Don't rule out an intermittent in the heater wiring, socket connections
or even inside one of the tubes. A bad joint or dodgy connection will
show up as low current and cause a trip.
Your guess at the operation of the current monitor is correct. 3A
through 0.3ohms gives 0.9V of AC across the resistor. The two 1N5711s
rectify and double the AC to give a bit over 2V DC, then the opamp adds
x1.5 gain so normal heater current gives something over 3V at the
monitor output.
Steve
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.
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|