UNI-T M890F Multimeter Repair (shows mostly random values)

[bdanny09]

Hello all,

been lurking these forums but never posted. I'm new at electronics (year 2 EE) and I have no experience actually fixing stuff.

I have this multimeter from my late father that I'd like to keep for sentimental reasons, the problem is that the values it displays are all over the place. For example, AC voltages are into the hundreds (when set on 700) or dozens when set on 200 etc. with the leads disconnected. The Ohmmeter is also showing random values, although it does show near-zero values when shorting the leads, but nothing accurate when connecting resistors (10M or 5M resistors both show 3.03M on the display)

I wouldn't know where to start, which is why I could use some help. Is this even salvageable?


Thanks

just an edit to mention I did try changing the battery.

[mzacharias]

I would start by using another multimeter to measure the input resistance (impedance) of the Uni-T while in DC volts (not milli-volts) function.

Most meters should measure about 10 MegOhms, although some cheapies are more in the 1 Meg range.

Most likely yours should be 10 Meg - if it's substantially off then you have a starting point. Could be a leaky MOV or the like.

Don't forget to clean the function/range switch contacts and circuit board switch tracks.

[bdanny09]

Hi, thanks for trying to help.

I've done the measurement and it seems to be OK, about 10MOhm, the only other multimeter I have has 2Mohms max so I had to put it in parralel with another resitor and calculate the resistance, made me feel really smart

I cleaned the contacts but it hasn't made a difference. I did notice, however that the soldering is pretty bad on the cable connecting the selector to the mainboard. I'm attaching a photo of this.

Should I try reflowing the connection or it looks alright to you?

Any other ideas on how to proceed?

Thanks again.

[goaty]

The ribbon cable looks good to me.
Maby it´s the multimeter chip itself.

Look at schematic, this should be close.

[mzacharias]

The solder probably just didn't make it through both sides of the plated through connection.

Soldering them is worth a try but don't expect a miracle.

[jdragoset]

Beckman/Wavetek HD-110's were notorious for that random display and were usually traced to the display driver IC/LCD interconnections, which were elastomer strips with multiple carbon tracks through from one edge to the other.
Sometimes the problem would stop if the IC and LCD were slightly moved on the board.
Others would require disassembly and cleaning with IPA.

[bdanny09]

Just had a chance to look at this again.

I've cleaned the elastomer strip, but I wasn't holding my breath as the display was showing correct numbers, I was thinking if it's that, it would show broken numbers, and indeed when I first placed it back it was all buggy and showing junk, but I replaced it again and it shows real numbers again (just not the expected values).

I tried looking over the schematic but honestly there's not much I understand. I don't have any real world experience with this, and I'm really confused by some stuff, for example the resistor ladder looking bit from R1 to R10 gives  values in F or D units (what even is D?) and not in ohm? Other resistors, such as R82 is in KJ? Then other resistor values don't correspond to the values on the board anyway, or their location.

For example in the picture attached, R75 through R92 are marked 100u (or 1004?) are they actually capacitors? Seeing as they're on the line going from the selector to the mainboard, I'm inclined to think they're capacitors.

Thanks again everyone for trying help.

[goaty]

The device you have is more modern, but may be is parts similar to the schematic I posted.
The resistors 1004 (1Meg) you show for example are not in the schematic.
See if there is a IC under the Display or just a Blob. If there is no replaceable chip, then chances are the device is dead.

[bdanny09]

Under the display is a blob... but placed on a raised tiny PCB that could be changed fairly easily, if only one could ever find a replacement part. I guess this pretty much puts an end to this project.

On a side note, I had a look now into SMD resistor values notation and understand what all the resistor values are... it was quite confusing last night.

Thanks again to everyone for trying to help

[jdragoset]

If the blob under the display is also connected with elastomer strips, also clean them and contacts with IPA.