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Old 10th June 2006, 11:16 AM   #1
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Default Screwed up!!!!

I was checking a 600W Rockford audio amp.
I found a bad filter cap( 2200uF 35WVDC ) There are 2 of them that share a neg.
I pulled them both, tested them and one was bad. When I went to replace them, I noticed the board the board was not marked for polarity. Looking at the common (neg) I installed them accordingly. The cap that was bad,( not the common lead , ) goes to the input of an LM337 (- VR) .
My question : being that it is a -VR input , would the cap have been reversed polarity?
( neg to input of LM337 Pos to neg common with other filter cap)
My problem , lol , 2 blown filter caps now.
I am also getting power to input of both Power Mosfets but no output from either, no power to anywhere else on the board.
If I can avoid it I don't want to experiment with cap polarity, these things are $4.95 each, lol.
Im not looking for anyone to solve my problem, just some advise on this Cap polarity.
Any advise would be appreciated.
I learned not to remove components without marking polarity first!!!!!!!! lol.
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Old 10th June 2006, 03:05 PM   #2
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Default Re: Screwed up!!!!

Quote:
Originally posted by amelecsol2000
I was checking a 600W Rockford audio amp.
I found a bad filter cap( 2200uF 35WVDC ) There are 2 of them that share a neg.
I pulled them both, tested them and one was bad. When I went to replace them, I noticed the board the board was not marked for polarity. Looking at the common (neg) I installed them accordingly. The cap that was bad,( not the common lead , ) goes to the input of an LM337 (- VR) .
My question : being that it is a -VR input , would the cap have been reversed polarity?
( neg to input of LM337 Pos to neg common with other filter cap)
My problem , lol , 2 blown filter caps now.
I am also getting power to input of both Power Mosfets but no output from either, no power to anywhere else on the board.
If I can avoid it I don't want to experiment with cap polarity, these things are $4.95 each, lol.
Im not looking for anyone to solve my problem, just some advise on this Cap polarity.
Any advise would be appreciated.
I learned not to remove components without marking polarity first!!!!!!!! lol.

Are you sure? The positive side should have, at least, a square pas as opposed to a round pad.

As for -VR, this should be of a lower potential than ground, so the negative side of tha cap should connect to -VR while the positvie side goes to ground. You should not have to experiment, just measure.

If you are getting voltage on the mosfet but it is not passsing through, there is no voltage at the base to turn the devices on. This would be a problem with the PWM chip. SG3525? Do you have a scope?
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Old 10th June 2006, 06:53 PM   #3
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Default Screw up, lol

I do have a scope and am heading that way right now. I will post my findings.
Thank you for replying.
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Old 10th June 2006, 08:13 PM   #4
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To blow the original caps, you would need excessive rail voltage, too much ripple current or reverse polarity.

Excessive rail voltage could have been due to failure of the switching power supply's regulator.

Excessive ripple current could have been due to low impedance loadng of the power supply (either low load impedance on the audio outputs or a fault that caused the output section to draw too much current).

What is the model number of the amp? 35 volts doesn't sound like it's enough for the rails of a 600 watt amp (unless it's a 4 channel amp). They used 50 volt caps in their old 500 watt amps.

To avoid blowing components in the future, use some sort of inline current limiter in the B+ line. A $10, 2 ohm 25/50 watt resistor (tubular ceramic preferred) can save you a lot of money if you do this type of work.

As TO-3 noted, the square pad is the positive pad for a capacitor. For diodes, the square pad is the cathode (striped end). For ICs and transistors the square pad is pin 1 (except on a few Orion amplifier jfets).
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Old 12th June 2006, 11:17 AM   #5
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Default screwed up

Ok got the new caps , 50WVDC ,had to do some driving.
The model is Punch 201s

The PWM is a TL494C

I have not had a chance to go out there yet . I will be out there in a little while.


Thank you.
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