Altair 680 power supply...

Bill Sudbrink wh.sudbrink at
Sat Jan 16 19:48:56 CST 2010

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, there was
AC all over the place (where there should have been DC).  I
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
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
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
of sync?  What could be wrong with a transformer that it would
produce something like:

        /  \
        |   \
       /     \_
      /        \
               |   /
               |  /

bent and very squared off at spots?  My scope shows nice clean 120
coming into the transformer.  How do I figure out a part number for
a replacement transformer?


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