I have a model 65 that has a modulation hum. The hum increases as a station is tuned in and fades between stations. I installed two Y type line to ground safety capacitors to each side of the power line. As well, I have an X safety cap across the line. The set is grounded to an 8 foot copper clad ground. Oddly the modulation seems nearly non existent at times. It really works well otherwise. Any suggestions would be appreciated. Thanks.
Are there any neon lights on in your house when the radio is on?
No neon or fluorescent lights on at the time. The line to ground and across line safety caps should eliminate most of that kind of interference
Plasma or LCD TV? My Samsung Plasma puts out a bit of hash.
No sir. We don't have a tv like that. It acts just as Elements of Radio Servicing describes on page 438 as modulation hum; silent between stations and hum when tuned in. I'm considering checking the chassis grounding post, but this model 65 is a heavy beast. It's very solid. It receives well.
I'd check and tighten the ground connections on the chassis. Sounds like you might have a bad one.
Terry
Also, your "Ground" source may not be realy Earth Ground.
If you're using the ground screw on an outlet, that may take a high resistance path to real ground and cause a "ground-loop" through your electrical system and cause hum. Depends on your house electrical system. Try a ground clamp to a COLD water pipe, clean, no-paint.
Chuck
It's grounded with a single #14 gage wire screwed to an 8 foot copper clad grounding rod driven all the way into the earth. When I removed the wire it REALLY hums, but when it's connected it's hums moderately but too much.
Wierd stuff sometimes happens on old sets, like a poor ground connection through a rivet or nearly crapped out antenna transformer or coil. Tapping around gently with a dry chopstick when there are problems just might lead you to a mechanical problem, if there is one.
Something else came to mind if you still can't find it check your RF coils for (green primaries) corrosion. You may have a sightly bad coil that isn't giving you a good ground return for the grid of the tube. Also you could have a bad 24 tube. It could have a short from the cathode to grid 1 or a heater to cathode. If you've got a spare I'd swap it and see if that does the trick. Either 24 could be at fault. The pilot lamp may affect the overall amount of hum. It may have a bit more of less with or without it in the circuit.
Just a thought.
GL
Terry
Heater to cathode leakage in one of the tubes can cause this kind of hum, although it is not usually of an intermittent nature.
Stating obvious again this evening, check antenna coil. Open will garble every time.