both dipped SM (Silvered Mica) style caps and *both*
were getting hot! (Be careful when testing to check this! You can die!)
When I removed the old caps I found that the leads out of the caps
could actually be "turned" very easily - something was very wrong with
these caps internally. With new caps the Transmitter output power is
now back up to snuff (over 50 Watts out with B+ at 450 VDC).
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