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Old 21st August 2006, 05:43 PM   #1
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Default Reversed polarity on Sprague Powerlytic

Today I met Murphy and his law...

I was almost done re-building the power supply of my amp. I guess I wanted to finish too soon, because I accidentally connected one of the capacitors the wrong way (reversed polarity). During testing, I noticed that the voltage off one channel was a bit lower than on the other. While investigating the cause, I soon discovered my mistake.

Fortunately, the capacitor didn't explode or anything, but the capacitance is now a bit lower than it should be. With a resistor of 940 ohms, I have an RC-time of just under 30 seconds; this should be 35 seconds.

The cap is a 37000 uF Sprague Powerlytic. I have 4 of these in the amp. I would really like to continue using all 4 of them, but is it save/advisable/etc.??
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Old 21st August 2006, 07:17 PM   #2
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Default Re: Reversed polarity on Sprague Powerlytic

Quote:
Originally posted by starbase218
Today I met Murphy and his law...
That SUX. I hate when that happens.

Quote:

Fortunately, the capacitor didn't explode or anything, but the capacitance is now a bit lower than it should be. With a resistor of 940 ohms, I have an RC-time of just under 30 seconds; this should be 35 seconds.

The cap is a 37000 uF Sprague Powerlytic. I have 4 of these in the amp. I would really like to continue using all 4 of them, but is it save/advisable/etc.??
Ditch it. Even if it didn't blow up, something's not right internally. It might work for awhile, but it will probably poof after some use. PS filter capacitors have to carry considerable AC, and that's a pretty tough assignment. Bite the bullet, get a new one, and save yourself more troubles in the future.
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Old 21st August 2006, 09:36 PM   #3
poobah is offline poobah  United States
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Agreed,

Reverse voltage boils the electrolyte... even if it still works its days are numbered... an accident waiting to happen.

BTW... don't do that again. My old boss has a 1 inch scar on his cheek from a piece-o-cap that wound up IN his mouth; through his cheek. Little caps... no big deal. The big boys are different story though...



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Old 21st August 2006, 10:02 PM   #4
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even the little caps can be quite nasty when they go, i had a nasty gash in my hand from the top of an electolytic cap.. it exploded because i conected it the wrong way and ofcourse my hand just happened to be over it..
OWen
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Old 21st August 2006, 10:22 PM   #5
anatech is offline anatech  Canada
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That's a fact. Consider all electrolytic caps are bombs just waiting to detonate. I've almost been got a few times in the shop. I have ended up looking like Santa Claus once. Everyone had a great laugh because they all came-a-run'in.

I've had some small ones leave dents in wood. None of them are safe.

-Chris
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Old 21st August 2006, 10:41 PM   #6
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if a 220uF cap can leave a 1mm deep gash in my hand.. imagine what it could do to your eye! When i power up an amp for the first time or even after makeing changes to it i always cover it with something like a large tupperware tub for about 5 minutes or so incase anythign is going to go bang.. better safe than sorry i say!
Owen
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Old 21st August 2006, 11:08 PM   #7
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We require safety glasses working around live open circuits,
and electrolytics is the primary reason.

With regard to reversal on large electrolytics, we tend to catch
them quite early because we watch the current at low voltages.
I keep the extracted capacitors and use them on in-house
projects, where I have yet to see one fail. In Starbase's case
I would toss them.

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Old 21st August 2006, 11:24 PM   #8
poobah is offline poobah  United States
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OUCH!

That's where my hand would have been as well!



Good point about the glasses! Even a little cap could leave you bummin'

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Old 22nd August 2006, 12:20 AM   #9
lndm is offline lndm  Australia
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I've had a small film cap shoot a wad of epoxy across the room, with my wife explaining she thought it was a gunshot. (though on later inspection I found the batch to have detriorated cases)

In my younger years my teacher brought a 1F cap to class. I secretly (foolishly) charged it to 12V to give my friends a scare. They weren't the only ones. Teachers from nearby classes were in the hallway trying to figure out what happened. My reprimand didn't compare to how I already felt.

The point is, this serves to remind me of the potential (no pun) living within these devices when I might otherwise look at them just sitting there looking peaceful
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Old 22nd August 2006, 08:25 AM   #10
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Just when I had the PS the way I wanted to... this happens. Well I guess I've only got myself and eagerness to blame.

The pic shows the power supply. The top-left cap (underneath the power switch) is the one that had its polarity reversed.

Would you know any good (web)stores where I can order these caps? They don't have to be 75 volts (the PSU delivers approx. 35V).

Thanks in advance.
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