I'm trying to get an A77 (PA77) working ... need some advice. The previous
owner had applied 200w plus to input. After replacing the soft start
resistors which cooked and one of the HV RFC's (L9) which vaporized, I
applied power and the amplifier made good power. Then I noticed there was
plate current with no input. I discovered that the bias switch transistors
Q4, Q5 were shorted and D17, D18 were open. Replaced them and bias was now
normal and switched with input RF. To replace these components I had to
pretty much remove the "T/R" PC board. Then, after reassembly, although
bias is correct, no power out! Upon applying drive, grid current swings
very high as I turn thru resonance and the amp trips on excessive grid
current even with a few watts drive. When I manage to keep it on line, I
note that there is not much of a plate current dip. Power out is actually
less than exciter power.
What did I screw up?
73, Bob
W0YVA
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