put it under load for a decent amount of time and check the voltages when its hot
here is some useful text about the subject:
If you have some experience with a soldering iron and have a multitmeter,
you can make a tester to safely evaluate a Commodore "black brick" power supply
without risking damage to your computer. You need a 7 pin female DIN connector,
some hookup wire and a couple of lamps to use as an artificial load for the
supply. If you have a junked C64 board laying around, remove the power connector
from it and wire it to a pair of 12 volt car bulbs. I use #1003 12 volt lamps,
one each for the two outputs: +5V and 9VAC. The lamps each draw under one Amp.
An artificial load is recommended to keep a failing supply from damaging your
computer. Connect it to the computer only after tests show it's working OK.
Run the supply with the lamps connected for about half an hour or until the
case of the supply gets warm. Monitor the 5 volt supply with a DC voltmeter to
see if the voltage is too low or (and here is what to really look out for) too
high! It should measure almost exactly 5 volts. Low voltage (4.7VDC or less) will
cause problems like a blank screen and program crashes with the computer. Less
than 4.9 volts usually means the capacitors in the supply are failing. A failing
supply may still read 5 volts if not properly loaded! On the other hand, if the
voltage starts rising, going up past about 5.2 volts (loaded or not), it means
the regulator chip in the supply is failing. Excessive voltage can and will
damage the computer. I've seen bad ones that would always work when cold, then
fail when they warmed up. The lamp on the 5V line will get very bright if the
regulator shorts because the output voltage will rise to about 11 volts. RAM
chips in the computer are "TTL" devices and seem the most sensitive to
overvoltage. Their "absolute maximum rating" is 6 volts. A leaky regulator may
cause the voltage to rise slowly, so you need to monitor it with the voltmeter
as the supply warms up. Also check the 9VAC, which will either be there or not.
Loss of the 9VAC will produce a blank screen on early C64s even though the red
LED is on and the +5 volts is normal. Because the circuitry inside the later
C64C is different, loss of the 9VAC or an open fuse inside the C64C will still
allow the computer to work! However, you will not have sound, the cassette will
not work, and there will be no TOD clock source for the CIA chips, so some
programs may not work. Loss of the 9 volts AC will not damage the computer.That
voltage will never be high or low... it's there or it isn't.
Although you could use large resistors for the tester, I prefer lamps
because they give me a constant visual indication of what's happening. Both
voltages should be nice and steady. If either lamp flickers, goes out, or goes
up in brightness, time for a new supply. Since the brick is potted with epoxy,
it is considered unrepairable. Resistors can be substituted for the lamps, as
follows: a 5 to 8 ohm, 20 watt resistor for the 5 volt line, and a 10 to 15 ohm
20 watt for the 9VAC line. Keep in mind both lamps and resistors will get hot
after a few minutes. Don't set them on any surface like plastic that could be
damaged by heat, or on metal that could cause a short.
- Code: Select all
9VAC -----7 6----- 9VAC
ground -----3 1----- ground
+5VDC -----5 4----- +5VDC or no connection
2----- ground