48-1270 Phono Hum - Printable Version
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48-1270 Phono Hum - Philco47 - 05-19-2020
Hello All,
I am working on a Philco 48-1270 console. I replaced all the capacitors, all but 2 resistors, and the electrolytics, I didn't touch the Mica caps. AM, FM, SW all work well no hum at all, they have their own electrolytics. As soon as you push the phono push button it hums like a bad electrolytic. The turntable plays, you can faintly hear it under the hum. It hums when the turntable is connected and disconnected. So the problem is in the chassis.
It uses a 7AF7 as the phono amp. - phase Inv. tube. If you pull the 7AF7 tube the buzz stops, but so will the phono output. I traced out every wire and connection from the 7AF7 socket and see no problems. I tried 2 different tubes, getting the same result. I had replaced the original 10mf electrolytics (16 A & B on the schematic) with new 10mf electrolytics with a stronger working voltage. The phono push button is not shorted. I'm stumped.
Should I boost the 10mf a little higher? Triple check the 10mf for errors? (A safer bet) Maybe have 2 bad 7AF7 tubes?
Any ideas are welcome.
Thank you,
Steve
RE: 48-1270 Phono Hum - morzh - 05-19-2020
See this: the turntable metal base should be connected to the chassis. Or so I think.
See the adapter wires coming to the amplifier: if one is a shield (and another is shielded inside it) see that the shielded gets to connect to the chassis.
I cannot see the adaptor in the sch.
RE: 48-1270 Phono Hum - Philco47 - 05-19-2020
I believe I have it all hooked up as it was in the cabinet.
RE: 48-1270 Phono Hum - NostalgiaRadioTime - 05-19-2020
On my 47-1230 I had to ground the metal phono frame with a wire to the chassis ground screw and the hum went away (as Mike / Morzh said). Not sure that will help you with yours, but that did the trick on mine.
RE: 48-1270 Phono Hum - Philco47 - 05-19-2020
So just a wire from an unpainted part of the phono frame to the radio chassis? If so, that sounds easy.
Thanks
RE: 48-1270 Phono Hum - NostalgiaRadioTime - 05-19-2020
Yes that's what I did. You can try it with a gator wire for an quick test...clip one end to the frame, the other to chassis ground and see if it makes any difference.
RE: 48-1270 Phono Hum - Philco47 - 05-19-2020
I'll go give it a try now. Let you know in a few.
Thanks, Steve
RE: 48-1270 Phono Hum - Philco47 - 05-19-2020
IT WORKS!!!!! NO HUM AT ALL!!!! Thank you SO MUCH guys. I have no idea what I would have done.
Very appreciative,
Steve
RE: 48-1270 Phono Hum - NostalgiaRadioTime - 05-19-2020
