Electronics > Repair
HP 3478A Multimeter - Parts for overvoltage protection circuit
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11mrjones:
Thanks pqass, I'll check it out
Kleinstein:
If the problem is with the 3 K range and not the higher ranges like 300 K, the diode used FET is likely not the problem. Its more a thing of the BJTs Q201-204, like one of them shortet / lost collector connection or too low gain.
Wallace Gasiewicz:
In my 3456, the ranges affected were the ones with the highest volts used for the resistance measurements and the problem was definitely the diode in the over volt circuit.
11mrjones:
Well an interesting development.  I removed Q201-Q205 and replicated the protection circuit on an external breadboard.  Was planning to swap out individual xtors to see what happened.

With a simple multimeter diode mode test all xtor junctions appeared to be normal in circuit when tested the previous day. When a removed them all and tested device junctions out of circuit today, again nothing abnormal. I checked both forward and reverse bias for all junctions and all were ok.

So I then built the protection circuit on an external breadboard (see pic) with all the original xtors and after meter power on everything is working normally,  ie, no shorting clip to bypass the protection  circuit required to get proper readings.

Perhaps as Kleinstein suggested a collector lost connection, cold solder joint?, i definitely dont know.

Should I just resolder all the xtors back in place and call it a day. Or should I run some tests on the various xtors while I  have them out before putting them back. What is advisable?

See attached pic.

Thanks
Victor





Kleinstein:
It could also be one of the 240 K resistors that went too high. With the transistors removes one should be able to check the resistor in circuit now.

While still on the breadboard one could check the voltage drop at the protection with the 1 mA and maybe some 1.5 mA test current.
A check of the transistor gain could also be a good idea. The transistors work as pairs, and swapping them could have avoided using 2 weak ones together.

I would consider butting the transistors back,  and maybe get new ones only when ordering other parts anyway.
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