Brymen BM786 power on failure - Page 1
Forum section: electronics / repair

author: Mat219
author_rank: Contributor
timestamp: January 29, 2023, 10:20:29 pm
content: Hi.My BM786(EEVblog edition) has stopped powering on.I've contacted Welectron where I bought the meter, and they sent me a schematic of the part of the circuit that might be causing the problem.There's one component that could be causing the problem (I think it's a blown transistor, which provides a 3.5V rail) and I can't find the datasheet for it.Welectron has stopped responding to me after I've written back with the problems I found (to provide evidence and ask if I should send it in).The component is on the Q18 and is marked B039K.Does anyone know what this component is?EDIT: I'm aparently blind and I didn't notice that the transistor type is written right below the Q18 in the schematic. DOH!And should I just bug welectron until they give me a refund? (the failure happened randomly on turning on, after 3 months of use, so it can't be my fault, and I should still be in the warranty period) or should I try to fix it myself?Thank you.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 30, 2023, 12:13:40 am
content: if you have the schematic, share it here it would greatly help ?Is it on it ?

author: Mat219
author_rank: Contributor
timestamp: January 30, 2023, 12:17:30 am
content: Quote from: coromonadalix on January 30, 2023, 12:13:40 amI can share it, but it's only a small part of the power circuit, so I don't know if it will help much.

author: floobydust
author_rank: Super Contributor
timestamp: January 30, 2023, 12:34:01 am
content: It is a mosfet, part of the power on soft switch. Dave poked around it in his video around 17:42 but I can't read the SMD markings.https://www.eevblog.com/forum/blog/eevblog-1520-troubleshooting-a-faulty-bm786-multimeter/Q18 P-ch B039K (smaller than SOT-23) SM2403Q19 might be 603G N-ch somethingConsiderAO3401, orAO7401etc.

author: Mat219
author_rank: Contributor
timestamp: January 30, 2023, 12:47:09 am
content: How is it possible that a 3.7A continuous and 15A pulsed current mosfet fails in a multimeter which has a consumption in the 10mA range?

author: bidrohini
author_rank: Regular Contributor
timestamp: January 30, 2023, 12:57:27 am
content: I am just wondering how this company is providing the customer schematic to help themselves, ruther than providing replair and replacement service!

author: Mat219
author_rank: Contributor
timestamp: January 30, 2023, 01:06:03 am
content: Quote from: bidrohini on January 30, 2023, 12:57:27 amIt's possible they would replace it, they just stopped responding after I answered them after my investigation.I might just have to open a new issue.

author: floobydust
author_rank: Super Contributor
timestamp: January 30, 2023, 02:27:58 am
content: Quote from: Mat219 on January 30, 2023, 12:47:09 amThe problem can be the P-ch mosfet Q18 having a lazy VGS(th)so with say 3.4V it's not on enough. You could measure its voltages (multimeter loading is an issue). Or see if temperature is aggravating things, warm it up or toss it in the fridge.Q19 is typically a NPN but Dave did find a diode junction across it. It has a base resistor - but appears to be a N-ch mosfet?What are your expectations for Welectron? I think they've done a lot giving a possible fix and schematic snippet. They don't have time to give personalized repair advice.If it's still in warranty, I would send it out unless you can troubleshoot and ID the problem for sure.

author: Mat219
author_rank: Contributor
timestamp: January 30, 2023, 11:53:12 am
content: I just wanted them to tell me, If I could send it back and how.I've never returned a product internationally, and I don't know if their "withdrawal form" is the right thing to use.

author: Antrus
author_rank: Contributor
timestamp: October 26, 2023, 08:19:14 am
content: hello, can you share eeprom 24cs04? It's on a board with buttons.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: October 26, 2023, 06:12:13 pm
content: Isn't C65 cracked (shorted )?

author: KissAnalog.com
author_rank: Newbie
timestamp: November 17, 2023, 03:54:36 am
content: A 10 uF at both the input and output of that regulator seems interesting. I'd like to see more of the schematic to see where that 1 Mohm resistor goes to. It is a large value to have from Gate to Source (I assume to keep it off until the Gate gets pulled low). There could be some voltage ringing with that large value resistor and that 10 uF tantalum. Also there is a zero ohm resistor that seems to have been put there to possibly dampen this voltage ringing - maybe. I'd consider dropping the 10uF to 1 uF and lowering the 1 Meg resistor to 100 k or even lower. But then I'm guessing as I can't see enough of the circuit...