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PS problem

Discussion in 'Electronic Repair' started by exray, Apr 18, 2004.

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  1. exray

    exray Guest

    Am still trying to get my scope (HP 1222A) back running and this power
    supply has me baffled.
    Basically I'm getting -2 volts out of the -12 supply.
    Larger at

    All of the components shown check as good with the exception of U4 which
    I cannot verify. I did clip and lift pin 4 and the defective -12 line
    rose to -21-ish so it seems like it may be doing its job.

    The manual indicates R23 to be a current sensing resistor.

    The note on the schematic indicates that I completely removed the load
    but I'm not so sure of that. Its a double sided board and I cannot get
    to the underside without complete disassembly of the scope. I did
    however poke around all the other circuitry on the same board looking
    for any anamolies that may be pulling too much (or no) current and came
    up empty.

    Any guesses? Could U4 be the culprit? I would simply change it out as
    trial-and-error if it weren't for the inaccessibility problem. It
    appears to be an LM301AN type. Agree?

    Or am I still looking at some load irregularity?

    Not much else left to check!

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