Model 65 Modulation Hum - Printable Version
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Model 65 Modulation Hum - Sourbone - 10-20-2012
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.
RE: Model 65 Modulation Hum - TA Forbes - 10-20-2012
Are there any neon lights on in your house when the radio is on?
RE: Model 65 Modulation Hum - Sourbone - 10-20-2012
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
RE: Model 65 Modulation Hum - LASJayhawk - 10-20-2012
Plasma or LCD TV? My Samsung Plasma puts out a bit of hash.
RE: Model 65 Modulation Hum - Sourbone - 10-20-2012
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.
RE: Model 65 Modulation Hum - Radioroslyn - 10-20-2012
I'd check and tighten the ground connections on the chassis. Sounds like you might have a bad one.
Terry
RE: Model 65 Modulation Hum - Chuck Schwark - 10-20-2012
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
RE: Model 65 Modulation Hum - Sourbone - 10-21-2012
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.
RE: Model 65 Modulation Hum - codefox1 - 10-21-2012
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.
RE: Model 65 Modulation Hum - Radioroslyn - 10-21-2012
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
RE: Model 65 Modulation Hum - peteivan - 11-02-2012
Heater to cathode leakage in one of the tubes can cause this kind of hum, although it is not usually of an intermittent nature.
RE: Model 65 Modulation Hum - codefox1 - 11-02-2012
Stating obvious again this evening, check antenna coil. Open will garble every time.