50+ years ago, 400 Hz power supply plugged into 60 Hz source. Runs for a few minutes and then throws its guts all over the bench. Back before the days of power supplies that work from 48 - 440 Hz, 100 - 240 vac. But you only do it once ?
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On Mar 5, 2025, at 9:13?AM, Jim Ford via groups.io <james.ford@...> wrote:
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Yep, I have seen the fireworks in the lab, too. ?Been a while, fortunately. ?More than 25 years ago we had a technician who installed a few tant caps backwards into a VMEbus chassis. ?Blam, molten metal shot across the lab! ?Fortunately nobody was hurt.
Jim Ford?
Laguna Hills, California, USA
On Mar 4, 2025, at 9:30?AM, Dave Daniel via groups.io <kc0wjn@...> wrote:
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I'll add that since, in general, modern caps are smaller than vintage caps, make sure the the voltage rating of the new cap provides sufficient margin over the voltage rail. For instance, Tektronix often used caps rated for +15v on +12v rails. In all the cases where I have recapped equipment, a modern +25v cap was smaller in physical size than the original +15v caps.
Arnaud, you can replace Ta caps with alternatives in most applications, so there's no need to spend too much for an original type. Usually ones that burn out are in supply bypassing spots, which are not critical. The main issue then is space available for new parts, since the common fix is to go with a much larger capacitance (and physical size) aluminum electrolytic type to get ESR comparable to the Ta ones. I'd say put in an Al cap of at least 2-10 times as much C as the original Ta, with enough V rating for the supply voltage. Or, use the biggest C that can physically fit into the space available. You can look at various brands and families of caps to choose the lowest ESR you can find, but they won't be dramatically different.
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Another option is to go with an OSCON organic dielectric type Al cap. These are built for low ESR and can beat Ta in this respect, so fitting is easier. The problem here is that (I think) OSCONs are only available in radial mount style (can with leads out the bottom), so trickier to fit in axial-lead spots. If axials are available, then no problem.
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Remember this all only applies to supply bypassing spots. Sometimes Ta is needed for particularly low DC leakage and better stability like in timing circuits. Ones in these kinds of spots hardly ever fail because they're not stressed. Supply bypassing ones fail a lot, but value stability and leakage hardly matter here, so bigger Al types or OSCONs (which have the worst leakage) work just fine to get decent ESR.
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Ed
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