My Altair 680 has achieved the position of primary attention on
my workbench. All of the ICs are socketed. Not having seen
power in over 20 years, I did not trust the power supply as far
as I could throw a bus. I pulled all of the ICs, attached dummy
loads (automobile bulbs) and powered up. Sure enough,
AC all over the place (where there should have been DC). I
Do you mean AC, or DC with excessive ripple. Did any of the lines go the
wrong side of ground? (my guess is they didn't, but it's best to be sure).
replaced all of the electrolytic caps and that got rid
of the AC
but some of the voltage levels are very wrong, at least as compared
to what is written on the schematics. The worst offender is just
off the "plus" side of the full wave bridge (BR-1). It should be
nine volts but is actually at 11.5. This makes what should be the
I wouldn't worry about that one. It's an unregulated supply, and will
drop on load. Assuming it's the input to a regulator (like that 7805 you
mention), it'll be fine.
main five volt rail a little more than 9 volts... the
far side of
VR-1 (a 7805). The wave form coming off the transformer is really
Now that does worry me, for obvious reasons. You need to correct that
before you put the ICs back (but you knew that ;-)).
There several reasons why this voltage could be high
1) The ground connection to the 7805 'common' pin is open. The 7805 treis
to get the output pin 5V _above that common pin_, so if the latter is not
ground, the output will be wrong wrt groud too. A few manufactuers used
the mouting screw of the TO220 package as the common conneciton and
didn't connect the middle pin anywhere. Others used the heatsink of a TO3
packaged regulator as part of the common connection (I've seen a version
of the latter in an HP instrument!). Check the ground connection, if it's
made through the mounting screws, check they're clean and tight.
2) THe 7805 is defective. They do fail, alas, which is why you chack the
PCB on a dummy load :-))
3) There is resistor in parallel with the regulator. Some manufacturers
connected a high-wattage resistor between the input and output pins of
the regulator, of a suitable value to carry most of the load current at
the expected voltage drop, thus leaving the reguator to carry (and
control) a much smaller current. This is an evil trick because the output
voltage will rise (and the rewgualtor can't do anything about it) if the
load is too small. Again, I've come across this in an HP device, it was
fitted with a crowbar circuit, and I wondered why it tripped with the
supply on dummy load. The reason was my load was not large enough. Do you
have scehamtics? If so, that'll show if the resistor is present or not.
ugly not a smooth sine wave. I hate power supplies.
I don't really
understand, looking at the schematics, what this transformer should
be doing. My guess is that it should be making a nice nine volt,
60 cycle AC sine wave with each of the two outputs 90 degrees out
No, assuming it's a centre=-tapped winding, the ends will be 180 degrees
out of phase.
With no load on the transforemr, the outputs should be approximately
sinusoidal. With the sort of load you're giving it (rectifiers and
smoothing capacitors), the waveform may look horrible.
Provided the transofrmer is not getting exessively hot, I'd not worry
about that too much.