Electronics > Repair
HP 3478A Multimeter - Parts for overvoltage protection circuit
11mrjones:
On initial checks when first looking at this the resistors measured per the schematic in circuit and also the same now with transistors removed.
Regarding swapping transistors and possibly avoiding using two weak ones as a pair, I rebuilt the external circuit exactly as it was in the meter in terms of transistor placement. That is, Q201-in-circuit => Q201-external-board., Q202-in-circuit => Q202-external-board, etc. (The resistors were not removed, so the external board has new resistors from inventory parts.)
I will see about measuring voltage drop and transistor gain .
Thanks,
V.
11mrjones:
Measured Hfe was 81, 79, 94, 90 for Q201, Q202, Q203, Q204, respectively (at Ic=10mA, Vce=10v). Spec is 30-300.
Also measured voltage levels of protection circuit with a 3k resistor load. See attached pic. Two voltage levels are given corresponding to 1.0 and 1.5 mA test currents. For example at test current 1.0 mA Ve(Q201)=5.3v and at 1.5 mA test current Ve(Q201)=7.0v.
Thanks
Victor
Kleinstein:
The measurements look good and even with the higher 1.5 mA current there is not much voltage lost at the protection. Chances are the defect before was something like a missing link or similar that no longer there. Even if the error would happen again, it would not be a damaging defect, but only wrong readings. Chances are one would want to have a reasonable good 3 K resistor at hand for an occasional check anyway - with the old parts as well as with new ones.
11mrjones:
Well thanks for all your advice, looks like I'll put things back in place and include some replacement parts in any next digikey order for backup.
Thanks everyone else also.
V.
11mrjones:
Resoldered all transistors back into meter and all works proper.
Bad joint somewhere initially? I don't know.
Is the lesson here to reflow all the joints as first step even if the look good?
Anyway , success, job done
Thanks all
V.
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