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Old 5th November 2004, 05:06 PM   #1
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Default A Tale of 2 Capacitors!

A Tale of 2 Capacitors!

So, last night I was in the shop, working on a Soundcraftmen power amp I picked up in non working condition and I thought I would share my tale of two capacitors.

The previous owner said the amp worked fine for years then one day started blowing fuses. My first instinct after years of amp repair was blown output devices. But in all the years I have fixed these little amps. I have only seen 1 with bad outputs mosfets.

So I started digging in. I disconnected the amp board from the Power supply and found the unloaded power supply still drew a lot of current as I raised the variac. Next I disconnected the secondary side of the transformer from the PSU board and raised the variac, no current draw and AC measures fine.

Next I looked at all the SCR’s in the rectifier circuit. The SC amps use a Phase Control Regulator that uses SCR’s for the rectification of the AC to DC. Sometimes these fail and short out causing problems but they all tested fine. Still I replaced them all. I raised the variac again and still drawing lots of current.

So out of curiosity, I side step to test the amp board. I grabbed 2 can caps and a standard bridge rectifier and connected them to the transformer secondary, then to the amp board. I raised the variac watching the meter to +/-70VDC and everything is fine, inject a signal and raise the level until clip. 44 Volts RMS no load, all is good, so the amp board is good. Back to the power supply board.

Next I disconnected the Power supply caps from the board and connect my test bench cans and raised the variac. Slowly I raise and no current draw, all looks good. I raise it some more and the current meter comes up slightly….hmmmm then all of a sudden the current slowly climbs higher and higher. I shut the variac down. Hmm I check everything again, recheck the new SCR’s all good. I raise the Variac again slowly and monitor voltage. But this time I can hear some strange noises. Sounds like a quiet crackling all most. I grab a scope probe and look at the DC at the cans. Hmm all good. One can has slightly more ripple then the other. I shut everything down. Inspect the board. Test all the diodes and transistors etc etc. 2 of the SCR’s for one rail are warm. I then grab the bench cans to move them and DANG one is VERY hot!!! What the heck????

I flip the PSU board over and noticed the board is Marked + & - and guess what! I have the cans hooked up backwards!!!! I have now just fried one of my bench test cans!!!! And am very lucky I didn’t blow myself up!

I inspect the amps caps and guess what, one of them is backwards I assume from the factory! In my haste, I simply looked at the first can, saw the + terminal and connected up my bench cans which are set up with a buss bar between them and Positive, Negative and Common test leads attached to them assuming that the + was in fact + out of the supply! WRONG! One can was backwards from the factory and had worked for years as such without problems. Then one day for whatever reason it shorts out blowing the power supply fuse!

So, I crab a new pair of cans reconnect my test leads the correct way and raise the variac, guess what. The PSU voltage comes up, no current draw, voltage looks good and everything works just fine now!!!

2- Power supply caps, about $40.00
Lessons learned – Priceless!!!

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Old 5th November 2004, 05:23 PM   #2
roibm is offline roibm  Romania
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Default Re: A Tale of 2 Capacitors!

Originally posted by Zero Cool
One can was backwards from the factory and had worked for years as such without problems.
What brand was that cap? It was good
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Old 5th November 2004, 05:55 PM   #3
markp is offline markp  United States
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Wow, for a big cap to take 70 reverse volts for that long is a miracle! It should have spewed.
If it sounds good... it is good!
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Old 5th November 2004, 06:05 PM   #4
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Originally posted by markp
Wow, for a big cap to take 70 reverse volts for that long is a miracle! It should have spewed.

I dont understand it myself. I am of course Assuming it has been that way since new. the amp is very clean and doesnt look as if it had been monkeyed with. BUT. it is possible. someone was in there before me and reversed one of the caps and that is why is stopped working. but.....it doesnt appear so. no marks, etc to show someone else being inside....

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Old 5th November 2004, 07:23 PM   #5
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Did you do a polarity check on the bad cap? Maybe it was just marked wrong from the factory. It could be that the person who originally assembled the amp check and fond it ot be marked wrong and so installed it with the proper polarity, ignoring the markings. Seems unlikely that it would work for any length at all hooked up backwards.

If that' not the case, I'd bet someone switched it around when trying to get the amp up and running.

Thanks for this thread. I have a non-working SC being shipped to me as I type. I will definately check out the things you've listed here.

Blessings, Terry
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Old 6th November 2004, 01:12 AM   #6
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40$ ouch..

nice story
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Old 6th November 2004, 02:19 AM   #7
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Location: 3RS
Default Had similar experience.

I once came across a Denon CD player which played OK for sometime after purchase . Must have been a few weeks.
Then it suddenly stopped .
When I opened it up and one power supply cap attracted my attention as its top was severely convex , possibly about to blow.
I thought that they must have put in a lower voltage unit. But surprise , surprise it was soldered the wrong way around.
I couldn't believe it because it worked for so long.
I replaced the device with a new one connected right and the machine was fine.
How they goofed that during assembly I find very hard to understand. I 've been on Jap assembly lines and these things just can't happen.....but it did.
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Old 6th November 2004, 03:39 AM   #8
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You are indeed very lucky as I was years ago when I had accidentally connected a computer grade electrolytic capacitor up backwards. Just moments after removing my face from looking into the assembly, wondering why the dang power supply wasn't working the pressure release plug shot up like a bullet hitting the ceiling followed by a geyser of boiling electrolyte. I will never forget it and will never place my face directly over any electronic device again. If you have ever noticed, all my avatars have had goggles on. Just a reminder to myself of what could have happened if I took some boiling electrolyte in the face. I don't need to be any uglier than I already am.
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Old 6th November 2004, 04:17 AM   #9
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I was carefull to monitor the current draw. I knew something wasnt right. the faint crackling sound from the cans is what gave it away. i thought it was just static noise on the supply line from something in the circuit that i was hearing in the can.

I tested those caps tonight and both measure within tolorance for capacitance and i ran them up to rated voltage with a power supply on the variac through a 4 watt 120V bulb to slowley let the caps charge. worked well. the caps seem to be functional. at least for bench testing.

But i think i am going to try and find a clear plexiglass project box to mount them in with binding post's mounted on top and a switch connected to a discharge bulb.

Interesting notes. A 4 watt night light 120V bulb measures about 500 ohms cold. a 20 watt microwave oven bulb measures about 60 ohms cold.

it takes about 4-5 minutes to charge a 10,000uf can to 70 volts through the 4 watt bulb and about 30-45 seconds to discharge the same can with the 20 watt bulb.

I wonder how fast you can safley discharge a cap? obviously a dead short is not a good idea, but how do you calculate the maximum rate of current draw from a cap and factor in that fact that the voltage is dropping???

I noticed that the 4 watt bulb limited the charging current on a pair of 26,000uf caps wired together to about 11.5ma for most of the charge cycle.

And that 52,000uf takes about 4-5 minutes to discharge from 100V to .5 volts. using the 20 watt bulb.

I imagine 500,000uf would take about an hour to discharge with a 20 watt bulb. I bet you could run a 60 watt bulb for quite some time....

More fun with caps later.

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Old 6th November 2004, 04:02 PM   #10
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Originally posted by Zero Cool
More fun with caps later. Zero

Howcome i am visioning you sitting on 240 very big caps, welding spectacles on, on your way to some planet ?
It doesn't count how one deals with winning, but how to handle a loss (© DjT)
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