Encountered a "Death Capacitor" today!

They sound scary, and most people bring up mental visions of an old CRT television, but those aren't what they really are. (BTW...TV caps are WAY MORE dangerous!!).

The old "death caps" were developed to filter out RF noise from amplifier circuits. How they did it was by creating what amounts to a low-pass filter in front of the amp power supply. This involved using a capacitor, and life was good (sort of) until that capacitor fails. It's an old design, probably last used in the late 60's or early '70's.

I probably should have titled this "how not to blow up your testing stuff", but I digress. Or, "always use a MM to check your ground". Anyway, a guy brought an old guitar amp (really old) to me today to take a look at. It wasn't big, but it was big enough. Disconnected all the right stuff, and got ready to start going through it with the scope. FIRST, I decided to check the potential between earth ground and the chassis. WOW...over 100 volts difference! Yes, it was a two prong plug (an old one) with no polarity indications. Good thing I checked it! Bad cap on the low-pass filter circuit.

Some of those transistors are running at 350v+ and I had my fingers all in that, one at a time of course. One wrong connection and I would have smoked some gear!

Always pay attention to your ground potential! Sometimes, it's not what you think!!!

Over-all good advice!

I got bit pretty good once while changing the ballast in a 277v florescent light.
Damn near welded my lineman plyers to the fixture. Young and dumb at the time....

Did you fix the old guitar amp?

I made a bank of 10 supercaps at 350 farads(each). Not microfarads, not nanofarads. Farads.

I plan on over charging it from about 40 yards away. I'm confident the explosion will be pretty freaking big.

originally posted by: Flyingclaydisk(BTW...TV caps are WAY MORE dangerous!!).

from a young age my dad always impressed upon me how lethal touching certain parts of old TV innards could be.

originally posted by: defuntion
...

Did you fix the old guitar amp?

No, not yet. That freaked me out a little bit. It was easy enough to fix (well, beyond just the correct polarization), but after that I just wanted to think about how to eliminate the potential noise without that kind of a circuit, and without completely altering the original amp.

a reply to: RoScoLaz5

Some of those old TV caps are 600-1,000 volts or more! And, they discharge in less than one second! You can't move your hand that fast. Even some stereo amps have 650-750 volt caps, and those will light you up too if you're not careful.

A lot of people think, 'hey, it's only 120vac going in, so how bad could it be?'...heh, well, you might be surprised!

RF amps are even worse!

originally posted by: Flyingclaydisk
a reply to: RoScoLaz5

Some of those old TV caps are 600-1,000 volts or more! And, they discharge in less than one second! You can't move your hand that fast. Even some stereo amps have 650-750 volt caps, and those will light you up too if you're not careful.

A lot of people think, 'hey, it's only 120vac going in, so how bad could it be?'...heh, well, you might be surprised!

RF amps are even worse!

I have a few SET amps that have 700-800v running through them, you definitely want to make sure caps are discharged before tinkering. There are some exotics running several thousand volts..not for the timid.

a reply to: Flyingclaydisk

Stay away from the starters on a Central Air compressor,
too Clay. I about welded half my outboard gear to a hood
discharging onea those cans, 220 single or not 'twas special.
And that afternoon I played bass slower than usual.

a reply to: Flyingclaydisk

Sounds like quite a zap and jolt you would get.

But I would not worry, just don't stand in water and wear rubber gloves. The real enemy though you got to watch out for is? You know it. The dreaded corded lawnmower.

I used to sell caps to TV stations and in the warehouse, the big ones had copper bars across the terminals. If you didn't short them out, they'd pick up a charge from the air and surprise you. Old metal picture tubes would hold a charge, too.

Old TV sets were notorious for zapping people. All repairmen used a suicide cord so they could power the set with the back off. After the back was off and before plugging in, take a screwdriver and ground the things that would get you. Then, remove the side from the high voltage power supply cage and plug in the set. The 1B3 high voltage rectifier and the 6B6 were both capped. The 6BG6 was high frequency high voltage AC for picture horizontal deflection so a shock from that went through the surface of your skin and left cone shaped burn. The 1B3 was a half-wave rectifier, usually putting out 20,000 volts DC, driving the picture tube, so a zap from that hurt.
What hurt worse was that getting zapped caused the zappee to juke. The 6x9 speakers on the old sets were sometimes held on with long screws and it was entirely possible to impale one's hand from the shock.
I had the scar for years.

Worked arcade repair, crt repair is a dying art...


One of the jokes was these capacitors and frequent stories of accidents abound... techs often joked they could tell the exact charge by the shock they got...


Btw rip the arcade industry, killed by covid.

a reply to: Flyingclaydisk

My dad went to Alabama Polytech (Auburn Univ) right after the war, on the GI bill and got a degree in EE, graduated, and went to work for RCA right out of college.

Our house in suburban Chicago was the only one with a color TV, in the late 1950's. The Wizard of Oz was a neighborhood event over at out house.

I remember dad occasionally working on that old TV, and telling me touching the wrong place would be a bad deal. That TV lasted forever.

He got it for free, after it ran non-stop in the lab for a really long time.

So no testing with the tongue if it has a charge?

originally posted by: merka
So no testing with the tongue if it has a charge?

If it's your own tongue..no!

I'm curious what amp it was. I've worked on many amps before, it was a job once a long time ago. Always had some chopsticks on my bench...

When i was in the navy i took small caps and charged them with a crank type megohmmeter and left them lying around for idiots to pick up.

Years later I got a laser capacitor that was clearly marked in big red letters "deadly when charged"

there is a capacitor device out there called a USB killer stick that looks like a usb flash drive that will Kill any computer, external usb ssd hard drives that computer criminals have for use encase of a police raid to destroy computers or hard drives.
en.wikipedia.org...

a reply to: ANNED

LOL! I had a shop teacher in Jr. HS who did the same thing. He was always late for class, but he always had something new sitting on his desk which he was going to talk about. The students would constantly meddle with the stuff on his desk before he got there, and it used to drive him nuts. So one day he put a fully charged cap on his desk. It was a pretty good sized cap too! Not enough to kill someone, but certainly big enough to "educate" them.

Two or three people picked up the cap without discharging it and set it back down. The class clown, Mike Debben, got up there and started playing with it and imitating our shop teacher who had a very unique way of talking (he said "aaaaaaahhhhhhhhhh" a lot, like several times in every sentence). So Mike was holding this thing up in front of the class, and imitating our teacher, enjoying the laughs he was getting. Suddenly Mr P. walked in and Mike hurriedly tried to put the cap down when...BAM!!...it lit him up! Knocked ol' Mike right on his butt, and the class roared! Even Mr. P. was laughing so hard he couldn't talk. I'll bet that was the best laugh Mike ever got.

First words out of Mr P's mouth were...."Aaaaaaaahhhhh, I bet that'll aaaaaaahhhhhhh teach y'all a lesson about aaaaaaahhhhhh touching stuff on my desk, now won't it???"

LOL!!! I'll never forget that!

a reply to: rktspc

It's some el-cheapo thing, says Silver Marshal XL (or something like that) on it. It's some kind of an import knock-off on the real "Marshall" amps. It's definitely seen better days, but he likes this thing (no idea why, it's not a tube amp or anything, but he says it sounds cool). Looks like it sat in a garage for about 30 years. (and inside it looks like someone made it in their garage too! LOL!)

I'm just glad I checked the chassis on it before I did anything else. Then I looked around and saw the cap. I'd never seen one before. I'd heard stories, but never actually saw one.

a reply to: ANNED

Regarding the killer USP gadget, I doubt it would actually wipe out a HDD. It would definitely jack up a computer, no doubt about that, but I don't think it would wipe out the data, at least not in a way that it couldn't be recovered by data recovery specialists. It might do a number on a SSD though. It's surprisingly difficult to destroy a HDD to where data can't be recovered, and those data recovery guys are wizards when it comes to that. At work we have the only 100% foolproof way (that I'm aware of) to destroy a drive. It's a shredder specially made for shredding HDD's. Just drop the drive in, shut the lid and turn it on. You hear a little crunching, but that's about it (not loud)...and that puppy is turned into millions of tiny little pieces. It's practically dust. Not recovering anything off one of those drives! And that bin is HEAV-Y too! Takes a special truck to lift it up and empty it.

a reply to: Flyingclaydisk

When I was on a industrial electronics course they did that joke of charging a capacitator and leaving it around to see who would be the first to touch it where they shouldn't.

It worked several times.

The worst shock I got was not from a capacitator, was from the primary coil of a transformer that was connected to direct current. On that occasion I really felt that my heart almost stopped with the shock.