Subject: Re: Meltdown (was Re: musical processing)
Reply-To: johnd (John DuBois)
Date: July 12 1990
In article <email@example.com> firstname.lastname@example.org (Matthias Urlichs) writes:
+And then, after two months of uninterrupted operation, the power supply looked
+kind of saggy -- it was starting do melt. Because a lump of plastic with some
+metal inside conducts heat even worse, it started to flow off my desk faster
+Luckily, a cheap-and-dirty power supply for it needed only a 7805, a
+transformer (of course), and two condensators. Those were the days...
+Matthias Urlichs -- email@example.com -- firstname.lastname@example.org
+Humboldtstrasse 7 - 7500 Karlsruhe 1 - FRG -- +49+721+621127(Voice)/621227(PEP)
I imagine discussion of the Commie power supplies has gone around a
few times in this group. My own experience was this...
A year or two after buying my '64, it began to behave strangely. After
running for a while, "waves" of color changes would begin scrolling up the
screen. It continued to operate, but character colors would begin changing
in such a way that it looked like bands moving up the screen. Occasionally
a character would change value. After running for a couple of hours or so
it would die down and stop. I opened it up and looked at the video chip.
One of the pins was bent, so it wasn't in the socket. I removed the chip, bent
the pin back and reinserted it. It didn't help, but I suspected it might have
been damaged by operating without the pin in. I looked for a replacement
chip for a long time before finding one. But, the price was ridiculous
(something like $20). I didn't feel like replacing it when I wasn't *sure*
that was the problem.
I diddled around with the machine for a while before deciding that the
video chip seemed to be running pretty hot. I sprayed some coolant on it.
It had an obvious effect, though sometimes it just forced it from the state
where it had *stopped* misbehaving to the state where it was. I was then
sure that it was the culprit. But, before buying the replacement I felt
the other chips, and they seemed to be pretty hot too. I finally got a clue
and put a DVM across the main 5V supply bus. At first I didn't believe it;
I had to put the DVM on the supply pins of one of the chips before I was sure
I was actually looking at the supply bus. There was NINE VOLTS across it!!!
Not just MOS and such which I'd expect to take it for a while; there was also
ordinary TTL being powered at 9V for hours on end (actually, I had gotten
in the habit of leaving the machine on at night so that it would stay in
the state where it stopped misbehaving). The only symptom was the video
problem! TTL is supposed to be operated no more than a quarter volt above
5V; somehow I would have expected rather more serious problems (like "silicon
haze") when exceeding that margin by 16 times!
Of course, I was pleased that it would be something simple to fix. Hah.
I grabbed the power supply and immediately discovered, as had many others,
that it is glued together; no screws to make it easy to take apart. OK, split
the base off (crrack!) with a screwdriver and hammer. Fat lot of good that did
me; again as everyone else knows it was a potted supply. I found that
replacement power supplies were going for $30. Yuck. It produced (was
*supposed* to produce!) 12VDC, 5VDC, and 9VAC (I think there might also have
been -5VDC in there). I was ready to try putting a new regulator on the output
of the dead one, or if that didn't work build a new supply, but just for fun,
since I had little to lose, I tried putting a big 5V Zener across the power
bus. Since it would heat up, I didn't want it inside the machine; I crimped
Molex connectors to the pins and slid them over the Gnd and +5V pins of one of
the game ports.
Since it had completely lost regulation, I thought that clamping the
output to 5V might fry the transformer, or at least the rectifiers. But no,
it operated just fine! There must have been enough impedance left in the
regulator. With all the extra current flowing through the Zener, the supply
probably got even toastier, but it didn't seem to heat up to the point of being
a fire danger, so I kept it that way. The only real problem was that the Zener
got terribly hot, and I was too lazy to move it, so it stayed right there on
the game port next to the power switch. So all too many times when I reached
over to turn off the machine, I burned my hand. yowch!
About the best thing I can say about Commie machines is that, at least
up through the 64, they included complete schematics and specs in the
Programmer's Reference. If only I had them for my '386 :-(