13 Jul
2012
13 Jul
'12
10:34 p.m.
On Fri, 13 Jul 2012, Chuck Guzis wrote:
I'm getting around to doing some work on my
veneered and generated
HP5307A frequency counter. Looking at all the gold-plated PCB
goodness inside, the first thing that jumps out at me is a bulging
electrolytic. It's a 940 uF, 40V unit. Not 1000 uF, but 940. Not
50V, but 40V. I'm going to substitute a pair of 470uF, 50V units
paralleled as a substitute, but this had me wondering if anyone knows
why the strange values, particularly since +/-20 percent tolerances
are common on electrolytic caps.
The 940uF 40V capacitor just dates to before we had the modern preferred
values system. If it is just a filter capacitor, a 1000uF 50V should work
just fine, and would likely be more reliable (and have less leakage) than
two smaller capacitors in parallel.
I replace odd value capacitors all the time in older SMPSUs and as long as
the values are close, substitutions are generally not a problem. The hard
part I've found is finding modern parts with the same lead spacing.
Sometimes you have to go up a case size and/or voltage rating to find an
electrolytic that will fit the existing location without putting excessive
bends in their leads. This is especially true with old 3-lead "computer
grade" capacitors (typically two ground leads).