"timlad28" wrote in
message ...


Long thread snipped..................

Hi Richard,

It's been a while but today I finally got time to run
through the E773
doc for the AR88.

Testing the local oscillator stage at tube 3, I measured
the D.C.
voltage across R12 and got readings of -3.0v to -4.5v, not
the 3 to 3.5V
indicated. So something not right - which is good. Checked
the voltage
on tube 3 and it read 90-100v. Replaced with another tube
and got
similar readings. Does this mean the fault is in AF stage
or this just
symptom?

Regards,
Tim

P.S. I hope to have a signal generator shortly.




--
timlad28

I don't have the complete E773 document. The one I have
and other AR-88 manuals do not specify a voltage across
R-12m a 1K resistor. Its shown as a series resistor in the
regulated voltage to the oscillator plate. Its probably part
of a decoupling resistor. I would not expect there to be
much of a voltage drop across it and the polarity will
depend on how you have connected the meter. You should see
about +150v to the chassis on one side and nearly that on
the other. The oscillator plate resistor is 10K (says 10M on
the schematics, that is an old abbreviation for thousands,
can be confusing). Because R-12 is 1K the voltage across it
will be directly in milliampers so 4V would be 4ma, not out
of line for the tube. This translates to a 40V drop across
the plate resistor. I don't think this is the problem.
I looked at four versions of the AR-88/CR-88 schematics
and the resistor designations appear to be consistent. The
document you have is a British army one so its just possible
its different, I don't have their version of the schematic.
Unfortunately, RCA does not have stage gain measurements in
their instructions, they would be helpful.
I don't know what to suggest next. If you have checked
the tubes for correct voltage and resistance at their
sockets. It seems to me we discussed an error in an earlier
part of this thread, something to do with the AVC and bias
measurement, I think the British manual has the return point
wrong.

--
Richard Knoppow
Los Angeles
WB6KBL