Replacing caps in test equipment.

[intp1000]

I've seen a lot of posts on the yahoo HP group that suggest replacing the PS caps on newly acquired old test equipment. Question. If cap size is NOT an issue then should one try to match the ESR of the new with that of the old cap (can this even be determined)? Can one just use a good 105 degree C low ESR cap; or would too low of an ESR cause a problem? I seem to think that too low an ESR would cause too high a charging current in the caps and thus a higher than expected peak current in the, typically bridge, diodes thus leading the old diodes to an earlier failure than expected. What do you think?

[stj]

Re: Replacing caps in test equipment.

esr is for switching supply's,
most old gear uses linear suppy's with a big transformer.
on those it's the ripple-current that matters - higher=better.

you often need to re-drill the pcb with these repairs.
modern large caps use a 10mm pitch between the legs - older stuff has bigger non-standard spacings.

[intp1000]

Re: Replacing caps in test equipment.

Thanks stj

So when replacing caps in the linear supply I'll have to go for replacements with the highest ripple rating. As to the lead spacing I'm replacing axial types with the modern radial type so that won't be a problem since their very different to begin with. As to the pcb hole diameter I hope I don't have to drill them out since the boards may, or not, be multilayer. Now that I think about it I think the boards are not multilayer. So thanks again for setting me straight about ESR.

[stj]

Re: Replacing caps in test equipment.

axials are better - use axials
there is a company that makes very high grade axial's that i use.
VISHAY BC COMPONENTS

[Krankshaft]

Re: Replacing caps in test equipment.

I use them on old stuff too specifically tube based equipment.

They seem to be the only company that makes those oddball values in axial packages like 16uf at 150V .