More Faulty BM786 - Page 1
Forum section: electronics / testgear

author: EEVblog
author_rank: Unknown
timestamp: May 12, 2023, 04:32:22 am
content: Two faulty BM786's with what looks like the same fault. Intermittent power on problem, and it's NOT the switch.And they are 1yr apart in manufacture.EDITED VERSION:Full live show

author: floobydust
author_rank: Super Contributor
timestamp: May 12, 2023, 05:18:44 am
content: I have seen crystal oscillators not start up reliably with issues on the IC (transconductance) or crystal or tuning caps being off. Try touch a pin to poke that circuit. It can explain the wonky power consumption if the gate is floating in linear mode. Or is it an RC oscillator now, the old 786 has no crystal populated (by the DMM IC) in your video and I forgot if you took it off.Handling the meter does warm it up which might be why leaving it alone (cools off) makes a difference.

author: gabiz_ro
author_rank: Regular Contributor
timestamp: May 13, 2023, 06:53:38 am
content: There seems to be a little delay when switch off until current drop to 0,so some part of soft on-off seems to work,at least for off part.Most probably is something on power switch taht provide not enough power to mcu or even mcu hang on boot sequenceMaybe power switched by rotary isn't all that is required to power on.I don't own this model so I can't test.With no batteries switch rotary to some position and insert battery,is it powering on?If in sleep mode switching rotary to another position wake it from sleep?There may be some tracks on rotary switch that send some pulse during movement to mcu that may be required to reset sleep timer counter but also maybe required at power up.Can't remember from older video but I think programmer cable powered multimeter even it was in off position,so simple check just by connecting cable to PC.

author: EEVblog
author_rank: Unknown
timestamp: May 13, 2023, 07:07:59 am
content: NOTE: I've now posted an edited shorter version of this on the 2nd channel, linked above.

author: EEVblog
author_rank: Unknown
timestamp: May 13, 2023, 07:24:01 am
content: CORRECTION: Serial numbers are only 1 week apart. Fist two digits are year.

author: PA0PBZ
author_rank: Super Contributor
timestamp: May 13, 2023, 09:14:34 am
content: Quote from: EEVblog on May 13, 2023, 07:24:01 amThat's what I wanted to comment when I watched the recording of the live session, unfortunately your timing is not compatible with my day/night cycle

author: Muttley Snickers
author_rank: Unknown
timestamp: May 13, 2023, 10:06:37 am
content: I don't have this meter and only watched the short video but wondered if you could wake up the display by pressing any of the buttons, or if a button or buttons were making intermittent contact during power cycles?I have a Fluke 83 which prior to repair would do weird stuff at random due to the buttons making contact on power-up, once booted it wouldn't switch to continuity mode which indicated a stuck button, it works a treat now.

author: EEVblog
author_rank: Unknown
timestamp: May 13, 2023, 10:35:30 am
content: Quote from: Muttley Snickers on May 13, 2023, 10:06:37 amNope.

author: EEVblog
author_rank: Unknown
timestamp: May 13, 2023, 10:37:08 am
content: The faulty unit powers on fine using the external programmer and programs fine. Reprogramming makes no difference.So it seems the programmer is bypassing the power-on or reset circuit or whatever the problem is.

author: gabiz_ro
author_rank: Regular Contributor
timestamp: May 13, 2023, 11:00:19 am
content: Cable may be some serial connection,looks like few pins on connector.May Try some powerbank USB connection not PCThen only power come from cable no reset or other signals.It may be not reset problem but power problem instead.If some mosfet not fully switch then Vcc may be under some level so brown out reset may be detected