Greetings - I recently acquired a small tombstone radio (12 X 14 X 8 inches)with only one identifing mark: a metal label on the front that says "Concert Master". The tube line-up is: 80/58/58/57/47. Everything appears to be near original condition, but one feature seems unusual: There are two 8 microfarad/450 volt filter capacitors, one of which has the positive (!) side tied to ground (the chassis). I slowly powered up the unit (no antenna attached) and got the usual hum from the speaker -- no explosions. Since I don't know the make/model I don't have a circuit schematic. Any thoughts/comments regarding the one capacitor having the positive connection to ground? THANKS!

I don’t know what kind of radio you have but my RCA R-74 has an electrolytic with the positive to ground. It’s not unheard of.
Brett

Brett -- Thanks for the feedback. Now that I know that this situation (positve to ground) is not in error, I'll proceed with the restoration.

:I don’t know what kind of radio you have but my RCA R-74 has an electrolytic with the positive to ground. It’s not unheard of.
:Brett
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You should draw out a schematic. It will give you an idea of the circuitry involved. Find the center tap of the high voltage winding. Is there a resistor between it and the chassis (perhaps even the speaker field coil)? If so, then there is a point in the radio more negative than the chassis. For filtering, this point must be connected to the negative side of the filter capacitor, and the chassis must be connected to the positive side. This negative point, and perhaps points in between, might be used for grid bias throughout the various parts of the radio (most likely the audio section).

T.

Thomas - Thanks for the comment. I have started tracing out the circuit and you are correct - the speaker field coil is between the HV winding center tap and the chassis, so I now understand the orientation of the filter capacitor. dwc

:You should draw out a schematic. It will give you an idea of the circuitry involved. Find the center tap of the high voltage winding. Is there a resistor between it and the chassis (perhaps even the speaker field coil)? If so, then there is a point in the radio more negative than the chassis. For filtering, this point must be connected to the negative side of the filter capacitor, and the chassis must be connected to the positive side. This negative point, and perhaps points in between, might be used for grid bias throughout the various parts of the radio (most likely the audio section).
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:T.
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