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In Reply to: Re: M60 output cap short? posted by JAMES on November 13, 2000 at 17:45:49:
4,700 uF caps, especially a few in parallel, will not show infinite resistance on a typical DVM. No matter. You're looking for a short here - I think. First, you must be sure your caps are completely discharged before applying the ohmmeter. Turn off the power, get out a pair of clip leads and hang about a 10K, couple watt resistor, across each capacitor bank until you measure no voltage with your DVM. It doesn't need to be exactly 10K. Plus or minus 200% is fine. Once you're sure your caps are really measuring no voltage, then stick your ohmmeter across them. The resistance should slowly drop as the DVM's internal voltage slowly charges up the caps. This is normal. You'll never see infinite resistance with caps this large. During this test, wiggle things around and see if you should suddenly read a short. If so, you've got it. I'm convinced you have a mechanical intermittent, based on your description. A real short should have taken a fuse. You might have an open circuit instead. With one output supply offline, you will hear a lot of hum. You might want to carefully check the cap's insulation with respect to the mounting hardware. Many large electrolytics have their "can" hooked up to the negative side of the electrical cap. The can may be at a very real potential. It's very unusual to damage a cap from soldering, but possible. Any such failure mode could cause a short and also an open.I hope this helps, Mike.
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Follow Ups
- No, not infinite resistance... - mark 19:11:53 11/13/00 (1)
- pop a cap?? 8-( - Mike K 21:28:42 11/13/00 (0)
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