I like that you keep investing failures of "your" meters. Makes interesting videos and improves probably not this but multiple lines of devices. Keep on going Dave!

Did you notice its doing it in a pattern? It goes: 0 5 0 5 0 5 0 5 pause, repeat. It's extremely consistent. Something in the logic is definitely doing it. Looks like it's timed with the refresh of the LCD too....

5:31 "Light at the right angle" - no no no Dave, it's all about getting the tongue at the right angle 😁

there is an electrician imprisoned in a workplace somewhere and he reprogrammed it for SOS but didnt get it done right...

try the range button. if it is a 5 V issue then the "5" will jump a decimal, if it is a leftmost digit issue then the "5" will stay on the left

Dave don't see enough sellers doing this, levels of honesty in new nevermind older products that you show.

Hey Dave, 3:30, you can see why they discharged 'unevently'. Top is a NiMH while bottom is alkaline!

Looks like it's generating some noise somewhere and it is trying to autoscale to the fast fluctuations in the noise. EDIT: So, it shows the correct voltage and toggles range. Then it can't be noise. I'm probably wrong. Definitely looks like a processing issue now.

At 2:30 My guess is there is a broken capacitor attached to the voltage reference

And this is why Dave is able to sell meters, absolutely no BS .. I have two already; thinking about a 121gw :-)

Notice the 5 cycles on regular timed intervals.

Internal bondwire fracture - maybe on something related to the voltage reference. Making just enough contact to supply a reference, then draws current and the failed junction goes non-linear, add a tiny amount of capacitance and it oscillates?

Please spend some time on finding the fault on the BM235, this subject is interesting.

I have had meters that a voltage reference ic takes a dump after the meter gets hit with high voltage...dose similer things

I bought a 235 directly from Brymen and 1 month after the warranty expired it developed a calibration error that prevent it from measuring any voltages. Shipped it back to Brymen and they re-calibrated it and sent it back - it did take about 90 days to get it back but at least they repaired it.

nice job of mixing batteries!... noticed one is rechargable (the low voltage one)

You must have another scrap unit around that you could swap the processor with just to see if that cures it. it doesn't matter about calibration as you are unlikely to put the unit back in to service. It would just be good to see it through to the end.

It is very regular. The pattern repeats. I guess some cross talk from some counter, on a high bit causing a saturated value.

The white lead on the input connector board looks a little toasted (visible between 9:11 and 9:20)

It's an interesting and very unusual fault, the fact that it is a repeating pattern makes it fascinating ! I would like to see a scope reading to measure the frequency of that pattern. Most of these guesses here show that most people really don't understand what they are seeing and either didn't watch the whole video or are assuming that Dave knows nothing about electronics and that he can't see what we can see in the video !

Always very interesting videos. Thanks Dave for the time you spent for us. I just have a quick question : I saw on your PCB some resistors that is labeled RM in place of R. What is the meaning?

the way the 5 flashes from 5 to 0 is almost like some sort of error code.. like it flashes 4 times then pauses then flashes 4 times and so on. almost like the check engine light on a car

My theory is the LCD driver is damaged. The segments change each time the processor refreshes the display, more often when measuring resistance than when measuring voltage. It looks like some bit of some internal counter (frequency divider or memory address counter) has a short to memory area of first digit's segments.

Ive got a similar lower model to this one, a 210A I think but in the Greenlee brand, and so far its been perfect, love it.

Maybe ESD damage to the micro. Might be worth scoping the meter supply and comparing its current draw against a known good one. If the chip is damaged, its supply current may be higher.

"Let's switch this puppy on", bless, not heard that for too long, brilliant. Thank you for that happy nostalgia thought.

Both fuses good? Voltage spike perhaps trashed the asic driving the display? I had a Uni-T that someone blew the fuses which caused a voltage skip that got past the input protection and took out the main IC. Spot on except it had all the wrong ranges. Replaced the IC and it's been perfect.

Just guessing. It is showing the correct voltage intermittently, so the downstream of the ADC is ok. It is weird how the display is messing up which may show an electrical issue, like voltage leaking on a line, or dropping. It could be some kind of ADC communication breakup which is going to the max value.

It could be a problem with the voltage reference. Or it could be an op-amp with a cracked solder joint or feedback resistor somewhere. Those are the only 2 things that could shoot the reading to the roof and back I would guess.

Hi, I've had a *very similar problem* with APPA305. Reason was, it somehow managed to erase its' own calibration EEPROM. Because I got it free with this error and it had some decent amount of counts, I decided to dig into this. It took me several hours to reverse engineer eeprom data mapping, and in the end, I've mapped all calibration data (I think except of temperature) and recalibrated it.

Is there a voltage reference chip in there somewhere? If Vref is dropping to zero the display might read full scale.

Is it possible its switching in a reference voltage for self calibration and the switching signal is not functioning? Or is there a pin on the ASIC that's pulsing at that frequency? Is there any ghost signal coming out of the input connections?

might be an issue with one of the registers getting an improper clock and/or ref voltage.

Can you measure the input to the logic side to see if something is feeding 5v into it? Although this kind of failure doesn't explain the A/mA weirdness to me. I'm brand new to electronics, green as grass, so I'd love to learn more about this, and if this develops into anything. Will scoping any of the logic test points (if there are any?) help at all? I'm probably talking complete nonsense, but I'd love to see more diagnosis of this.

Just a random thought. But could there be a 5v rail connected to a mux? I assume it's in package package on die so there's not an external chip to replace but might that be doing it?

When I saw this, I wondered if the design has a auto-cal, or sanity check circuit, that’s executed on boot up. I can imagine a built-in circuit, like BIT, that switches the input between ground, 5V, etc. If the control signals for such a circuit went berserk, it might cause these symptoms.

If the measurement is spot on, could it be the communication between the processor and the lcd? if it's an spi or i2c, could the processor be reading the values correctly but experiencing noise when sending that over to the lcd controller? 5 is 0101 in binary btw

Maybe a stupid suggestion, but could it be the most-significant bit (MSB) of their custom ASIC going weird? IDK how they make their ASICs or what each of the pins does, but maybe a bond wire somewhere that's related to the MSB is faulty. Not that I've ever heard of them failing, but it's all I can think of. The randomness makes it look like a floating pin or something.

@eevblog2 I would probably take another meter out of the scrap bin and swap the processor. Not too many pins so shouldn’t take too long I don’t think. Would be interesting to see if it persisted, might be the silicon.

I was going to buy one of these, but I got no reply to my question on the EEVBlog Store, or on the eBay listing. So, I figured that support communication would probably also be nonexistent. So, didn’t end up getting one.

That's a weird fault, I'd be fascinated to see more in depth troubleshooting...

Could the clock frequency on the processor be off enough to stuff up the ADC? Or could the clock be erratic?

My first guess on seeing its behavior, and the fact it happens in any mode, is that it has dual ADCs that it alternates between to get measurements and one of them is not working anymore for some reason.

The meters that get sent back for repairs, do you then sell them on at a reduced price? If so ide be keen on one.

What happened to "always check power supplies"? If the processor is given an out-of-spec supply voltage then it may misbehave.

I had a fault in a scope, where a rubber key was stuck inside and made a permanent contact. Couldn´t be seen from outside. So the reaction on any input was weird. Check all rubber keys for correct operation.

Seems like a internal silicon issue to me. Not sure how the main processor and LCD driver communicate and the format of display data, but it could be just a display data corruption considering it only affects the most significant digit and in a repeatable way.

Shot in the dark here - Is the reference crystal on spec? Can they be fractured internally yet still 'function' ? Will replacing the crystal still require re-calibration?

@Dave if you can, try giving it a re-flash. What's the betting of a random bit flip in the firmware that causes a jump to the wrong part of code or some crap?

Did the broken inductor issue you made a previous video about turn out to be a common issue or was it a one off?

Very nice that you sent him a new one

I've got the same problem with my old school Keihtley 175. Don't have clue what it is yet.

Swap the asic (processor) with a good meter and see if it fixes it. Might be an internal fault in the asic, perhaps even reflashing it, if possible, with new firmware would fix it.

Hello, did you try freezing the processor a little bit to to see if there is any change?

The thing is it doesn't appear to be random. On voltage mode you can see a pattern.

8:10 - The surface of the black component in the lower screen just right of center.

I wonder if that half dead battery is a clue, ie, is something dragging the processor voltage down so it doesn't work correctly.

Try swapping the main chip with one from a good meter?

p.s. could this be made into a how to use your oscillisucope lesson to trace down the fault?

R33 near the lower fuse looked as if it had a hole in it?

Definitely look like exceeded range measurement due to different ASIC chip properties in this particular multimeter or it can be software bug too. I'm curious are this glitch is only BM235 specific or it affect all BM2xx line? I ordered BM257s at yesterday and hope it will not show these funny 5-s.

Swap the asic with another meter, my bet's on a damaged asic... It toggles at a consistent frequency

Do the eeprom chips carry any write data. I had an incident where a character byte was accidentally written to the wrong address on an eeprom and it caused the display to output the wrong character when called. Just something to check.

I'm not sure if' it's affecting the other digits, since you didn't connect up anything to measure, they show as zeroes (with the least significant digit 'bobbling'). Measure, say, 1.234 volts and see if it alternately displays "1.234" and "5.000"

Very interesting issue. Following with interest.

Can you put it under the oscilloscope and see if any of the controller's pis are oscillating?

My other theory is a short circuit on the zebra stripes connecting the display to the board. My meter alternately displays 0.000 and -0.000. Maybe in this Brymen there is a short circuit between the driver lines of the minus symbol and the G segment of the first digit.

How does the reset line of the chipset look?

Not that it matters, but what firmware is in that meter? I noticed the RM1, RM2, RM3, and RM4 positions on the PCB. They look like option jumpers. I wonder what would happen if you played with them. I have two of the BM235 meters and they have different firmware revisions. That is one thing about the 121GW over the 235. Easy update of the built-in firmware.

ADC bias fault? My audio ADCs usually run with 1/2 VE bias.

What happened to: "though shalt always check voltages"? :P seriously though, isn't 5v a standard digital voltage value? Something being polled at regular intervals and it's somehow leaking onto the main sensing line/one of the grounds being weak and unable to sink enough stray voltage, would be interesting to see whether the relevant pin on the micro was actually seeing any stray voltages? One interesting thing to note: the volts reading is dc, and the millivolts is AC, but they both show something that looks like 5 to some exponent, that's important, because the only waveforms that have the same peak to peak and RMS values...are rectangular waves, i.e. some kind of logic and it's quite a regular pattern too

Dual slope integration lost its integrating part (cap)?

The alternating between 0 and 5 does seem to have a bit of a rhythm to it...no idea if that's relevant.

Is there a bit from the ADC that is flickering?

Possibly get a new asic and solder that in. As everyone said it is flickering at regular interval. Atleast putting a new one would eliminate some faulty silicon

Op amp blown or open pull down resistor

What about an unstable crystal o/p? Worth a quick probe ??

Seems the best solution is to replace main chip with another good DMM ic's and do some comparison ...

Maybe give it some crazy vibration and see if anything changes?

Probably a stupid solution, but have you tried connecting it in parallel with another multimeter? Maybe it's working fine but leaking power to the measuring side somehow?

What happens when you switch ranges? Let's say, 50V instead of 5? Will it flash second digit or first?

It could be a both programmatical and hardware error. Because the digit 5 blinks with a constant pattern. (It blinks 4 times then stops for a while) Why don't you replace the MCU to see if it helps?

Dave, check the EEPROM. Read and check its contents several times. I have a 25q series EEPROM with similar weird behaviour, it bit flips at MSB every exactly 16 bytes sent. It was used as a font lookup table memory and a particular character on the display changes, as your multimeter does. I exchanged emails with the manufacturer several times and they refused to believe that their EEPROM was delivered as faulty, they carefully test them, etc. The EEPROM was delivered with factory programmed fonts and I never used it. Just plugged it in my flash reader to check if their fonts are fine and bootloaded the fonts in my code. I had instantly saw the weird glitch, with exact repeatability. I copied its content in a blank EEPROM and it works flawlessly. It's not my circuit design fault, as the manufacturer claimed. Maybe you have similar issue.

replace the chipset. take one from another broken bm235. would love to see you repair it and give it away to someone in need!

If it should measure the reference, the result would obviously be full scale. But I don't know why it would measure the reference unless it alternates between signal and reference using a switch.

I would be nice if you would try to swap chips, just to confirm whether the fault follows the main CPU or the PCB, calibration wouldn't be important for that.

It's a consistent pattern, that's for sure.

I would try to resolder the cpu chip to see if this could fix the problem.

I did it. I bought you meter. It's really nice. Great for the price point! :)

Looks like it is losing its ground. Bad pull up or pull down resistor?

Why are some traces un solder masked? 7:20

The fault produces a repeating fixed pattern not random, internal cpu crosstalk fault??

I'm looking to buy either the BM235 or the 121GW, just not sure which to get. I mostly do some Auto/Home DIY and minor electronics repair, nothing too deep.

At round about minute 10 you can see a hand above the input jack, look like making high 5. I guess this is what the meter want to say. 🖐

posible pul up / pull down R... ???

1.check OP amps input stage. 2. check Ref. Voltage input stage (ref diide). 3. clean the contakts of the rotary switch (oxyde). 4. Glitch in the memory or cpu register?. 5. re-solder pins specially Opamp and mpu.

I've used a Mantis for years and I agree they are the 'Ducks Guts'....RM3 looks almost unsoldered on the left side (11:39) got to be a problem

14:40....If you get a proper reading when an input is connected and an erratic one when nothing is connected....suspect an input ground fault or an input fault of another kind.

Batteries are mixed, rechargeable and ordinary, thats why voltages are different.

Question: I see that some traces on the PCB are without soldermask the whole length. Why is that? What benefit does it provide?