Hi folks,
I recently bought a "basket case" PA-77. (single 8877) The OM decided
to shoehorn a Dahl A77S transformer (1.2A CCS) and a 32uf filter into
the case. He succeeded mechanically, but moved most of the other
PS components around, and created more than a few electrical
problems. :-) As a result, I'm in the middle of a ground-up rebuild.
I've summarized where I'm going so far below, and would appreciate
any comments you might have.
(1) The amp uses 8.2V RF-switched bias. With the current combination
of B+ components, there's 3900V at the anode, (only bleeder current)
and I'll -guess- it'll drop to around 3700V on SSB. That 8.2V bias
@ 3700-3900V puts the operating point rather far up the curve toward
class A during speech intervals. Because the bias/QSK/grid trip board
is blown to smithereens, I'm thinking of raising the bias to 12V or so,
(TR, rather than RF switched) with a "BIAS/INPUT then OUTPUT" relay
sequence.
(2) Since I'm headed toward a new controller, I'm toying with
the idea of using a microcontroller, rather than RC constants
and such. The question here is since the anode current is
referenced to B- instead of ground, and the A/D converters on
the uC board are ground-referenced, anyone have a good idea
on how to monitor the anode current? The only thing I've
come up with so far is a galvanometer coil. Grid current,
anode voltage, and directional coupler I have worked out.
(overkill, but the idea here is to have the uC calculate
operating efficiency, and provide trips for grid current,
anode current, and SWR)
I'm in no big rush to get it running, the fun is in the
project! :-)
Thanks,
Jim Barber, N7CXI
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|