RadioBanter - View Single Post - AR88D

timlad28 · September 20th 10, 12:20 AM

Richard and Scott,

I checked V-12 again this time with the BFO turned on and it reads 39v. I used a copy of E773 (came with the set) which had a table of tube voltages. Sorry, no oscilloscope or no s-meter. Downloaded more documentation from the web. I will try alternative tubes and double-check resistor values. This time I'll do my homework and try to be a little more specific.

Again, many thanks.
Tim

Quote:

Originally Posted by Richard Knoppow

"timlad28" wrote in
message ...

Richard Knoppow;718630 Wrote:
"timlad28" wrote in
message ...-

Hi,

The R.F gain on my AR88D has to turned almost fully
clockwise to hear
any stations. I can pick up stations across the bands but
can only hear
them clearly when the R.F gain is full on. I thought the
pot (R46) may
have failed.
I detached the RF gain pot and tested, the pot measured
from 0K to 68K.
The two resistors to ground are both in tolerance.
--
timlad28-

Have you checked the tube voltages, if not do that
first. I think there are also stage gain values in the
handbook. These will help in finding where the signal is
disappearing. Using a good scope as a signal tracer will
also help.
RCA draws schematics with the tube bases shown as they
are which often makes them difficult to decipher.
Sometimes
re-drawing a part of the circuit helps.

--
Richard Knoppow
Los Angeles
WB6KBL

Hello Richard,

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.

- Tim

First of all which handbook do you have? The ones I
have show V-12 as the BFO with 40V on the plate (pin-3). The
BFO must be turned on for this voltage to be present. If its
turned off there will be nothing there. In any case, even if
there is a problem at this point it should not affect the
overall gain of the RX. The symptom is so general its hard
to point out any one thing. It would be helpful if you could
borrow an oscilloscope (I assume you don't have one) and use
it as a signal tracer to see where the signal drops off.
Also, don't assume tubes that check good on a tube tester
really work, sometimes tubes which check good don't work in
some circuits.
Unfortunately, RCA does not seem to have provided either
socket resistance measurements or stage gain measurements in
their data. You can find a lot of additional info on the
AR-88 and its relatives by doing a web search, There are
Canadian and British military manuals available with more
info than the RCA handbooks. If your receiver has an S-meter
installed its indication may give you a clue.

--

--
Richard Knoppow
Los Angeles
WB6KBL