power supply problem

[>>>marjan<<<]

Replace the 7912 with new one. Also check those 15uF caps (I guess they
are tantals wich are prone to failure). I'd replace all elcos in PSU
(hey how old they are? - they go dry over the years), add some 10-100n
ceramic caps in parallel to elcos and protective diodes (one across
each 7xxx - with cathode to input for 78  and to  output for 79),
also one between output and gnd (cathode to output for 78 and to gnd 
for 79) maybe at input too. Why? Strange things happen during switching
on/off transients. I doubt diodes went bad. Unsolder one end and test
them
with DMM. It's a shame for such a instrument to have such a lame PSU...
If you ask yourself what those 2K7 res do, well some 7xxx brands behave
only with some load applied to the output, and you want to test it
without
rest of the cct connected...


marjan