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Old 6th April 2013, 11:43 AM   #1
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Default Motor run cap failure

My supplemental cap has leaked all its oil out and i dont know why. Its a 100uf 450v item. The leads showed some signs of getting hot but no other obvious tell tale exists.

I will admit, this was after an evening of use during which i noticed i hadnt put the jumper wires in a in the 3rd and 5th terminals ftom the left along the rear edge of the board (basic triode mode).

Any ideas? The amp works fine without it and interestingly the small amount of hum that wss oresent has now gone!

Last edited by mudchute; 6th April 2013 at 03:53 PM.
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Old 8th April 2013, 02:33 AM   #2
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You probably had a defective one with an internal short. That would explain why you heard hum...it was loading the PSU. The amp's on-board caps are sufficient for very quiet power supply. These types of caps some times self-heal, but it sounds like yours got hot and the oil expanded until the seal failed. Did the can have any dents in it?
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Old 10th April 2013, 02:25 PM   #3
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Have you verified that it's a motor run and not a motor start cap?
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Old 10th April 2013, 02:40 PM   #4
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It was definitely a motor run cap that I ordered... In hindsight, its a 100uf run cap that is half the size of every other 100uf motor run cap out there and very cheap & Chinese looking. I used it in a bid to keep my amp as compact as poss as comparable equivalents were enormous.

To demonstrate my inability to learn I've ordered another (with solder tags so I can use my own cable) in the hope that the previous one was defective. If it goes wrong again I'll have to get another cap tge same physical size but invariably a lower value in order to fit the space I've allowed for it.
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Old 10th April 2013, 02:41 PM   #5
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Oh, and it had a small dent (2-3mm deep x 5mm long) in it.
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Old 10th April 2013, 04:52 PM   #6
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Quote:
Its a 100uf 450v item.
That is suspicious. Motor run caps are intended for use on AC and usually rated for 330, 440 or 660 VAC. I have never seen one marked 450V. Where are you getting these?

Quote:
My supplemental cap has leaked all its oil out and i dont know why.
Where did the oil come out of the cap. Any of the motor run caps that I have used did not have a vent, and there is no place the oil could escape from without puncturing the can.

Several years ago there was a nearby lightning strike that killed my home AC. The motor run cap had eaten the surge and died. I cut it open. The top of the can where the terminals are had bulged upward thus severing the internal connections from the wound core to the terminals causing the cap to go open circuit. It looks like this failure mode is intentional to prevent the cap from exploding.

If space is an issue, use a smaller valued cap. You need anything 370VAC and above. Even a good 20 uF is better than a Chinese firecracker. My Lexan TSE uses a 20uF because that's all that will fit.

If you try the same cap again check to see if it is getting warm. If so remove it immediately. A cap that gets hot (from internal electrical leakage) WILL leak....or explode....and it will fry your power transformer slowly.

A cap that gets hot because it's mounted next to a tube will have a shorter life than the same cap that stays cold, but it is not bad.
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Old 10th April 2013, 05:07 PM   #7
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CD60 100uF AC 450V Motor Run Aluminum Electrolytic Capacitor Dchcj | eBay does that work?

Its funny you mention the pt as mine was getting a bit of a buzz on as well as warming up quite a bit.

The cap is an ally can with a plastic end cap which the terminals go through. The oil leaked past the end cap seal where the can is swaged over slightly to create a seal. It hadn't expanded or deformed at all, I think it was just s**t. I'll give the new one a blast (not literally I hope) and see how it goes.
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Old 10th April 2013, 05:42 PM   #8
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No, you want a film-in-oil cap. That is just another electrolytic cap. Is that what you used?

Edit: ok I see that this must be the same as what you ordered last time. What leaked was not oil but electrolyte. Don't bother with it. The goal of the aux cap is to provide low-ESR filtering via a real film cap. You don't actually need any more capacitance in the power supply. Adding another electrolytic in parallel with the one on the PCB won't give you the benefit of a real film cap there.
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Old 10th April 2013, 06:18 PM   #9
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This will fit in the gap in my amp DUCATI 30 uf CAPACITOR MOTOR RUN 450v 30mfd FLYING LEAD | eBay is that the correct type?
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Old 10th April 2013, 06:54 PM   #10
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That looks like the right type of cap, yes. Not the most pretty, but it has the right specs.
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