NC-300 audio faded away

[WA2ROC]

While listening to 40 AM last night, I noticed that the audio level was slowly
decreasing. After 30 seconds or so, it was almost gone. I plugged in a set of
headphones and could hear signals but they were not all that loud.

The signal strength on signals according to the S meter were fine, just the
audio was really low.

I tested the 1st audio 12AX7 and output 6AQ5 tubes on my emission tester and
they were OK, but I suspect they may be bad. I plan to swap them with tubes
from my working Heath Mohawk.

Any other things I can look for before yanking it out of the case?

[KM1H]

I'll ask the same question I asked yesterday on the National reflector and didnt receive an answer:

To wit:  When was the last time the radio had been recapped and out of tolerance resistors replaced?

While a tube may restore it for awhile the bad components are likely dragging some stages down.

Carl

[Jeff W9GY]

Gotta agree --- coupling caps, resistors all need to be checked.  Of course, a tube can be a suspect, but lots of other things need to be looked at on a receiver of that vintage.

[ke7trp]

They got it right. Mine needed almost all the caps and a dozen resistors changed.

C

[w5rkl]

Check the voltages at the audio stage(s). I had a similar problem with a receiver where the audio was fine one moment but not the next. After checking the voltage in the stage, I found a bypass capacitor had shorted, dragging the screen voltage down, shutting the audio stage off.

73s
Mike
W5RKL

[WA2ROC]

Swapped audio tubes from my working Mohawk with no results.

Pulled the chassis from the case and started checking voltages around audio output tube.  Everything within 10% of values shown in manual.

Went to audio driver and found zero volts on plate. Hmmmm.  The plate resistor failed open. Replaced it with a similar value (270K) and the audio is better than ever.  That resistor was probably failing for years and finally gave up.

While in the neighborhood I did some searching and found several resistors that were not originals, judging from how they were reinstalled.  Did more voltage measurements and nothing was found out of tolerance.

I did add 3 new caps, 2 in HV power supply and the other for cathode bypass in audio out.  Easy to add and I left the original in place, minus wires of course.

Probably get back in there when winter comes and do more resistor checking.

Thanks to all for suggestions.  Most were spot on!

[W3GMS]

Swapped audio tubes from my working Mohawk with no results.

Pulled the chassis from the case and started checking voltages around audio output tube.  Everything within 10% of values shown in manual.

Went to audio driver and found zero volts on plate. Hmmmm.  The plate resistor failed open. Replaced it with a similar value (270K) and the audio is better than ever.  That resistor was probably failing for years and finally gave up.

While in the neighborhood I did some searching and found several resistors that were not originals, judging from how they were reinstalled.  Did more voltage measurements and nothing was found out of tolerance.

I did add 3 new caps, 2 in HV power supply and the other for cathode bypass in audio out.  Easy to add and I left the original in place, minus wires of course.

Probably get back in there when winter comes and do more resistor checking.

Thanks to all for suggestions.  Most were spot on!

Congrats Dick on nailing the problem.  Most of the components with rigs that old are on borrowed time.  I prefer just to go in and replace everything.  I use metal film resistors in most cases.  Beware of buying cheap bargain basement type caps.  Its just like when your working on your house.  Buy the best materials you can afford since most of the work is in the labor of doing the job.  Do it once and be done with it otherwise you likely to be wearing off the paint on the lower edge of the cabinet pulling the receiver in and out of it. 

Joe, W3GMS 

[KM1H]

I overhaul several 300's a year and replace all the paper caps, usually the yellow Astrons, which all have some leakage when tested properly. Dont forget the ones for the Tone circuit up against the front panel.  Lots of resistors get changed also, even under the bandswitch.

Add a 1M 1/2W resistor between the plate of the audio driver and 6AQ5 and by pass the driver cathode with 47-100uF to restore the lost gain and give a bit of bass boost; use 25uF if more bass not wanted. The resistor drops distortion quite a bit.

Ive gone even deeper with my own 300.

Carl