[Boatanchors] Filter cap question

[Brian Clarke]

Hello Ray,

100 uF at 500 V is not a readily available capacitor nowadays. So, yours
must be very old, and they probably don't have a temperature rating. The
difference between 100 uF and 125 uF is not significant. What will be
significant is leakage at your chosen operating Voltage and temperature, and
ESR for handling ripple. The AC-3 uses Voltage doubling for all three
outputs, and so, rectification is effectively half-wave. That is far more
stressful on the filter capacitors than full-wave rectification. The AC-3,
on its 650 V rail, uses two of those 125 uF capacitors in series with 150 k
resistors in parallel in an attempt to balance the Voltages across the
filter capacitors. Under steady-state operating conditions, those resistors
dissipate 0W7 each, and the design calls for 1 W rated resistors. They're
under-rated from a safe-design perspective and have probably drifted. So,
check those resistors. If it were mine, I'ld replace them with 2 W rated
resistors. That the design uses half-wave rectification, would make me
suspect that the 150 k resistors fell off the back of a Chinese truck, that
is, price was the main purchase criterion.

I would suggest that before installing your replacement capacitors, reform
them on a current-limited power supply till leakage falls to less than 5 mA.
Even at 5 mA, heating just from 325 Vdc alone will be 1W6 per capacitor.
Heating from handling the half-wave ripple will be on top of that.

As you're contemplating replacing the HV filter capacitors, have a careful
look at the other two DC rails. Half-wave rectification is a cheap and nasty
accountant's design, not an elec engineer's design. Though the phasing of
the windings isn't shown on the circuit diagram, I suspect all the half-wave
rectification is in one direction, which means that DC is injected back into
the mains. Not even 1st year elec eng students could get away with that kind
of illegal design. Yes, I know that most of our PCs' power supplies use
half-wave rectification - but that only says that resellers have more
political clout than the lawmakers. Do you know what happens when you have
DC on your pole pigs?

73 de Brian, VK2GCE.

On Wednesday, 4 September 2019 11:52 AM, you asked:

Question about filter caps.  I'm recapping a Drake AC-3 power supply.  Calls
for 125 mfd at 400 volts.  I have some 100 mfd at 500 volts.  Is 100 close
enough to 125 to work??