Craftsman 82357 DMM bad resistance measurement

[bkenobi]

I have a Craftsman DMM model 82357 that I bought in the late '90s or early 2000's. I believe it's equivalent to one of the WENS 20TRMS units. I like the unit but recently I noticed the resistance values are incorrect. My best guess as to why is last year the probe touched a ground plane while doing a continuity test with the board powered. The GFI receptacle kicked off so I assumed no damage to anything. I generally look at continuity rather than absolute resistance so perhaps something else caused the issue.

In any case, I opened the case and overall I don't see anything clearly burnt. I did locate a chip with the label "14053B PMG044" (apparently an analog multiplexer/demultiplexer based on the datasheet) that appears to have gotten hot at the pins as the solder has a reworked look.

It looks like I can get them fairly easily so I could just replace it and see what happens, but I was curious if anyone has suggestions on this model or repaired a similar type of DMM issue? Yes, I can replace it with a cheapo Uni-T or a nice Fluke but I'd like to see if this one can be resurrected for fun and maybe extend its life.

[redwire]

After a big overload (mains) to Ohms function, it can damage the clamping transistors or PTC thermistor or the ohms source resistor, causing wrong readings.

What is it reading on Ohms when the test leads are open or shorted. I see a 220Ω resistor is reading low at 144Ω. Does DCV work?
Somebody was in there and grossly resoldered the U2 14053 but likely not what happened.

It could be repairable, just need more info.

[bkenobi]

Nobody has been inside the unit because I've owned it since day 1 and I've not worked on it. I think a fuse blew once (not sure of circumstances) so I recall having to open it around 10-12 years ago to replace the large ceramic fuse (socketted so I wouldn't consider that "working" on it). Otherwise, nothing has been worked on. I used my air rework and some solder flux to clean the ugliness up but I imagine sufficient heat to reflow that solder could have been enough to damage the chip.

The attached pictures show the DMM with open and shorted leads in both AUTO and continuity (beep) modes. I also included images for 0, 5 and 12VDC with a Uni-T as a reference. It appears that in AUTO mode, the resistance is large but not "0L." as seen in continuity mode. Now I'm second guessing but I don't believe that it showed up that way before (large resistance rather than "0L.").

[redwire]

I would suspect the protection clamp transistors, if they get damaged or short they will give a phantom ohms reading with open leads.
That is Q2 and the one next to it, small SOT-23 parts.
You could remove them, and then try the Ohms function to see if it works (with open leads). The multimeter will work with them pulled but will not be protected against overloads. Or use another multimeter to test them.

[bkenobi]

Thanks for the suggestion. I'll take a look and see if they look like I can remove them. Can these be tested prior to removal or do they need to be isolated to properly diagnose their function. Assuming these are the bad components, where would you recommend sourcing replacements? I've used Mouser and Digikey in the past but seems like 1 or 2 transistors might be too small an order for them to consider.

[redwire]

What markings are on the two transistors? I've seen typically SS8050 (in SOT-23 package not TO-92) with marking code "Y1". Order more than 1-2 (just shipping is the main expense) and one will fall on the floor or vanish if you sneeze.

You seem to be displaying 1.8kΩ ohms with open-circuit test leads, so try going across the E-B pins and see what you get for a reading. The C-B are shorted with a PCB trace as they are diode-connected transistors.
Try the multimeter on Diode Test I think that would also not work properly is a damaged transistor.

Or just pull the parts and hopefully that Ohms function works. I'm just guessing at what got damaged in the multimeter.

[bkenobi]

I pulled the transistors. I tried to get a picture of the markings but couldn't get the lighting right with either my phone's camera or my SLR. After pulling, I was able to get one on each that seem to indicate BR25. A quick search suggests it could be a shottky diode?

I reassembled the DMM but the contacts on the display seem touchy so some segments weren't working. I don't think it has anything to do with the transistors though as I removed and reassembled and the unit and it was a bit better (more segments). I read in a forum post somewhere that a similar unit had an issue like this and required shimming the display. Anyway, the segments that were working seemed to suggest that with open leads it was showing "0L." and when shorted "0.000".

[bkenobi]

This is the post I found that said the board required shimming after inspection. It happens to be the location where the software has been archived so a dual benefit in posting! I will investigate this next time I get to it. I don't know if it's a shimming issue or a bad contact to the grounding plane on the back of the case. If bad ground, I imagine a little dielectric grease on the spring/contact location might resolve.

Less pleasing was my having to shim the circuit board with a hard object in order to get the display working properly after I'd had the meter apart for an inspection.

http://greyghost.mooo.com/82357/

[bkenobi]

I got a few minutes to test the DMM this afternoon. Although the display is still missing some segments, it's better after adding dielectric grease and a few layers of electrical tape on the back side of the board where the screen contacts the pads. I can confirm that with the SMD components removed, I now see the correct resistance reading.

Assuming I can get the display segments back with improved shimming, does BR25 offer enough info to help source new modules? It seemed like Mouser has a lot of modules in SOT-23 but not clear on what the ideal replacement would be since BR25 doesn't seem to exist there or Digikey and I haven't yet found a datasheet with Google.

I appreciate the help so far in getting it mostly working again!

[lotas]

Br - 2sc4081 (sot-323)

[sam_sam_sam]

Could also be this one as well

BCW60DR the data sheet says that it has the markings of “BR” the 25 might be a date code

https://www.s-manuals.com/pdf/datash...cw60_zetex.pdf

Here is where you can find some

https://m.aliexpress.com/i/225183280...apt=4itemAdapt

Unfortunately it does not show an actual picture of the device

Looking for something on AliExpress is sometimes very difficult to do

You can also use this website page to locate some as well

https://www.digipart.com/part/BCW60DR

One note I have not use any of the companies that are listed

[lotas]

The case in the photo is SOT323, while the BCW60DR has SOT23.
https://www.alibaba.com/product-deta...787871541.html

[bkenobi]

I have a few extra transistors from a project that are the same size. I'm curious if these would be acceptable replacements? I can order the ones in the link but it appears I have to order 100 and wait a month so if I could use what i have, that would be nice.

https://www.digikey.com/en/products/...3904-TP/717395
https://www.mccsemi.com/pdf/Products...04(SOT-23).pdf

I don't know how to read the datasheet to confirm compatibility.

[sam_sam_sam]

You would have to compare the data sheet to know for if they are similar or not

[lotas]

https://alltransistors.com/transisto...ansistor=15736

https://alltransistors.com/crsearch....ps=TO236&cf=on

[bkenobi]

Doesn't look like the MMBT3904 is on that list. That compatibility list looks like it's pretty common transistor but curious if it has a "standard" part number that would reference all typical of that type. I tried looking on Mouser and digikey thinking I could get them from the US quicker. But I quickly found that I didn't know what specs I should even be searching for. I can put the new part on, but I'm lost on how to sucessfully find a compatible replacement. I'll use the list in that compatibility list to try again though! I suppose I can also try them on Aliexpress to see if anyone is selling one in a smaller quantity that isn't 1 month delivery and/or $10,000 shipping (I'm not joking on that, I found one transistor with that for shipping and double if I wanted 2x).

[stj]

here is a schem of a meter with the same chip

[bkenobi]

I found a vendor on Aliexpress that only has a lot of 50 and reasonable shipping but will have to wait a while for it. The DMM works as is so not a problem. I won't get it from these guys though.

[redwire]

They look like vanilla SOT-23 package and the marking code for 2SC2412 "BR" is mid-rank for hFE 180-390. It's not as strong as other parts. I find these protection transistors can be whatever was lying around, so sometimes they are oddballs.
The clamp transistor special requirements are low leakage current IEBO (0.1uA or less) and lower breakdown voltage BVEBO (5V or less), and high current. Fluke will hand select them for low IEBO leakage current but who has time for that.

I would use SS8050 family parts. Mouser has MMSS8050 or SS80850-G etc.
pic is common chinese transistors and their marking codes.

[bkenobi]

I ordered some 2sc4081 yesterday but it's good to know that there's lots of options. I figured they were pretty generic, but one transistor certainly isn't the same as every transistor obviously. Thanks for the further feedback!