[Eric Adams]

Because at low volume it hums
The latest subject is a DeWald C-800 . With the volume turned all the way down, I have a steady hum. It doesn't get any louder when the volume is turned up. It just hangs back in the background when tuned into a station.
All caps replaced (except micas). Resistors checked and replaced as necessary. All tubes tested ok. Alignment done. On both AM and FM stations come in loud and clear.

Any ideas? This has really got me . I may take a ride over to freezing lake Michigan and jump in-like someone else we know  

Thanks,

[morzh]

Eric

There have been several threads lately addressing the similar problem.

All I can tell you:
1. Check all tubes for shorted Filament and Cathode (uually output ad Det/1st Audio)
2. Bad grounding - rivets etc
3. Shorted Gnd to chassis if separate, or shorted Filament wire to chassis.
4. Mechanical noise from the power transformer


You have to poke around.

Like, pull tubes one by one, see if it stops and with what tube. Then go from there.

[Radioroslyn]

Agree with Mike but when you start pulling tubes leave the rectifier in. If you pull it you will loose all HV Also replace the tube back into it's socket before going to the next one. You don't want to have with just the rectifier tube in it at the end of testing.
Terry

[morzh]

Yep, that's the point - one at a time while having the HV, we need to know which one it stops at.

[Eric Adams]

Thanks for the ideas, guys. I don't think that I should pull tubes because it has no power transformer. I have swapped out each tube one by one with no change. I'll check grounds. Anything else you can think of for this AC/DC beauty?

[morzh]

Rotate power pug?

[Eric Adams]

Nope. has polarized plug - common to ground.

[codefox1]

Don't bet your life or your reception on wall socket being wired correctly. Try in another room where a similar set works OK.

[morzh]

Although I try to wire chassis following Wide-Neutral convention. I would never rely on it for safety. For exactly the same reason - it cannot be guaranteed.

[Arran]

 I would loose the polarized plug, at least temporarily, I see that this is an AM/FM set so I would go back to how the factory had it wired until you can rule that out. Some AC/DC sets do not like having the neutral and hot side swapped on the power switch since it's often on the back of the volume control, I also noticed that the FM and AM detector tubes have their heaters connected closest to common negative/AC Neutral at the end of the filament string. If it is not being caused by this, or a heater to cathode short, then there must be an open ground connection, or something incorrectly wired.
Regards
Arran