Fluke 189 multimeter - functions button not working after a short

[techboy]

Hello guys,
New here, I am glad I found an electronic forum covering all aspects of electronic repairs.

I had an issue with my Fluke 189, where accidentally measured low voltage 5-12V while I was on Diode mode. I am glad my Fluke survived and working fine, although the blue circle button (refer to the picture) to switch to functions like Diode or Continuity mode stop functioning.
I took it apart and reset the memory by resoldering the battery but still not functioning. Run quick measuring on the board but resistors and transistors seem to be fine. No burn marks as it was low voltage.
All other functions of the multimeter seem to be fine. Fuses are all fine.



Does anyone have experience with repairing multimeters? I spoke with Fluke authorised service, but this model is discontinued and no parts available, unfortunately.

I can take higher resolution photos of the PCB if it's going to help?

Regards

 

[Bluejets]

I take it you meant "in general" selection blue button not working as on diode it switches to cap, not continuity.
Sounds like it might have damaged the uC.

 

[techboy]

Hi Bluejeans, thanks for your reply. Yers that's correct, the button is not switching between modes on all switchable sections.
I was hoping to be blown components by the voltage provided while in Diode mode.
Do we know more about how multimeters in general measure voltage and diode mode?

I cannot find even a generic schematic of a multimeter to reverse engineer this fluke one. All chips are Fluke made.

 

[Bluejets]

ah..that's bluejets pullleasssseee...left over from flying days...

Anyhow, had a good search for a possible schematic without any luck.
I did read (skipped through) a 96 page online manual but you probably already have one.
I'll stick the link here anyhow.
https://www.manualslib.com/manual/453766/Fluke-187.html?page=3#manual

Apart from that, my initial thoughts were that good quality meters such as yours generally have well designed circuitry that should really have some sort of protection against a cock up...and a bit further searching in that respect brought up this particular bit of info that pretty much supports what I had initially thought.

Might be of some help to you at least........ it's a bit like when we'd have a r/c plane go in the deck.....break the plane a bit was no real concern, but break parts of the engine or radio gear and that always led to a bit of knuckle biting.

http://mrmodemhead.com/blog/fluke-87-fusible-resistor/

One further note, some fuses on these boards might actually take the physical properties of other components, always a good idea to be aware of that.

 

[techboy]

Hey Bluejets, my apologies I was one eye scanning at work and misspelled it

Yeah I went through the manual, but that seems to be operational only. I however found a schematic for Fluke 17B and it seems like these lines goes straight to the microcontroller for analysing. I wonder why this recent model has schematic available but not others. Perhaps chinese leaked it.


I have multiple sizable chips on Fluke 189 and bit more transistors and resistors, but no generic ICs.

I am out of ideas, apart than to take high res photos and try to retrace inputs back.

PS: Thanks for modemhead's link I will go through it and compare with my fluke.

 

[Bluejets]

Something else I found was this video.
Now the bloke tends to ramble on a bit and takes ages to get to the point, however, this particular meter had a wallop of overvoltage applied and survived so I actually hold out some hope for your unit.

He also mentions a "calibration" routine which I did see somewhere also in a pdf so most files for these things are freely available.
Just a shame the schematic doesn't seem to be the same.
I think the fusible resistors referred to in the mrmodemhead article might just be part of which this bloke had to replace.

Anyhow...the video...

Calibration manual link.......
http://www2.produktinfo.conrad.com/...-an-01-en-Fluke_187_Kalibrationsanleitung.pdf

 

[techboy]

Thanks Bluejets for all your suggestions. Especially Dave's video on multimeter input protection was very insightful.

I found the issue, a user in another blog suggested to check if the blue button makes a good contact. So i soldered two wires on the pads and shorted them to see if it will switch and it did. It was just matter of unclean button pad. I remembered that on your first video link with the british guy, he mentioned that he put silver paint on the pad as they often get worn out. Perhaps it was just an coincidence that it failed at the exact same time as I mismeasured.



Thank you again for all your efforts and guidance.
Regards

 

[Bluejets]

Great stuff......must admit it did cross my mind but as that would have been my first check I tended to overlook it.
Was also reminded when the video showed the bloke recoating the button but didn't think to mention it.

Cheers Jorgo

I tend to remind most that electronics are largely fairly reliable these days and more often than not to look for mechanical defects so there ya go as they say.
Good to see it's going again.