Hello -

I am working on a Delmonico PB-741. It's a Japanese (JVC?) copy of German-styled radios of the 50-60's.

As received, the AM band was dead, and there was significant hum on the FM and Shortwave bands, as well on the phono inputs. I was able to get the AM section working again (a break in the coil).

The radio does not have a transformer in the power supply. I have replaced the electrolytics. The humming is very much improved, but is still annoying, especially during quiet passages of music on FM.

I am expecting too much from this radio, or does anyone have any suggestions about what my next steps would be in trying to tame the hum? The radio actually sounds pretty good - it compares well with my Nordmende.

Thanks in advance -

Dan

Have you tried reversing the line cord plug?

Rich

Yes, Rich. No difference.
…and you need to bring that W3 callsign back east!


Dan

Hi Dan

Have you tried extra filter capacity? Try replacing audio tubes, especially 50C5's. Any shielded wire around the volume control? Do you know if the radio had hum when new? There will be a way to reduce hum to an acceptable level. You could cancel hum by inserting some 60 cycle in the opposite polarity.

Reducing the hum level can be difficult-- first make sure you have all the foil shields in place, including the one on the bottom cover. Then maybe try parallelling in some bigger power supply filter capacitors. Then look over the routing of the AC line, it should be kept away from the audio circuits.

It is important to determine how much ripple is on the B+ (B1, B2, B3)
Use a scope and look at all 3 points.

If it at acceptable levels there the power supply filtering is not the problem
Perhaps heater to cathode leakage in one tube or something like that.

However if the ripple seems high on B+ you can try increasing all the cap values in the filter to reduce it.

But remember that when it was designed.. I doubt there was hum.

All previous suggestions good ones to try. Find out the source of the hum. Does it disappear if you turn the volume all the way down? If so, look for lack of shielding or bad lead dress from detector to volume control. Is the first audio tube shielded? Check all grounds in the audio section and make sure they are securely soldered.

If turning the volume down does not get rid of the hum, use a clip lead and ground the grid of the first audio tube. If hum goes away, the first audio and output stage should be OK. Look for hum pickup from the first audio grid back through the volume control to the detector. If hum doesn't go away, try adding more capacity to the last electrolytic, as already suggested.