RadioBanter - View Single Post - AR88D

timlad28 · October 24th 10, 05:04 PM

Quote:

Originally Posted by Richard Knoppow

"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.