Oh my, am I losing it?

[Randall]

Something happened today on my bench that has me unnerved. I was doing a re-cap on a '82 100 watt Marshall 2203, and had just clipped out the last can, the one under the board. I set it upright next to the other 5 at the corner of my small bench next to my Weller station. I had the socket side of the board propped up and was tinning the last of three wires when suddenly pppfffzzZZZZTTT! Flash! Set me back in my seat with a very real WTF was THAT?

The amp was not plugged in and had not been for a week, the caps that were in it were fresh and not charged up. Nothing else on the bench was powered on except my Weller station, but it was working fine, with no sign of arcing on the tip. In fact, I could find no sign of arcing anywhere on the amp, or anywhere else.

It scared me, so I'm not 100% where it came from, but I think it was from the other side and behind the chassis, which is where the old caps and solder station were. Looked at and measured caps, they seem fine with no obvious damage. I even took apart my Weller to see, but it is clean, I thought being in sub tropical SW Florida maybe a crawlie got in there, but no.

I opened up my wall outlets, nothing. I powered up the amp when finished, and it has no issues, no smoke marks, nothing. So I submit, how is this possible? My only reasonable guess is the last can I took out was still charged and shorted to something (it was close to the amp). But if so, why no arcing marks anywhere, and how is it possible for it to remain charged enough to make a light show after a week? I don't understand this.

[The Dude]

Did you have tacos for lunch?

[rjb]

SW Florida?
Ball lightning is my guess.

[Leo_Gnardo]

Old Marshall caps are famous for holding charge for weeks. But if you discharged those caps, even the main ones while the lower-voltage filters were attached, there shouldn't have been enough charge to put on much of a show. Film caps on that board may have held charge too but similarly, not enough to fzzzzap!, just a tiny snap at most. So, it's a mystery. New caps, although you didn't charge 'em, maybe somehow took a charge.

The ol' ham radio guys often sling a wire across terminals of stored unused hi voltage oil caps, because they can pick up a serious charge from static electricity, unplanned but still unpleasant for anyone who takes a hit off 'em.

[gbono]

Not losing it you just experienced dielectric absorption in an aluminum electrolytic capacitor. I had a conversation with a Kemet Applications Eng several weeks ago about this very issue and it is not uncommon to see "soakage" in high voltage capacitors. This is not due to static electricity but it is caused by the chemistry of the electrolyte.

[Leo_Gnardo]

Not losing it you just experienced dielectric absorption of an aluminum electrolytic capacitor. I had a conversation with a Kemet Applications Eng several weeks ago about this very issue and it is not uncommon to see "soakage" in high voltage capacitors. This is not due to static electricity but it is caused by the chemistry of the electrolyte.

Yup, that too, but I've not seen more than maybe 30-40V "bounce-back" result from this phenomenon in caps that are ordinarily charged to 450-500V. Enough to throw off resistance measurements in an amp but not enough to put on a little fireworks show.

[gbono]

15 - 20% of rated working voltage is common - Kemet ships their high voltage caps with shorting bars. I have seen over 100V develop on supply caps. A ghost in the machine is the other plausible explanation

[Randall]

I don't know what I experienced. I do know that the hum, buzz started and ended with the zap and flash. And I didn't discharge or measure the caps before I too them out. I had my fingers on some of the terminals, but as I think back, not on the last one. I simply snipped the wires and pulled it out, so I guess it is possible it was still charged, but what happened after that? I don't know.

[tedmich]

+1 ball lightning !

[Leo_Gnardo]

+1 ball lightning !

How can we avoid? Briefs or boxers? Talcum powder or graphite?

[tedmich]

most guys do a bit of "ball lightening" on a regular basis...Never talcum powder! It gives women cervical cancer (no shit) and as long as I'm alive there's always the possibility of...transfer.

[Alex R]

the shop i rent is on a farm and plagued with flies in the summer. So I bought one of those bug zapper things, no flies any more. But every time it zaps one it sounds like an arc, and a minor wave of panic seizes me, I have never got used to it. The fact that it's behind me doesn't help.