RE: C-64 PSU

From: Bil Herd <>
Date: Tue, 24 Apr 2012 15:30:48 -0400
Message-ID: <>
I mentally replaced ”calibrated” with True AC Voltmeter which tend to be of
better calibration by design compared to a cheapo meter with an AC scale
representing a diode rectified version of the signal.  :)

The ratio of the output voltage at full load vs the output voltage at no
load is the regulation value of the transformer, more expensive
transformers typically  have a better regulation.  Guess which kind we used
at CBM.

This used to be one of the bitches of linear design as you have to run the
numbers at low loads in addition to high loads  since poor regulation means
higher voltage drop across the regulator even though the current is less.
Add to that the variables of powerline regulation (105-132VAC), transformer
tolerance, VR tolerance etc etc and a power supply that works at all
extremes can be a little tough to thread the needle…. Without spending
extra $$$ at least.

*From:* [mailto:] *On Behalf Of *Ed Johnson
*Sent:* Tuesday, April 24, 2012 1:30 PM
*Subject:* Re: C-64 PSU

Seriously, we need calibrated equipment?

On Tue, Apr 24, 2012 at 6:57 AM, William Levak <> wrote:

On Tue, 24 Apr 2012, wrote:

A question..

Since I have now quite a bunch of stuff sticking long way out of my
EXPANSION port and also some from the tape port - I am thinking to
eventually replacing my 30 years old and still reliable PSU. I have the one
with two fuses and a power switch, which is quite rare and I wouldn't like
to make it fail as it seems to get running quite hot with everything
attached. Now - what's the problem you may ask. The problem and question is
always the 9VAC. ALthough all the PSUs are rated at 9VAC, I recall that
they supply somewhere between 11V and 12V. Moreover I recall that back in
the days, when I supplied a "true" 9V of AC some things didn't work
properly on the 64. I don't remember today what was failing but I remember
that delivering a proper 9V didn't work out. Does anyone know why:

a) why PSUs rated as 9V (have to) deliver 11+

Power supplies rated 9V appear to be at 11 or 12V, but actually are not.
The 9V rating means 9V effective voltage delivered to the equipment at full
power load.  To measure this, you would have to put a resistor across the
power supply of sufficient wattage to match the maximum rated output of the
power supply, and then measure the voltage drop across the resistor.  Of
course, you would also need a calibrated voltmeter in order to get an
accurate reading.
SDF Public Access UNIX System -

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Received on 2012-04-24 20:00:32

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