Motorboating question for the experts - Printable Version
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Motorboating question for the experts - NostalgiaRadioTime - 01-12-2013
Just finished a total re-cap, and out of tolerance resistor replacement on a PT-2 Transitone (the tubes all check out good too). When the volume is wide open, the radio picks up all the local stations faintly (at what would be low volume). As soon as you turn down the volume, nothing comes in but major motorboating. Tried reversing the leads on the volume control thinking it was hooked up wrong, but it made the motorboating problem worse. So that wasn't it. My question is, could the motorboating be caused by a faulty volume control, or possibly a bad speaker transformer? Or is there a certain way the volume control wires need to be placed to eliminate interference? I did spray the volume control with DeOxit a week ago, so the mechanism should be clean. Thanks in advance
RE: Motorboating question for the experts - Arran - 01-12-2013
One of the most common causes of motorboating in an AC/DC radio is an open bypass cap, usually a .22 microfarad one between the chassis and the B- bus. In the case of Philcos this is the big paper cap connected in series with a choke, sometimes made out of wire wrapped around the body of the capacitor. It could also be an open output filter capacitor, or rather the B- connection to same.
Regards
Arran
RE: Motorboating question for the experts - NostalgiaRadioTime - 01-12-2013
Thanks Arran. I know I replaced that cap with an exact value / voltage new one, but I didn't wrap the wire around it, as I read elsewhere on this site it used to be for marine and other beacons around IF frequency, that the shielding is no longer needed. Still think it's something with the volume control or its wiring?
RE: Motorboating question for the experts - Elijah - 01-12-2013
I can't say this is the problem but I had the same situation at the low end of the dial. I did an alignment and it went away.