Is this 3478A defective? - Page 1
Forum section: electronics / testgear

author: Miti
author_rank: Super Contributor
timestamp: September 27, 2025, 09:35:32 pm
content: Hi,Can I safely assume that this 3478A is defective, or at least leaky, most likely in U102? I have another one that doesn't cycle through ranges like this does.This one keeps switching between 3V, which is high impedance, it charges from a leak somewhere, then switches to 30V, which has a 10M Ohm impedance, input discharges and the cycle repeats. Please see the video.Cheers,Miti

author: TERRA Operative
author_rank: Super Contributor
timestamp: September 28, 2025, 05:10:56 am
content: As a shot on the dark, have you tried cleaning the PCB etc in case there's some contamination?If it's a problem with the U102, good luck. The easiest way to find one is to just buy another meter, in which case fixing the first one is then somewhat redundant...

author: AVGresponding
author_rank: Super Contributor
timestamp: September 28, 2025, 08:29:55 am
content: Yeah that's what I'd look at first. Wouldn't take much, like from some capacitor electrolyte or even sweaty fingerprint residue, with the several GΩ of impedance on the 3V range

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2025, 09:14:30 am
content: If set to high impedance mode and voltage it is normal to have they kind of cycling. The input bias current in the 10 pA range will charge up the input capacitance and once the 3.x V are exceesed it will switch to the 30 V range to dischare via the 10 M impedance.The input current depends on the voltage, but it is rare that the input current reaches zero at some voltage in the 3 V range. So not having this cycle is more like the exception due to low bias or low input resistance.One could to a check. Already the speed of the charging cycle gives a hint on the input bias current - the slower the better.

author: Miti
author_rank: Super Contributor
timestamp: September 28, 2025, 11:50:10 am
content: Quote from: TERRA Operative on September 28, 2025, 05:10:56 amQuote from: AVGresponding on September 28, 2025, 08:29:55 amNo, I haven't done anything on this meter, other than using it as a lab rat for the OLED display and FRAM module but it was like that from the beginning, when I bought it for $75 CAD.It seems to work fine otherwise.Quote from: Kleinstein on September 28, 2025, 09:14:30 amSo where is that bias current coming from with the input open, does it leak out of the meter input? In this case it means that the other one is exceptionally good. It just goes up to about 0.9V and stays there.I willtryto measure if there is a leak current coming out of the meter, not sure if Fluke 8845A will pick anything.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 28, 2025, 02:36:32 pm
content: The input current is a sum of different sources: leakage of protection diodes, switch leakage, input current of the main amplifier, charge pumping from the AZ mode (from the switches and DA in the amplifier input capacitance in AZ mode), surface leakage.To measure the current one can use the meter itself: compare the zero reading with a short and 10 M mode. Alternative one can add some 10 nF of low loss capacitor (e.g. PP or PS) and look at the voltage drift rate.Stabilizing at 0.9 V could be from a rather low input current or from quite some dirt to get more than normal resistance to ground. So it could be a good or bad meter.