I recently acquired an RCA 3-X-521 bakelite tabletop (5 tubes.) Reception is very good, but there is a nontuneable 120 Hz hum in the background that doesn't vary with the volume control. This hum does tend to distort the sound, particularly on strong local stations. All tubes test good, but the 50C5 audio output tube does seem to have some leakage. I suspect the power supply filter caps are bad. I'd appreciate any input you may have, as well as a set of schematics.

Thanks in advance,
Larry Rickard

Hi Larry, I can't find a schematic for an RCA 3-X-521 in either Riders or some Canadian schematics I have. Can you verify the model number please. I think the 120 Hz. hum is most likely filter capacitors. You should be able to read the value off the originals and get modern replacements. Use the same or next highest capacitance and at least as high voltage rating. If it's a transformerless set the original caps were probably 150 volt working, replace with 160 volt. If the original is a multi cap unit you can use individual units, just make sure both the positive and negative connections go to the same place as the old ones. Make a sketch of the chassis connections before removing them, it will stop confsion until you get a schematic. If the leakage is heater to cathode in a tube it could produce 60 Hz. hum. I just realised that if it is a transformerless set and uses half wave rectification, supply hum will be 60 Hz. not 120. For it to have 120 Hz. it must use full wave, either a voltage doubler or power transformer, in that case the electrolytic voltages will be higher. Good Luck, Don Black.

: I recently acquired an RCA 3-X-521 bakelite tabletop (5 tubes.) Reception is very good, but there is a nontuneable 120 Hz hum in the background that doesn't vary with the volume control. This hum does tend to distort the sound, particularly on strong local stations. All tubes test good, but the 50C5 audio output tube does seem to have some leakage. I suspect the power supply filter caps are bad. I'd appreciate any input you may have, as well as a set of schematics.

: Thanks in advance,
: Larry Rickard

Thanks, Don. The model number is correct. I had a little trouble reading it myself, but I verified it in the Collector's Guide. It looks like a typical "All American Five," with all of the filaments wired in series. I checked the base diagram for the 35W4 rectifier tube, and you're absolutely right - since it is a half-wave rectifier, the hum I'm hearing is 60 Hz, not 120 as I thought. Perhaps a hearing test is in my future!

Regards,
Larry Rickard

Hi Larry, I'm sorry I couldn't find the schematic in any of the places I tried. If you look at the schematics for RCA here on Nostalgia Air you should find one fairly close. Since you found it in Bunis (I didn't check there) you should know the year and find a similar model. Perhaps Bunis lists another RCA of the same age that sounds close and you can check its schematic. Good Luck with it, Let's know how you get on and if I can be of any further help. If the electros don't fix the hum that leaky tube's a suspect (is it heater cathode leakage)? It's probably worthwile recapping it anyway. Cheers, Don Black.

: Thanks, Don. The model number is correct. I had a little trouble reading it myself, but I verified it in the Collector's Guide. It looks like a typical "All American Five," with all of the filaments wired in series. I checked the base diagram for the 35W4 rectifier tube, and you're absolutely right - since it is a half-wave rectifier, the hum I'm hearing is 60 Hz, not 120 as I thought. Perhaps a hearing test is in my future!

: Regards,
: Larry Rickard