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In Reply to: A friend just shorted my Sherwood S-5000! posted by Raver on December 29, 2003 at 11:48:39:
Raul,Your friend probably blew up the selenium rectifier. DO NOT OPERATE THE AMP RIGHT NOW OR YOU'LL SMOKE THE OUTPUT TUBES OR WORSE. NOT EVEN JUST FOR A MINUTE TO "CHECK" SOMETHING!! The capacitors are probably fine, but there may be other damage. The output tubes may well be toast.
The low plate voltages are probably caused by the severe load on the power supply. With no negative bias voltage the output tubes are running wide open passing as much current as they can, and are loading the power supply way beyond normal; those tubes will (if they haven't already) destroy themselves if this continues.
Where are you located, maybe I can repair it for you. I am really busy but I hate to see more damage done to the amp.
Follow Ups:
thank you for your offer Jim,
but unfortunately I live in Italy (and I'm a newbie in this field). I don't think that the problem is only the selenium rectifier. I hoped it was so, and I already removed it and replaced it with two 1N4007 diodes with the two positives (band) tied together going to the to the C3 capacitor and the two wires from the power transformer to going the two remaining sides of the Y (it as a three wires rectifier and I read on the Asylum that it can be replaced this way). The hum was still here. No more tests made. Just tested all the tubes with my emission tube tster. They seem still good, no shorts, no leakage and they are still in the "good" range.
nt
REVERSED CONNECTION. THE POSITIVE SIDES SHOULD GO TO TRANSFORMER LEGS SINCE YOU'RE HAVING NEGATIVE BIAS VOLTAGE.
Basically, similar approach as other inmates except starting up with variac after repair.Depending on your transformer configuation, you're right for B+ only with centre tap, hence two diodes will be OK, one at each arm of the transformer output.
For bias voltage without centre tap,(check your original rectifier circuitry) a full wave bridge rectifier is the way to go(four diodes configuation) with standard product from RS or similar mfy. A rectagular or bead like black subject with four legs.(two for ac input, two for dc output)
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