I can sure use the expertise of this reflector.....
Alpha 77D problems..
Background...77D (says PA-77 on rear, s/n 2986) Has been in use for
several years, treated well (I hope) and behaved well. Sometime ago,
occasionally it would fail to work (no RF out, 40m grid, high SWR to
exciter) when first turned on (after 3 min warmup of course). Usually
turning it off and back on would bring things to life and it would perform
fine all weekend in a contest environment. Later it became increasingly
harder to get it to work, sometimes having to give it a right jab, and
later by removing and reinserting the CB-2 and QSK control boards. But
once working, it would perform fine all weekend (contest, never turned
off)
Well, I haven't been able to coax it to life for some time now. A spare
8877 behaves the same way. The symptoms are still no plate curent, 40
mils of grid and very high swr to the exciter. With no drive there is no
grid I.
The warm up cycle is fine ad HV goes from 2500v to about 3900v after the 3
minutes.
Manual toubleshooting tip number 6 seems to desribe my problem, and points
to damaged RF sensors (D3 D4 area) or bias switch Q202, Q203. Some
measurements: cathode bias line (CB-2 pin 2) seems to work as it should
60v to 20v when keyed, then drops to about 8.5v when a small amount of RF
applied. I think this indicates the rf sensors and biasing works - it
knows the difference between no RF and a little RF.
I'm not using the QSK function but things appear to be OK on the QSK
board. Q301 pulls the 50v t/r line to gnd when keyed.
Continuity: OK between the plate and the B+; on the return side (cathode
choke L2 back to B-) is 55 ohms plus D203 which adds up. Therefore nothing
obviously wrong in the HV and return lines.
I've managed to waste most of the day poking carefully around, but
everything measured agrees with the way it should be or at least my view
of that, which may be the problem. In case this sounds vaguely familiar, I
asked the reflector this spring....and Tom, W8JI responded (Thank you!)
with:
"Sounds like a bad connection on the bias board. If the cathode or grid
looses most but NOT ALL it's dc return path continuity, you will have some
grid current, little or no plate current and output, and high input SWR."
I have been keeping this in mind - it makes sense but all the checks I've
made indicate that the return path is OK. The bias board (CB-2) looks like
new, but I did resolder everything just in case - remembering the right
jab that used to get it working - no joy.
Any suggestions - what to measure/confirm next or other suggestions
welcomed. I'm sure some of you know this amp inside out and any help would
be appreciated!
73 Don
VE6JY
VE6JY is Don Moman email: ve6jy@freenet.edmonton.ab.ca
Box 127 Lamont, Alberta
T0B 2R0 (403) 895-2925
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