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.

All bath tub capacitors were replaced. I had to replace dud capacitors in the RF unit to get all bands working and the odd resistors that had gone high. Up until recently the set was working OK. Then the issue with low RF gain.

All tubes have been checked and OK. I'm endeavouring to understand the RCA wiring schematic ;-) Has anybody encountered this issue before?

Many thanks,
Tim

"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.

All bath tub capacitors were replaced. I had to replace
dud capacitors
in the RF unit to get all bands working and the odd
resistors that had
gone high. Up until recently the set was working OK. Then
the issue with
low RF gain.

All tubes have been checked and OK. I'm endeavouring to
understand the
RCA wiring schematic ;-) Has anybody encountered this
issue before?

Many thanks,
Tim




--
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

"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

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


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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

"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