Old Power Supplies, revisited

From: Richard Erlacher <edick_at_idcomm.com>
Date: Sun Apr 28 11:16:25 2002

At that low voltage and low capacititance, I'd say you needn't worry about
"reforming" the capacitors. Many of my commercial S-100 supplies put out over
12 volts, at no load, on the so-called +8V supply, and over 20 volts on the
bipolar 16 volt supplies. They come down by 10% or more as they're loaded but
none of them come down below 8 or 16 volts even when loaded somewhat beyond
their specified current. It's nice to have the regulators dissipate as little
power as possible, but it's also necessary to avoid ripple voltage "valleys"
that drop below the 2.2-volt margin recommended by the regulator vendors.

You can check the unregulated output of your filters by loading the supplies
and measuring the output voltage in both DC and AC mode. The DC level will
reject (averaging it out) the AC component. The AC reading will reject the
DC. If the AC voltage is sufficient that it, when centered about (therefore
subtracted from) the DC level is lower at its minimum than the required
minimum input for your regulators, then you may have a problem.
Adding/substituting a larger capacitor may help, but if there's no resistor
between the cap and the diodes, it might be well to add one, to limit the
current from the diodes to the cap at power-up. Knowing the rating of your
diodes will help you decide whether this is desirable, and what value resistor
is needed.

Dick

----- Original Message -----
From: "Louis Schulman" <louiss_at_gate.net>
To: <classiccmp_at_classiccmp.org>
Sent: Saturday, April 27, 2002 9:07 PM
Subject: Old Power Supplies, revisited


> I am putting my SWTPc 6800 back together, after complete disassembly
> for cleaning. So, on to testing the power supply.
>
> It works! But here's the question, which I know has been discussed
> before with respect to S-100 machines.
>
> The power supply is a big transformer with one really big (91,000 ufd)
> capacitor, and the usual rectifier stuff, etc. The documents call for
> 7-8 volts unregulated, along with + and - 12V.
>
> I am getting something like 9.3 V, and + and - 14.3 V. Are these
> voltages too high? Of course, on the SS-50 and SS-30 cards there are
> voltage regulators, but I don't want to cause any damage. This thing
> probably hasn't been powered up in 25 years.
>
> How can I check the capacitor function without an oscilloscope or other
> fancy equipment? Should I worry about "reforming" the capacitor?
>
> Thoughts appreciated.
>
> Louis
>
>
Received on Sun Apr 28 2002 - 11:16:25 BST