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Old 4th May 2008, 02:56 AM   #11
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In this amp, the power switch turns on a large toroid which is rectified for use as the main supply to the output stages
the first thing that happens is that a feed from those rails goes through a 1 amp fuse and then is dropped down to 15V that feeds a switching regulator. that supplies 15V to the front end boards. It does not have a shut down mode in this basic first stage. this is just the very first stage of many stages and power supplies that get powered up in sequence.

During trouble shooting i went through 3 or 4 fuses and i was running out. I had replaced all the bad components i could find and i didn't want to waste any more fuses trying to find the problems down stream.

I needed a way to isolate and protect that one stage within the amp. just putting a bulb in series with the main supply wouldn't have worked.

I was just about to try a bulb or just use my current regulated power supply to supply the 15V to that stage with the current fold back set to one amp when i accidentally found the problem.
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Old 4th May 2008, 03:50 AM   #12
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one thing that might help in troubleshooting switching supplies. try replacing all of the small electrolytics in it. they have a tendency to cause the circuit not to oscillate, or even worse, cause the semiconductors to overheat and short.
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Old 4th May 2008, 06:26 AM   #13
Mooly is offline Mooly  United Kingdom
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Quote:
Originally posted by unclejed613
one thing that might help in troubleshooting switching supplies. try replacing all of the small electrolytics in it. they have a tendency to cause the circuit not to oscillate, or even worse, cause the semiconductors to overheat and short.
Most sensible thing I've heard all day. I would add to that , that I use a 'scope for all D.C. measurements as well, it's suprising how often this shows up the problem. So many times I've seen techs measure a rail on DVM, say 11.2 volts for a 12 volt rail, and they think it's O.K. - near enough, the 'scope shows all, about 8 volts with a large HF component.
Regards Karl
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Old 4th May 2008, 01:41 PM   #14
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Crown did mention replacing 2 caps that can cause problems. I definitely will be doing that.
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Old 4th May 2008, 06:32 PM   #15
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Quote:
Originally posted by Mooly

Most sensible thing I've heard all day. I would add to that , that I use a 'scope for all D.C. measurements as well, it's suprising how often this shows up the problem. So many times I've seen techs measure a rail on DVM, say 11.2 volts for a 12 volt rail, and they think it's O.K. - near enough, the 'scope shows all, about 8 volts with a large HF component.
Regards Karl
yup..... even on linear regulators, such as a 7812. had a projo tv that had all of the correct voltages, but had a nasty vertical problem. looked (with a scope) at the output of the 7812 that fed the vertical processor chip, and it was 10V dc with 4V p-p of vertical sawtooth superimposed on it. the 10uf/50V filter on the regulator output rail was open.
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Old 4th May 2008, 06:49 PM   #16
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Quote:
Originally posted by Mooly

Most sensible thing I've heard all day. I would add to that , that I use a 'scope for all D.C. measurements as well,
Regards Karl
I have a Sencore Scope that has built in meter that can measure DC voltage, Volts P-P or Frequency from the probe. which really comes in handy!
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