[SOLVED] Fluke 8920A only mV LED and random numbers - Page 1
Forum section: electronics / repair

author: Datman
author_rank: Regular Contributor
timestamp: October 24, 2024, 09:21:29 pm
content: Hi allI recently bought a Fluke 8920A true RMS multimeter. In the first days it was working, but now it doesn't measure anything. In Auto, with nothing or 1Veff @1kHz at the input I see 0.00 - 0.17 - 0.48 - 1.72 - 2.23 - 4.09 - 5.14mV (millivolt!) or similar values.At a first time, I noticed something strange in manual mode... Now I don't remember exactly (in the meantime my father died...), but scales were wrong in manual ranges. So I carefully opened it and touched the resistor and the jumper on the socket near the backside, after touching the screen to discharge possible static electricity. It seemed to change something in readings, but only a few...I removed the board (with power off) and replaced it, hoping for false contacts, but after some tests it stopped working.Now, between TP201 and TP202 I read only a few millivolt instead of 500mV. +5V, +15V and -15V are ok.Voltage selectors have been ok since the first power on (I switched them from 110Vac to 240Vac).Looking at the manual, I have done tests from Step No.1...5 (page 52) to 17, 33, 45 reading wrong values.Can you help me?P.S.: Which is the "ground"?... The gnd of the 7805 is different from the PCB metal shield! Why???...ThanksGianluca

author: Datman
author_rank: Regular Contributor
timestamp: October 25, 2024, 02:28:18 pm
content: Between the Gnd of the 7805 and the metal shield I read 29,4kOhms!

author: Datman
author_rank: Regular Contributor
timestamp: October 25, 2024, 09:05:46 pm
content: *** It works!!! ***Test here, test there, measure resistance here, measure there, turn on, turn off, ..., ..., it works, but I don't why!Oh... In manual mode I suspect it was out of scale (flashing decimal point).Gianluca

author: Datman
author_rank: Regular Contributor
timestamp: October 25, 2024, 09:33:30 pm
content: Note:I had desoldered the ground of C53 to follow a track... Maybe the ground upside was not making contact to the ground downside? Now the resistance between the two grounds is near 0 Ohms!

author: daisizhou
author_rank: Super Contributor
timestamp: October 26, 2024, 02:30:28 am
content: Please check the connection below, there is a schematic diagram, you should check the power supply voltage part first, it is best to shoot a video if you describe it accuratelyhttps://www.eevblog.com/forum/repair/please-help-how-can-i-adjust-this-transistor/msg5478322/#msg5478322Usually you need to connect a 50 ohm short-circuit load to the input interface (BNC socket),At this time, the screen will show 0.00, which is normal.If it is not the above phenomenon, can you take a video to show your fault phenomenon, because it will be clearer.

author: Datman
author_rank: Regular Contributor
timestamp: October 26, 2024, 08:36:19 am
content: Thank you, Daisizhou.I solved the problem. 🙂

author: daisizhou
author_rank: Super Contributor
timestamp: October 26, 2024, 08:48:22 am
content: So lucky。My 8920a also encountered humid weather, and the screen display would change irregularly. Finally, I successfully fixed it.