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46-480 Burned Resistor - Printable Version

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46-480 Burned Resistor - Jbaker - 11-24-2010

I have a 46-480 radio that I have been working on for a while. When I got the radio, it did not work and I found a burned up resistor. In the Philco service manual it is R308 and R17 in the schematic published by Rider. The value is 1K and it looked like a 1/4 watt size. It was replaced with a 1 watt resistor. The radio was completely recapped and the tubes tested. I can tune in AM stations but the resistor gets very hot and starts to smell after a minute or 2. The radio also motorboats if I turn the volume up too high.

Any thoughts on what could be causing the resistor to continue to burn up?

Thanks in advance for any advice.

Re: 46-480 Burned Resistor - rghines1 - 11-25-2010

The obvious suspect would be C37 but the radio has be recapped. Try pulling the 7H7 tube. If no help then have to suspect a problem inside the IF transformer can next. One of those internal caps inside the IF can may have failed. Richard

Re: 46-480 Burned Resistor - codefox1 - 11-25-2010

Agree with all. If the tube tests good, there could be a short somewhere in the wiring between the burning resistor and the plate, either in the IF Can or around the tube socket.

Re: 46-480 Burned Resistor - Jbaker - 11-25-2010

Thanks for the suggestions. You got me on the right track. I replaced C37 again and still no luck. Next I pulled both the 7H7 tubes and still the resistor heated up. After the 6H6 was pulled, the problem stopped. Even though the 6H6tested good, I replaced it with a new one. No luck. I have checked the wiring and the socket and haven't found any shorts. Next I will go after the IF can T6. That has to be the problem.

Can anyone suggest a source if I need a replacement?

Thanks for the help!! - Jim

Re: 46-480 Burned Resistor - rghines1 - 11-25-2010

Inside the IF can there is a cap that connects the primary and secondary center-tapped windings of T6. This may have failed. Schematic is blurry but think it is marked 100 below it. May be easy to replace this cap once inside. The "100' likely indicates 100 pF. Best to use a silver mica cap here. Richard

Re: 46-480 Burned Resistor - Jbaker - 11-26-2010

Indeed, the capacitor in the IF can was shorted. Replaced it and it worked. Thanks to all for the advice!!!
Jim

Re: 46-480 Burned Resistor - rghines1 - 11-27-2010

Glad we were able to help. Congrats!