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Electronics => Repair => Topic started by: truser on September 30, 2019, 01:17:05 pm

Title: multimeter hp 3478a not stable \ chatter \ jumping zero values
Post by: truser on September 30, 2019, 01:17:05 pm

All input connected together.
There is no zero and the values are jumping.
Power on a 5 hours, the result is the same.
 What problems ?
Where to dig?
Thank's
https://youtu.be/SEVjhzueS90

Title: Re: multimeter hp 3478a not stable \ chatter \ jumping zero values
Post by: Ringmodulator on September 30, 2019, 02:10:55 pm

Hi,

check the contacts between the 4mm front panel probe sockets and the cables inside the unit.

I had quite high Ohm display with the input shorted out for 2 wire measurement.
Cleaning and reseating the spade contacts solved this.

Regards
Chris

Title: Re: multimeter hp 3478a not stable \ chatter \ jumping zero values
Post by: truser on November 12, 2019, 05:43:23 am

Thanks, check it out.
But I do not think that this is due to my problem.

Title: Re: multimeter hp 3478a not stable \ chatter \ jumping zero values
Post by: Kleinstein on November 12, 2019, 08:08:46 am

The ohms reading problem could very well be a contact problem, especially if measuring 2 wire ohms. If 4 wire Ohms, it depends on how the cables are connected.

For the mV readings, there could be some thermal EMF from holding the DMM in the hand. The 3478 is a bench top meter, not a hand held. I would not expect that much trouble, but for best stability it should sit on a bench / board and not moved around.

The first points to check would be the supply and the line frequency setting (50/60 Hz).  Also check the integration time - the preferred setting for manual reading is something like 10 PLC or more.  Another point would than be looking at the DC zero reading in the 30 mV / 300 mV and 3 V range. This should tell if the problem is more with the ADC or the input amplifier.