I am taking every opportunity to learn more and working on my Alpha 87a
always provides a good lesson. I have been getting intermittent faults on
the filament monitor for the 3CX800A7's showing the filament current is low
- suggesting that one of the tubes has an open filament. I know this is not
the case as it is intermittent fault and when not faulted I can get full
legal power. I figure it's an issue with the filament monitoring circuitry.
I understand the basic concept of current monitoring using a parallel
resistive shunt load and feeding that resistive voltage drop into an op amp
to provide a corresponding proportional voltage that could be read by a
microprocessor analog input. The 87a filament monitor per manual is looking
for 3 states, less than 2 amps or more than 4 A and anything in between so
it seems the current sensing method would not need to be too precise.
The transformer's 13.6 vac go via lines labeled 'FIL' and "FIL RETURN"
through the Low Voltage Board, these are passed through to the Tube deck
along with a 3rd line called FIL_SENSE GND. On the Tube Deck, FIL is
connected to one side of the filaments and the other side of the filaments
are grounded. FIL_SENSE GND is connected directly to ground and FIL_RETURN
is connected to ground through a series 0.3 ohm, 5W voltage dropping
resistor. If my take on the filament monitoring circuit is correct, it is
'reading' the current through this 0.3 ohm dropping resistor. First I
don't know why there is a dropping resistor since I read from EIMAC that
these tubes are designed to run only +/- 0.6v ( or better) from 13.5 but
that's a different story I guess.
I cannot figure out how the monitoring circuit is working and would
appreciate some advice. The following link has copies of the tube deck and
LV board schematics. I hope someone can volunteer to be an Elmer for me on
this issue.
https://www.dropbox.com/sh/qlih0j20xsv76dx/AAAY2tsr3Mnc2FG7N46kQ7Z7a?dl=0
73 Jerry
NY2KW (ex-K1JOS)
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