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HP 85662A Display: Power supply problems

Question asked by wpaperman on Apr 15, 2010
Latest reply on Mar 10, 2012 by biff44
Anyone on the list have experience with the power supply in this unit?

I'm having some real difficulties troubleshooting this unit. All the supplies are present (+15,-15, HV, etc.) but all show more than 5 volts of ripple at the test points, the same as that appearing across the filter capacitors..
They will not regulate!  When I bring the unit up on a variac the voltages rise to their nominal values at an input voltage of 70 VAC. When I increase the input the voltages keep climbing and exceed their rated values. Above 90 VAC the over voltage protector, as it should, shuts down the supplies,which are now above their rated values (i.e. +15 volts rises to 19 volts).
There was quite  bit of power supply damage when I received the unit, primarily due to a leaking filter cap on the 140 volt supply. I believe all that was corrected, to the point of needed to "grind out" part of the PWB where the cap had leaked.
I have replaced several of the IC's on the the A6 and A7 cards. This restored the missing +5.2 volt line.
The unregulated 6 volt line barely makes 2 volts. I cannot bring the unregulated 9 volt lineup to 9 volts without the over voltage SCR tripping. The secondary voltages of the power transformer seem to be within ratings.
I do have "the other half"(B) of the system. It appears to be OK as all voltages and clock seem to be functional. I cannot fully test the "B"  without the "A" connected.
Any HELP would be appreciated. I would really like to add this unit to my lab.
Dave, W5WP  

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