Arc'd/fused cap lead in power supply - Need Help!

[Gaz]

When you disassembled the cap did you find a short stub of the broken lead? If so, was it still attached to the foil plate?

Sluckey, as mentioned in my last post. The lead severed right where it enters the cap, not inside. When I opened it the connection to the innards was solid.

[Gaz]

I keep coming back to this because it's so odd. Any chance the plus side is shorting to the outer case? Radials weren't designed to have the leads bent sideways.

No, and as you can see in the photos, plenty of space between the can and lead, and no signs whatsoever of arcing to the can. The shrink label on the can is perfect.

[Gaz]

Maybe when you wrapped the lead around the turret, it strained the connection to the innards and with heat and use it failed.

Double check the adhesive to make sure it isn't conductive.

I cannot find any evidence of the GE Silicone II being conductive from under the bad cap, or measuring the silicone with voltage applied. I've actually used it between pins 2&3 on an amp with 700vdc on the plates with no problems.

[Phil_S]

I keep thinking I probably shouldn't be putting in my 2¢ on this because I don't really have the expertise. Tonight, I decided maybe that's a good thing because it allows me to ignore some things. Here's what I'm gathering from reading through.

The caps are mounted by bending the leads back in a manner that is atypical and probably not intended for radial caps. They are mounted with the lead side down (on it's head), on a dollop of silicone goo, another thing that's generally isn't done. A screwdriver shaft is used to make a "soft" bend, but the truth is it's a pretty tight bend as a screw driver shaft is rather narrow.

The problem occurred twice. That means it's not random, except maybe if the 2 caps are from the same lot and the leads are defective. Somehow, I can't believe there was a defect.

In my mind, all this adds up to user error. The cap isn't being installed as is typical. The installation stresses the leads, and then the cap fails.

Find another way to mount the cap. Place it on it's side. The goo will stick to the plastic cover of the cap and keep it in place without getting on the leads. I could be wrong. I suspect there is little to lose and something to gain. Give it a try!

[xtian]

I agree with the above.

And I totally disregard it. I've bent the leads of radial caps HARD, every which way, and never had a failure. Doesn't mean I'm not asking for trouble, but after installing close to 100 radials, I'm not sweating it.

[Gaz]

I keep thinking I probably shouldn't be putting in my 2¢ on this because I don't really have the expertise. Tonight, I decided maybe that's a good thing because it allows me to ignore some things. Here's what I'm gathering from reading through.

The caps are mounted by bending the leads back in a manner that is atypical and probably not intended for radial caps. They are mounted with the lead side down (on it's head), on a dollop of silicone goo, another thing that's generally isn't done. A screwdriver shaft is used to make a "soft" bend, but the truth is it's a pretty tight bend as a screw driver shaft is rather narrow.

The problem occurred twice. That means it's not random, except maybe if the 2 caps are from the same lot and the leads are defective. Somehow, I can't believe there was a defect.

In my mind, all this adds up to user error. The cap isn't being installed as is typical. The installation stresses the leads, and then the cap fails.

Find another way to mount the cap. Place it on it's side. The goo will stick to the plastic cover of the cap and keep it in place without getting on the leads. I could be wrong. I suspect there is little to lose and something to gain. Give it a try!

Phil, thanks because I do appreciate it, and my level of understanding on this matter is about the same as yours, I think, and so it's useful to hear your thoughts, which reek of common sense

For now I have to assume it's user error with how I'm bending the leads because no one, myself included, can think of one explanation why or how this could happen electrically without melting down everything else with it. Another hint is that in the other amp I replaced the cap in it hasn't happened again, and that was in October. No tube changes or anything else. Should be a good sign, right?

The only thing that freaks me out a bit is why this only has happened in the power amp section, when I do the preamp caps the same way. They are pretty much the same size.

Oh, and here's a photo of a test I just did with my amp that has 675vdc on the plates. The clip on right has the full 675vdc on it, and the other clip is spaced 1mm or so away, and I get a reading of 0.000vdc. Is it safe to say the 100% silicone is a good insulator?

[Firestorm]

Any silicone is an insulator once cured. I was curious how it behaved soon after application. I use silicone to stabilize leads in instrument cables and noticed that early after application, there's a partial short between the sides that goes away in 24 hours. If you're curious too, test it that way. That said, it's only happened twice. Coincidence? Three times would be a trend. As to it being in the PS, that's where all the current is, right?