#1
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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 Last edited by Paula; 08-25-2004 at 10:53 PM. |
#2
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Yes, this is a common negative. Componets are arranged strangely on the schematic, which does obscure the circuit a bit.
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#3
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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 |
#4
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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!
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#5
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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 Last edited by Paula; 09-04-2004 at 10:49 AM. |
Audiokarma |
#6
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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 |
#7
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Paula: As E1 and E2 come off the cathode of the rectifier, that line is positive.
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#8
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Paula: Red is usually the higher capacity value and likely the one which ties to the rectifier cathode. Is that what you are asking?
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#9
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Quote:
Paula |
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