Cheap MastechMS8229 failing. Need pointers

[Moriambar]

Hey, I'm a total beginner here, but really hyped from Dave's videos…
Recently my "trusty" cheap Mastech kinda failed: from time to time the Ohm (≥100k) and DCV measure (≤5V) are not correct. Especially the ohms are displayed showing a fricking high 30 M or thereabouts slowly climbing down (I never had the patience to let it settle, since it takes more than 1 minute).

Measuring its resistance from another (way cheaper) multimeter, shows around 600k (should be 10M), while the mastech cannot measure the other DMM resistance (altough well within its range).


Since the EEVBlog Brymen is on its way, I thought it would be quite nice to try and repair the old mastech and keep it around as a backup, not-so-reliable-but-already-paid-for meter. Also I find this "project" kinda instructive…

Do you have any pointers on what to check, please? I tried (perhaps you'll laugh at me, IDK) the fuses and the batteries (of course) but everything seems fine.

Thanks

[SeanB]

Probably first clean the PCB around the input, and also clean the PCB switch contacts as well, along with the area around the main chip.  Use some pure alcohol, Isoprpoyl or even methanol or ethanol, and a small stiff plastic bristle brush, and dry with clean paper towel and allow to dry. Otherwise the input protection diodes are leaky, and need to be replaced.

[Moriambar]

Probably first clean the PCB around the input, and also clean the PCB switch contacts as well, along with the area around the main chip.  Use some pure alcohol, Isoprpoyl or even methanol or ethanol, and a small stiff plastic bristle brush, and dry with clean paper towel and allow to dry. Otherwise the input protection diodes are leaky, and need to be replaced.

Thanks, tomorrow I'll check the diodes and try the cleaning with isopropyl alcohol.

[JFJ]

... from time to time the Ohm (≥100k) and DCV measure (≤5V) are not correct. Especially the ohms are displayed showing a fricking high 30 M or thereabouts slowly climbing down ...

Were you measuring the resistance 'in circuit'? If so, then be aware that other components on the PCB could affect your reading. For example, a capacitor on the board, still holding some charge, might account for the 30MΩ reading that slowly decays away. A common way to avoid this problem is to unsolder and lift one end of the resistor under test, so that its resistance can be checked in isolation.

[Moriambar]

... from time to time the Ohm (≥100k) and DCV measure (≤5V) are not correct. Especially the ohms are displayed showing a fricking high 30 M or thereabouts slowly climbing down ...

Were you measuring the resistance 'in circuit'? If so, then be aware that other components on the PCB could affect your reading. For example, a capacitor on the board, still holding some charge, might account for the 30MΩ reading that slowly decays away. A common way to avoid this problem is to unsolder and lift one end of the resistor under test, so that its resistance can be checked in isolation.

Nope.

My other (ok totally rubbish) meter was measuring the "correct" value, ie 300k or whatever. But anyway all of this was done out of the circuit (I was actually trying to pair up precise resistor values), and the reading was off.

Anyway I plan to check it out tonight, now that my BM235 has arrived.

Thanks for the advice, if you have any other pointers, please do tell!