Hi all, I recently picked up a job lot of equipment that included an HP 419A Null Voltmeter. So I decided to have a look at it today. Pretty clean, old style mins inlet and a broken fuseholder are obvious issues. I opened it up and its nice and clean inside but the Ni-Cad packs (4 x 6V DEAC coil cell packs) are dead. I pull them and connect it up to a bench supply. First issue is it won't zero. Off-set reading, zero pot has no effect. I check the chopper waveform and it's OK and I can see the neon glow around the leads of the LDRs. Other issue is the battery test won't work, it's all over the place, mostly over-reading. This should be simple, 1 meter, 1 fixed resistor, one preset and two switch contacts all in series across the +_ 13V supply. Switch contacts are OK, preset appears OK. Fixed resistor has odd readings. Disonnect supply and then notice slight negative reading in batt check, 0 in "off". Short _+ rails to ground, same effect. Measurement shows 200mV across the 390K resistor???? Pull the PCB and all is revealed, corrosion in the edge connector. The battery electrolyte had got into the socket somehow. Several cycles of cleaning with DI water, then methanol then drying with hot air seems to have cured the PCB.Oddly for HP the PCB is not gold flashed, not even the connector fingers. Now I've just got to dig out a 22 was edge connector.
Oe to watch out for! Robert G8RPI. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
