Digital Multimeters (Also Brymen BM869 repair)

[retiredcaps]

Re: Digital Multimeters

still dont know the voltage but i thought it could handle over 1kv without exploding.

What were you trying to measure? A step up transformer from what device/equipment?

[retiredcaps]

Re: Digital Multimeters

BTW, I started a thread on eevblog about this meter. There is lots of interest in Brymen meters there because Dave's current eevblog branded multimeter is a Brymen BM235.

The BM869 is also popular among some members there.

http://www.eevblog.com/forum/repair/...in-real-world/

[jasonbay13]

Re: Digital Multimeters

i did measure the values of the parts i took off jff, referencing pic 36, the bottom left resistor measures 1k, the others broke taking them off. both ptcs are open. movs are 200ohm, open, and the big one is 1k. the r76 is just some residue on it, tests ~12.7Mohm and reads 156. same as r75. cleaning up and pics now.

I have seen some abused meters, but this one is in the top 5.


If you are willing to put in 1 hour work, I suggest removing all 4 power resistors (green), 2 PTCs and 3 MOVs. No need to measure or test the components. They are all likely bad just by looking at the pictures.

After removing all these components, scrub the entire area with 91% IPA using a clean toothbrush. Scrub until all the carbon residue is gone.

After it has been cleaned and dried, please post pictures so we can assess the damage.

If the damage looks minimal, we can, temporarily, put in regular 1k ohm resistors for the 4 power resistors and 2 PTCs. For testing, we do not have to put in any MOVs.

So with 6 temporarily 1k ohm resistors, we can power up and see if volts and ohms work. If it works, then we can order proper replacements.

PS. Post a picture of R76 and measure it in-circuit.

[jasonbay13]

Re: Digital Multimeters

pics

[retiredcaps]

Re: Digital Multimeters

Except for that one resistor area, the rest of the pcb looks pretty good.

I would take a bit more effort to clean off that carbon residue. Here are two case studies by modemhead where that carbon can be conductive.

http://mrmodemhead.com/blog/ideal-carbon-footprint/

http://mrmodemhead.com/blog/yokogawa...03-multimeter/

After cleaning that area a bit more, I suggest soldering in the 1k ohm resistors as discussed and trying some low voltage measurements like measuring a 1.5 AA cell. Then try measuring a 1k ohm resistor.

[jasonbay13]

Re: Digital Multimeters

I cleared as much of the black away as i could without digging too deep. i soldered in those 6 1k resistors and upon power up in volts and ohms it correctly shows 0 volts and 0 ohms, then with a battery showed 1.4v and i had a 40k resistor that showed within spec of the resistor. even gave capacitance and diode a test. didnt try ac measurements as i have no readily available ac source besides the 120v line. dont wanna blow it up a second time.....

is it now time to order correct replacement parts?

Except for that one resistor area, the rest of the pcb looks pretty good.

I would take a bit more effort to clean off that carbon residue. Here are two case studies by modemhead where that carbon can be conductive.

http://mrmodemhead.com/blog/ideal-carbon-footprint/

http://mrmodemhead.com/blog/yokogawa...03-multimeter/

After cleaning that area a bit more, I suggest soldering in the 1k ohm resistors as discussed and trying some low voltage measurements like measuring a 1.5 AA cell. Then try measuring a 1k ohm resistor.

[retiredcaps]

Re: Digital Multimeters

I cleared as much of the black away as i could without digging too deep.

Can you post a picture of the updated pcb?

i soldered in those 6 1k resistors and upon power up in volts and ohms it correctly shows 0 volts and 0 ohms, then with a battery showed 1.4v and i had a 40k resistor that showed within spec of the resistor.

Does the 1.4V reading match another DMM?

even gave capacitance and diode a test.

So it sounds like you lucked out and the input protection components protected the main proprietary IC.

didnt try ac measurements as i have no readily available ac source besides the 120v line. dont wanna blow it up a second time.....

If all the other functions work, AC likely works.

is it now time to order correct replacement parts?

I would say yes. We need the help of the eevblog community to find out the values of the two PTCs. I'm not sure if the PTCs are connected serially or not?

We also need the markings from the 3 MOVs in order to get the proper replacements.

For the power resistors, we know they are 1k ohm, but I'm unsure what size and type (flameproof, fusible, metal, etc?). I'm guessing 2W from the size, but they could be 1W.

I'll post a question on the eevblog forum asking for the above values.

Now there is some concern given the damage to the pcb that the input protection might not have the same performance in a future "oops" moment. I can pretty much say with confidence that if you sent this meter back to Brymen for $$$ repair, they would chuck it in the recycle bin and send you a new meter. They have to worry about meeting UL standards, liability, etc.

[stj]

Re: Digital Multimeters

cant you ask brymen for a schematic?
you can get them for uni-t and fluke without problems, and it's not like you cant reverse engineer a meter.
so they have no real reason not to help you.

[retiredcaps]

Re: Digital Multimeters

cany you ask brymen for a schematic?

For Dave's BM235 meter, he asked but Brymen said it was proprietary. If they denied Dave, they will deny your average no-name hobbyist.

you can get them for uni-t and fluke without problems,

For older generation Flukes, you can easily find the info in their service manual. For the latest generation of Flukes, you cannot get that information. I have spent a lot of time reading the older Fluke service manuals.

For UNI-T, I have seen many schematics on the Internet. Whether they are accurate, up to date and correct is debatable. For example, we have seen multiple pcb revisions in their popular UT-61E over at eevblog so the schematic on the Internet may not be up to date?

and it's not like you cant reverse engineer a meter.
so they have no real reason not to help you.

Personally, I cannot reverse engineer a meter. I don't have the training and knowledge to do it. Other things however ...

[retiredcaps]

Re: Digital Multimeters

is it now time to order correct replacement parts?

BTW, I strongly suggest ordering these parts from a reputable distributor like Digikey or Mouser. Avoid ebay, aliexpress, etc for these input protection components.

I also suggest buying the big brand names. The components should be in the $1 to $2 each range so there is really no reason skimp on money here. You only need 4 power resistors, 2 PTCs and 3 MOVs or 9 components. I'm guessing the total + shipping should be under $20 USD.

[stj]

Re: Digital Multimeters

the fuses arent cheap.
SIBA or BUSSMAN HRC fuses command a price.

[stj]

Re: Digital Multimeters

BTW, I started a thread on eevblog about this meter. There is lots of interest in Brymen meters there because Dave's current eevblog branded multimeter is a Brymen BM235.

The BM869 is also popular among some members there.

http://www.eevblog.com/forum/repair/...in-real-world/

if everybody got a meter because dave liked it, we would all have this one.
https://www.youtube.com/watch?v=G9ChhjOKyLA

[retiredcaps]

Re: Digital Multimeters

the fuses arent cheap.
SIBA or BUSSMAN HRC fuses command a price.

Yep, not cheap.

There are some counterfeits on ebay so I would also avoid buying these brand name fuses off ebay, aliexpress.

Fortunately, I rarely measure current and have not blown a fuse due to an "oops" moment.

[jasonbay13]

Re: Digital Multimeters

this is completely separate, but my Milliamp range is inaccurate. by a lot. say my meter shows 200ma, but the load is really 100ma. this is based from an harbor freight cheapo and the 10A range of the 869. is it possible a slight overcurrent (say ~800ma) could damage the fuse without blowing it? or would this be something else?

[stj]

Re: Digital Multimeters

something else.

[TechGeek]

Re: Digital Multimeters

Sounds like either the probes/probe wires have been damaged and/or that something else in the meter has gone bad.

[stj]

Re: Digital Multimeters

if the mov's shorted and took out the resistors then i expect the probe cables to be untrustable.

[TechGeek]

Re: Digital Multimeters

How come? Because of the current draw of the meter with the input protection frying?

[stj]

Re: Digital Multimeters

exactly, the protection shorted out, so the cables had to handle the maximum current at maximum voltage till something gave - probably those resistors.

i dont know why meters dont have a fuse behind the main input to cut the current as soon as the mov's trip

[TechGeek]

Re: Digital Multimeters

Who knows... just another horror show because of price-cutting measures.
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