A TESTER FOR THE C64 POWER SUPPLY      
                  latest updates and/or corrections 7-7-05

     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.
     The pinout for the connector is as follows (taken from a C64 schematic)
shown facing the right side of the computer.

         9VAC -----7           6----- 9VAC

      ground -----3             1----- ground

         +5VDC -----5         4----- +5VDC or no connection
                         2----- ground

Note: although only four pins are actually used, the power connector is a 7 pin
DIN, as shown. Use pin 5 for +5 volts and pin 2 for ground or minus. The 9VAC
output is on pins 6 and 7. 


     Most electronic equipment nowadays is designed around "switching" type power 
supplies which are smaller, lighter, and are more efficient and so produce less 
waste heat in normal operation. The original Commodore 64 "black brick" power 
supply is a simple analog type, that is, it uses a transformer, series regulator 
and filter capacitors. It is inadequate to supply external devices such as RAM 
expansion units. It is internally "potted" with epoxy and therefore is unrepairable 
when it fails. The internal components are cooled by convection through the case... 
not a very efficient way to do that. The internal supply regulator IC runs hot and 
fails, sometimes taking chips in the computer with it due to overvoltage on the 5 
volt line. Anything you can do to keep the PS case cool while the computer is 
running is a good idea. For example, put the supply down near the floor where the 
air is cooler and where air is free to circulate around it. 
     There are actually several ways the original C64 brick can fail. The most 
common is that the computer simply will not turn on... no red LED indication on 
the C64. That means the +5V from the supply is low or missing. Strangely enough, 
sometimes such a failed power supply can be made to work by leaving it connected 
to the computer with the computer turned on, then disconnecting and reconnecting 
the AC plug of the supply. If the computer comes up, you know the supply is on its 
way out. Bad capacitors or an open regulator are possibilities, but since the 
supply cannot easily be opened for repairs, knowing what failed doesn't help much. 
     One other way the supply can fail is by overvoltage. If the supply regulator 
shorts out, that can put almost 12 volts on the 5 volt line. Since most of the chips 
in the computer can't take much over 5.5 volts, they are almost instantly destroyed 
by overvoltage. The problem is, you will not know the supply caused the failure. 
It's a silent killer. RAM chips in the computer seem most sensitive, and if you find 
one or more have failed (blank screen) and the chips are getting hot quickly, 
suspect a failed power supply as the cause. Whatever you do, don't connect a 
suspected bad PS to another computer until you get it tested. Keep in mind a failing 
supply may not produce overvoltage until it gets warmed up, but it can and will 
damage every computer connected to it! 
     If you have only the original black (or white) brick to work with, consider 
using "the computer saver", a protection circuit you can build yourself. It is 
designed to open the 5 volt line to the computer before the voltage goes high 
enough to damage chips. Until that happens, the saver is inactive. The saver can 
be constructed "stand alone" with DIN connectors on each end to be used inline 
between computer and PS, or the circuit can be built into the C64. I prefer the 
latter method because the computer can then be used safely with any compatible 
supply, and you don't need to hunt around for DIN connectors which are getting 
hard to find. The "saver" article is on my home website, and photos of typical 
installations are found along with schematic diagrams of the computer on my other 
     After-market C64 power supplies were designed "beefier" with heavy duty 
regulators and larger internal air-cooled heat sinks. Some supplies are not much 
larger than the CBM brick, and the better ones were made with metal enclosures. 
If the regulator IC is mounted on the metal case, waste heat is radiated directly 
into the air... a much better way to do it. All of those units kept the "analog" 
or "linear" design and so were large and heavy. Building such a unit from scratch 
is not too difficult if you can find a power transformer with two 9 volt secondaries 
that can carry at least 1.5 Amps each. As an alternative, two transformers can be 
used, but of course the final product will be that much bigger and heavier. The more 
power required, the bigger the transformer and regulator needed. Other components 
such as filter capacitors remain the same. Note that a beefier supply means the +5 
volt output can supply more current. The 9VAC has the same 1 Amp spec for all 
versions of the C64 and C128. The C64 supply 5 volt output is rated at 1.5 Amps 
maximum. The supply for the 128 has a 4.5 Amp rating, but because it is a switcher, 
it is not that much larger than the C64 brick. 
     A new Commodore REU (RAM Expansion Unit) came packaged with an upgraded power 
supply to safely run it and the computer. The new supply was a switching type much 
like the C128 power pack which can safely take the heavier load. The REU supply is 
rated at 2.5 Amps for the 5 volt supply and 1 Amp for the 9VAC, and its power 
connector fits the C64. If you buy an REU and it doesn't come with a power supply, 
a C128 supply can be modified to work by changing the connector to fit your C64. 
Even better, a "pigtail" arrangement (leaving the original 128 connector and 
attaching a C64 power connector along side it) would allow either computer to be 
used with the same supply, but not at the same time, of course. 
     Building your own switcher is not recommended. The design of such supplies is 
very complex and demanding of its components, and just finding the correct parts 
would be difficult. Places like DigiKey (www.digikey.com) and Mouser Electronics 
(www.mouser.com) sell ready-made, small, high current +5 volt switcher supply 
"modules" cheaper than you could buy the parts to build one. One of those plus a 
9 volt 1 Amp transformer and a case would get you a better supply than you could 
ever build. That's how I would do it if I were starting from scratch.

Ray Carlsen CET