"Scott Dorsey" wrote in message
...
timlad28 wrote:

Thank you for answering my post. Today, I managed to check
the tube
voltages and V12, Anode 3, measures -1.2v, when it should
be 30v. All
other voltages check out.

That sure would indicate something wrong. Check the value
of the plate
resistor in-circuit with a meter. Is it open?

When resistors go bad, they usually increase rather than
decrease in
value. If the plate voltage is way too low, it's more
likely to be
something on the plate side of the circuit gone up rather
than something
on the cathode side one down.
--scott

There is also a plate by-pass cap on that line, if it
shorted or developed a low resistance it would pull the
plate voltage down but its on the tube side of the BFO
switch. I just wonder if he measured this with the BFO off.
If all the DC voltages are as specified it suggests that
something is not right in the RF, IF, or audio path. Signal
tracing is the best way of finding out where the problem is.
Now, some isolation can be had by seeing if the problem
varies with the band. If it doesn't then its something
common for all bands, but that could still be one of the rF
amps as well as the mixer and IF stages. Another technique
is to inject a modulated signal into the IF to see if its
response is normal. It could be so many things that its hard
to know what to suggest. Bad caps, bad solder joints, bad
ground connection somewhere, resistor that has opened up.
Carbon comp resistors seldom fail catastrophically, i.e.,
going entirely open, but can change value substantially.
Usually they go up in value but not always. Wire wound
reistors often open up espececially due to corrosion at the
weld between the ends of the resistance element and leads.
An IF transformer may have gone open (unlikely).
The RCA 88 is a very good receiver and is worth some
effort to restore.


--
Richard Knoppow
Los Angeles
WB6KBL

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" 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