Most likely you would get away with a "slightly" higher voltage.
Distinctly remember what happens when you hook up a rather large 400
Volt electrolytic in the wrong direction. It made a noise you dont
forget. The paper cover with the polarity marked was on backwards.
After that I checked to verify that the paper cover was on correctly.
Years ago we had a problem with tantalum caps being inserted
backwards. The problems included major fires. The flame from an
exploding cap would short the inner planes of a card. When the card
is supplied by 1000A ugly things happen. Many solutions were found,
including three and four pin caps (+-+ or +--+), fused caps, and
differing lead sizes (presumably with differing holes to match).
Once a manager of the card stuffing line called to complain that his
"girls" were getting sore thumbs from pushing the caps into too-
tight holes. That's the amazing thing about fools; they're so
Once upon a time, I helped manage a PDP-11/45. The main memory was all
core; no MOS memory was in common use yet. And the DEC core was
expensive and took backplane space, so we had some third-party core in
an external box, with an interface that pretended to be a Unibus jumper
(so it didn't take any backplane space at all). I can't remember the
manufacturer anymore; maybe Dataram?
One time when the interface was out, I noticed that a tantalum bypass
capacitor had fried. I removed it and replaced it with a new one,
carefully matching polarity with the board markings. The next time we
powered the system, on, there was the lovely smell of a tantalum cap
Took a closer look at the board, following the capacitor traces to +5 V
and ground. Sure enough, the silkscreened polarity markings for the
capacitor were backward! So the original cap plus my first replacement
were both installed backwards, and both died. The original probably
died during burnin at the factory, with nobody noticing (or they just
didn't burnin any of the interface cards).
I had a /35, /34, and /45 (IIRC), built into some tektronix gear.
The /35 had a full boat of core. The others were a bit newer and
had semiconductor memory.
Makes sense. My boss did a major study of tantalum capacitors and
it seemed that no matter what they did they couldn't get the
incidence of backwards tantalums below about .5%. They even found
caps marked backwards from the factory.
Like I said, these were causing major fires. These were radial-
leaded ("tombstone") caps. When they blew they tended to shoot a
flame down into the board, causing an interplane short. Since the
boards were backed up with 1000A supplies the short caused the
boards to get hotter, causing the boards to short further,
causing... ...smoke. Management got upset when their (many)
Million$ system caught fire.
The old tube radios used wax impregnated foil & paper caps. After years of
sitting unused, they dry up. The first time you power them up, they seem to
work fine for about 10 or 15 minutes, then you get failures. I have seen
capacitors explode and blow out a long coil of foil, like a slinky, from one
end. Loud, too. Electrolytics seem to fail less spectacularly, but they
Of course, there were no circuit boards to burn up. Just the wooden cabinet
or whatever flammable surface the set was sitting on!
Benjamin D Miller, PE
The paper capacitors used a high acid paper, like books. they were
wound on paper, then dipped in wax in an attempt to keep moisture out of
the paper and foil layers. It deteriorates, with age, till the
capacitor develops leakage. Depending on where it is used, it will put a
positive voltage on the grid of the next tube, or short out a power
rail. Even the metal can Mil spec can capacitors were paper & foil, and
filled with transformer oil. These also developed leakage as they age.
The caps that explode are usually electrolytic capacitors that have
lost the formed layer. By turning the equipment on without reforming,
it draws more and more current as it tries to reform the electrolytic
layer on the aluminum foil, which generates heat. That boils the
electrolytic and pressure builds to the point a vent blows, or the
aluminum can ruptures. The proper way to reform old electrolytics is
to use a current limited HV power supply and slowly ramp it up to the
surge rating of the electrolytic capacitor.
If you have broadband, your ISP may have a NNTP news server included in
I recently needed a variable audio source, and remembered I had an old
HP Audio Oscillator, which I'd bought as surplus some fifty years ago.
I found it in the garage, (HP Model 2000AB, Serial 1651). The line cord
was deteriorated and had to be replaced, which took half an hour since
the power transformer had to be removed to get access to the input power
I know about reforming electrolytics, and thought about raising the line
voltage gradually with a Variac, but finally just plugged it in and
flipped the switch. It came on and worked just fine!
I even found the "Operating and Servicing Manual" as a .djvu file. It
was Copyright 1955, and for Serial 7725 and above, but the circuit in
mine used the same tubes and seemed the same.
I re-set the feedback, and re-calibrated it slightly and it played like
new. There was no line frequency hum in the output, so the two
electrolytics in the power supply must be OK. (At least they didn't
explode! I've had experience with ones that did--it makes a real mess!)
This was one of the first instruments designed by Hewlett and Packard.
They don't make them like they used to!
HP Model 2000AB? You mean the HP Model 200AB. yes, the old HP
equipment was built with premium parts. Most equipment wasn't. I'll
bet that if you took the time to test all the capacitors for leakage, at
least a few ate under 5 Megohms and need replaced.
I have a TS-382, which is the military version of that audio
generator. It also has a reed type frequency meter. It has no
electrolytics, but every oil filled metal capacitor is leaky. I am
restuffing the canned capacitors with HV mylar film capacitors that
should last 50 to 100 years.
If you have broadband, your ISP may have a NNTP news server included in
It is the HP 200AB. My eyes are not too good and the spell-checker
didn't catch it. Just last week, one section of the original 6SN7 tube
failed. I signal traced it and replaced the tube with one from my
spares box, also over fifty years old. When the oscillator still didn't
play, I was beginning to mis-trust my old vacuum tube diagnostic
abilities. It turned out the replacement tube had the same bad
section--a different one worked OK and is still working.
Mine seems to be even earlier than the one shown there. It has a
"fluted" knob on the frequency dial.
This looks like the manual I have. This Linux OS doesn't have a .djvu
program. I d/l'd it and will take a look using WinXP. Thanks for the
I'm sure both oscillators will outlast the both of us.
The feature of a damaged Cap depends on what kinds of it.
In my experiences, most of ceramic cap get short after destroyed, and
SOME of Tantalum Cap get open after destroyed - although they
publicize that All of the Tantalum can open after damaged.
P.S. most of my experience based on the Low voltage situation's
One major factor is the voltage source impedance.
With most capacitors, the dielectric fails in an over-voltage situation.
For a ceramic chip, the result is a fracture in the ceramic. This will
likely reduce the capacitance and the voltage at which future failures
occur. Depending on the source impedance, the result can be that a
conductive path is formed through the fracture ("short") or the
capacitor blown apart ("open"). Or that the only effect is a reduction
The same generally applies to other capacitors.
Some (eg air-spaced) have "self-healing" dielectrics and are virtually
unaffected, as long as the amount of energy released isn't too great.
Some are designed to have internal impedances such that the plate area
adjacent to a failure point will vapourise and thus the capacitor
Tantalum capacitors don't have any such measures. if the dielectric
fails, the result is almost invariably a dead short between the very low
A lot of energy released into an electrolytic used to result in its case
being fired out of the apparatus with enormous force. Certainly enough
to kill an assembly line worker that had just powered up the uncased
chassis in front of them. That was because the end seal was the weak
point. Modern ones have weak points on the top, to prevent this.
Generally, an extremely high source impedance only results in reduction
in capacitance. A high source impedance generally results in a short. A
low source impedance results in an open circuit - if you can actually
find the remains of the capacitor, that is.
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