A question of polarity
 

Here's one for the experts...

I'm replacing the electrolytics in a Silvertone 7251, and I am not sure about the polarity (they show up as E1 and E2 on the schematic below.) They were originally part of a 2-section cardboard unit beneath the chassis, with three wires coming out - black, green, and red.

My first guess would be that the black wire is the common negative for both caps, but I have seen exceptions to this. Does anyone know for sure?

Paula

Yes, this is a common negative. Componets are arranged strangely on the schematic, which does obscure the circuit a bit.

Thanks, Tom! :)

Interestingly, I just finished working on an Emerson R-152 chassis that uses a 2-section electrolytic with a common positive, though it's a transformer set. Still, confusing to a beginner like me.

Paula

That is interesting! I have recapped many radios both AA5 (series heaters) and transformer sets and have never seen a common positive. I am not saying that is impossible, however! Black color wire on the old multi-section electrolytics almost always indicates negative or common. On the chassis mounted metal cans the negative is the case of the cap. Reversing the polarity causes a "Big Bang" that is not a theory :) . Can you post the schematic section from the Emerson? Thanks!

Here it is. The electrolytics are C19 & 20, and were contained in a single can atop the chassis. The black wire connected to the xfmr center tap/field coil, a white (or yellow) wire went to chassis ground, and a red wire was the common positive (connected to the 80's heater).

The only reason I knew this was because a previous restorer had already replaced the electrolytics, and this is the way it was hooked up when I got the radio (and the radio worked!) I only re-replaced them because it was kind of a hack-job. I used more modern caps, and mounted them inside the old can, refilling it with Plas-T-Pair.

I saw a post on antiqueradio.com the other day that seems to confirm that this wiring configuration is correct.

Weird. Bottom line, I guess, is that it works!

Paula

Wow, I have never seen this before, but you are right, it works! This is one I will have to file in the back of my mind. The next radio I restore will probably be like this! (like when you hear a word for the first time you usually hear it again and again!)
Thanks for taking the time to post the schematic!

Mike

Paula: As E1 and E2 come off the cathode of the rectifier, that line is positive.

Paula: Red is usually the higher capacity value and likely the one which ties to the rectifier cathode. Is that what you are asking?

My question (with regard to the Silvertone 7251 shcematic) was whether the commen side of E1/E2 was positive or negative.

Paula