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My old Aleph 4 clone
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Old 13th May 2018, 10:39 AM   #11
KatieandDad is offline KatieandDad  United Kingdom
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I was thinking about testing the caps with a 470R 5W resistor in series with the existing transformer and diode. I'll check the diodes and the transformer beforehand.

I'd expect good caps to charge to about 35V (70% of Vcc) in about 15 seconds.

Is there a better way of testing them ??
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Old 14th May 2018, 12:06 AM   #12
Eric is offline Eric  United States
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Caps will have to be removed from the circuit for testing. Your proposed method of charging them should work well. What might be revealing is to see how long they hold the charge you give them. Charge them up, let them sit for a while (few hours?), then measure their voltage to see where they are. I suspect you'll have one or more that will be completely discharged in relatively short order. The faster they discharge, the worse they are.
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Old 14th May 2018, 03:24 AM   #13
PRR is offline PRR  United States
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Quote:
Originally Posted by Eric View Post
... ...The faster they discharge, the worse they are.
True; but fast discharge would not blow a fuse.

A fuse-blowing fault would be *instant* discharge; or inability to get a charge at all.
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Old 14th May 2018, 06:09 AM   #14
KatieandDad is offline KatieandDad  United Kingdom
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Sorry, I didn't make it clear that they would be individually tested out of circuit.
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Old 16th May 2018, 12:26 PM   #15
Eric is offline Eric  United States
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Quote:
Originally Posted by PRR View Post
A fuse-blowing fault would be *instant* discharge; or inability to get a charge at all.
True. I was trying to think of a good way to identify a bad cap with minimal equipment. Probably the best way I can think of is to test caps one at a time, directly connected to the power supply as usual, with a "dim bulb tester" in series with the transofrmer primary. When the light doesn't go out, you've found the bad cap. If one is bad and they are all from the same lot, the others are likely to expire soon as well.
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Old 16th May 2018, 02:27 PM   #16
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Eric View Post
True. I was trying to think of a good way to identify a bad cap with minimal equipment. Probably the best way I can think of is to test caps one at a time, directly connected to the power supply as usual, with a "dim bulb tester" in series with the transofrmer primary. When the light doesn't go out, you've found the bad cap. If one is bad and they are all from the same lot, the others are likely to expire soon as well.
Attached is a link to a video of how to test a capacitor ESR with a scope and a function generator.

YouTube

at 10:15 how shows a bad capacitor
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Old 16th May 2018, 05:56 PM   #17
Eric is offline Eric  United States
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Great video- thanks for the link!
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Old 16th May 2018, 09:12 PM   #18
PJN is offline PJN  United States
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Maybe disconnect the psus from the amps and see if the short is in the psus or amp boards. Might shorten the search.
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