Hi,
my 3325A is driving me crazy and no one could help yet, so I went over to a faulty HP3455A multimeter. It worked many years, but now it comes up with a OL display in DC.
Cal test stops at 11,10,9,8,5,4,3,2,1,0 and a digital value 0.062.
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All voltages are stable with nearly zero ripple. Vref is 10V, all perfect.
The good service manual (no extrem testing procedure with a signature analyzer as in the 3325A manual)? has many tables with listings of voltages for the individual cal number. The cal feature switches many fets to ground, vref etc.
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I found suspect voltages at the outputs of U6 a LM339 and replaced it, this fixed some cal errors. The meter now has only cal error #10. Checking the flowchart for #10 and the voltage matrix a found a suspect output on Pin 14 of U5, so I replaced it with no success. The voltage is 8.8V, that seems to be the Gate Bias voltage after 100KOhms, i lifted the pin and supplied shortly -24v, but this was not suceccessful either.
To pin 14 there is Q21 connected, i suppose it does not switch off, Pin 14 should be -24V.?
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Next I will? replace Q19, the part number is 5070-7074, but I can't find any information and it is not listed in the HP crossreference list. I has 4 pins, may be one pin is can and gate is tied to taht pin as i can measure the gate bias voltage at the can.?
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Has anyone a spare part or a hint which fet i can use to replace the Q19?
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Ralf
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