EEVBlog Multimeter (BM235) Repair - Page 1
Forum section: electronics / repair

author: brendan-petrol
author_rank: Contributor
timestamp: December 27, 2021, 10:39:34 am
content: Hey all!Would anyone, by any chance, happen to have a schematic / layout / documentation on the BM235? My multimeter appears to have developed a fault, and before I embark on a full blown fault finding mission, I was hoping to have any data I can have in front of me.The fault I am seeing is only on the Ohms function, when I switch to ohms (Auto range) my meter is measuring 0.08Ω. I cycle through the manual ranges and all of them are some riff on this value...600Ω Range0.08Ω6kΩ Range0.008kΩ60kΩ Range00.08Ω600kΩ Range000.9kΩ6MΩ Range0.008MΩ60MΩ Range00.08MΩWhat I've done so far:I opened the meter up and didn't see anything obvious (and I've used this meter successfully to measure resistances before, I think?)No popped componentsNo sign of heat damage on the PCBNo obvious solder whiskers, short circuits, missing or tombstoned componentsI've measured the open circuit voltages on the terminals for each of these ranges (with my Fluke 179), they are all around 1.6V as per the datasheetThe continuity tester mode works - the buzzer only sounds when there is an actual short across the terminals, but the display shows the resistance as above.With how consistent the readings are, I'm suspecting it's probably the ADC on the controller rather than hardware, since it's all ranges consistently.I can't imagine I'd have connected my meter across high voltages to damage the meter, unless it's been damaged by a transient from an inductor or something....Am I fighting a losing battle, or can anyone think of something I can test or measure....?Cheers - thanks for reading!

author: joeqsmith
author_rank: Super Contributor
timestamp: December 27, 2021, 12:43:13 pm
content: >The fault I am seeing is only on the Ohms function,Are you sure?There was that area Dave shows around the one connector that may have damaged one or more components. My particular meter has a notch in the plastic for clearance.If it is just the ohms you should be able to measure the open circuit voltage in the lower range as well as drive the input to determine which leg has the problem. There's not a lot going on and shouldn't take too much effort to trace out the front end.Of course, if the meter is new you may want to just try and return it.

author: brendan-petrol
author_rank: Contributor
timestamp: December 28, 2021, 02:32:47 am
content: Quote from: joeqsmith on December 27, 2021, 12:43:13 pmActually, honestly I hadn't confirmed it experimentally, simply from observation. I did use it to measure voltages after I identified the ohmmeter fault and it returned values in the ranges I was expecting. So I just assumed the rest of the meter was working. This was very bad form and thank you for the reminder to approach this in an ordered fashion!I did a couple of quick tests a moment ago to check the other functions.Tested the mV range by holding the probes in my left and right hands. I read approximately 40 Vdc between the two, then shorted out the leads and read 0 Vdc. I think this is a sufficient test of the mV range?Using my K-type thermocouple, I got an ambient air temperature of 20°C which seems consistent with my internal gauge for temperature. Holding the bead between my fingers caused the temperature to quickly rise to 32°C with the heat from my fingers.Taking a AA battery, I measured 1.6 Vdc on the 6 Vdc range (I don't have a good lab setup so I don't have any fancy test equipment to do a more rigorous test.... yet....). I also measured 1.6 Vdc on the 60Vdc and 600 Vdc range (although the 600 Vdc range took a while to measure it, almost as if measuring 1.6 volts on the 600 Vdc range isn't ideal)Using a mains outlet, I measured 250.0 Vac (quite a satisfying number for my OCD, actually). This is consistent with being in Australia - and I know that our voltage is typically quite high, our service was tapped high as we are a significant distance from the transformer and we were noticing some voltage drops during high load periods.Quote from: joeqsmith on December 27, 2021, 12:43:13 pmI did see that video in my research, however I didn't see any damage to L3 in my initial inspections. In actual fact, I didn't see any obvious damaged components or cold / failed joints. However, I didn't desolder the PCB that the banana jacks are mounted on yet as I was hoping to find a schematic or layout first before I get into desoldering things (starting off with the least destructive methods / easily reassemble-able methods) so I couldn't do a very good visual check of the components on the underside of that PCB and the SMD components on the main board under that extension board were difficult to sight properly too.I did also meter the input resistors and continuity test the input MOVs as Dave mentioned in the video and got the right resistances, MOVs were showing open loop.Quote from: joeqsmith on December 27, 2021, 12:43:13 pmWhen you say measure the open circuit voltage in the lower range, I'm assuming you mean something different to simply the open circuit voltage on the banana jacks when in ohms range (as I did mention checking this and that it was 1.6 Vdc as expected)? I'm not quite understanding what you're suggesting (although I am very keen to - my electronics repair skills are only beginner level!). With it just being on the resistance measurement (especially since continuity mode still works) I'm not sure it's going to be something on the analogue front end....?Quote from: joeqsmith on December 27, 2021, 12:43:13 pmUnfortunately it's not, it's been the workhorse for my electronics projects for a number of years now - so I'd love to get it back up and running even just for sentimental value alone (plus a replacement - looking at you, Mr 121GW- is currently outside my budget. My Fluke 179 is a work meter which I can use for now while I'm on Christmas break, but I think I'd get in trouble if I housed it at home permanently!). However, with how much of a weird issue it is, I'm not liking my chances of it being fixable / within my abilities. Also, trying to run the meter while having it disassembled looks like it will be a challenge, since the top PCB needs to be desoldered to access the components. I might be able to replace the standoffs with wires and the pin headers with DuPont wires perhaps, then I could run the meter measuring resistance while disassembled and under test.I'm not sure how I would power it up while disassembled - I'm not too keen on soldering wires to the pads that the battery compartment springs make contact with, I don't want to have to go fixing the pads every time the solder develops an oxide layer.... I suppose I could try using alligator clips grabbing the pads from the side of the board with a sheet of plastic on the other side stopping the clips from shorting anything else out, but I'm not sure the pads are big enough....As I was writing this, I looked at the pictures I took of the PCB while I had the case off and realized there was a plated through hole next to each pad, probably for just this exercise...Here I was imagining jury rigging a power source with alligator clips and a bit of plastic underneath to stop it from shorting, and here was a solution all along...Although I do like what I've started doing on my projects - I've been adding a square pad near the edge of the PCB for each voltage line. That way if I want to quickly power the board up on the bench, I can just clip onto it with alligator clips that have one side covered with heatshrink. It looks ugly as, I haven't found a way yet to make it look anything but ugly, but it isveryhandy for quickly powering up a device under test!Thank you kindly for your reply, and I hope you're having a lovely holiday (if you celebrate any at this time of year) or just a lovely day!