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In Reply to: RE: A shot in the dark (Levinson ML-11). posted by YRY on January 13, 2010 at 13:46:46
Sounds like one of the power supply rails has lost a fuse so that one channel is getting half its power (either the negative or positive rail, but not both).
All externally and internally accessible fuses checked out OK. The rail fuses were located on a small board along with 8 diodes and what I believe are some caps (pinkish rectangular components), but I can't be sure since there were no units printed on them. I also checked for continuity between the traces and the diode leads.
I think the 8 diodes must comprise 2 bridge rectifier circuits, and I believe that if one of the diodes in a bridge were to go open, then the rectification degrades to half-wave rather than full-wave.
When a diode fails, does it typically go open?
Could this cause the symptoms I described?
Normal rectifier diodes usually open, SCHOTTKY diodes can and do go dead short.
I at least feel like I am not fumbling in total darkness, and I may even learn a thing or two.
Assuming a bad diode is the culprit, how best to find which one is bad? I can follow wires/traces to determine which bridge rectifier to test, so that should eliminate 4 out of the 8 diodes as suspects. I have a volt/ohm/ammeter, but not sure how to utilize it in this case. I did measure resistance across ea. diode (in place), in both directions, but none stood out as different, so I guess this is not a valid diagnostic. Could I ferret out the problem by powering up the amp to look for discrepancies in voltage drop across ea. diode?
A shorted diode would measure same low value in both directions (swap probes Red and black and remeasure).
If your multimeter has a DIODE Setting use that, otherwise, lowest ohms setting that works.
Analog Meter? a good diode will be low ohms in one direction and 10X higher than that the other direction (when swapping probes to either end of diode).
A digital ohmmmeter with a diode setting will be OL or infinity ohms in one direction and between 200 and 700 ohms in the other direction. Of course checking components in the power supply, discharge all capacitors!
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