Hi
I have old multimeter Uni-T M890C+ that is acting weird these days. I know that I should buy new and I already did, but the thing is that I got it for 15 years maybe more and it is kind of part of me. It will be very nice if I can make it work again. It also has a very rare and expensive feature it measures up to 20 amps. So let's get into the problem.

It show very strange symptoms, when the dial is at Diode/Beep position everything on the display is fine, but if the dial is at any other position it fills the screen, every possible pixel is on digits, letters and signs. If I try to touch the leads while on Diode/Beep mode it fills the screen again and no beeping. Sometimes the Ohmmeter is working. I already checked the fuses and cleaned the dial's pins and the display, but the behavior is still the same. I have some pictures to understand more the problem. I also put in the post the schematic for M890G which is basically the same except that the G model has frequency measurement and the C+ doesn't.

 

I'll take a shot..
The continuity / beeper function looks to be separate from the DVM chip (ICL7106). That could explain why it works when the rest of the functions don't.
Since all the other functions need the ICL7106 alive and none of them work, I would suspect that the 7106 is not working. Assuming it is not completely roached, I would check the power to the chip, and the oscillator (OSCx pins). Shoot some contact cleaner at the switch contacts, it looks like V+ is switched and maybe that contact isn't making. Note that since it is a single phase backplane, all segments could be 'lit' by DC bias between an energized backplane and non-functional segment drivers with a non functioning 7106. That is a bad thing since eventually the DC will burn the display.

Looking more, be sure your battery is good and that the circuit in the Q2 area is OK. That circuit is driving the segment test which would turn all segments on if it drives pin 37 high. See pp 7 of the datasheet.

ICL7106 datasheet.

Good luck!

 

Hi

I have an update on the repair of the M890C+! I checked everything connected to pin 37 of ICL7106 and everything works fine. I also changed the Icl7106 with new one and the condition is still the same. I forgot to mention that the cause of the problem is dropping the device on the floor from around 1.5 meters, so if you have any suggestions to that I am open. Also it turned out that the schematic that I found for it is very different from the circuit board itself in terms of components numbering and their values. Completely different cause the schematic I posted is for Mastech's instruments on for Uni-T once, despite the fact that the schematic is for the G model I found one that is for the C+ one and it is still the same!

 

Are the any specific points on the circuit board that respond to pressure? I had a capacitance meter whose display only worked when pressure was applied to the LCD display. ... It could be a crack in the board or something similar. ... Contact cleaner on the rotary switch might help. But you need to try to locate a specific point of malfunction.

 

Yes I thought about that and tried to twist and bend the circuit with some little pressure but with no luck. I cleaned everything possible even before I post here the zebra on the display and the pins of the dial.

I noticed something else this evening. When I am at any position of the dial and turn off the device with the ON/OFF button it show what it is suppose to show on the display for a split second it is so fast that is hardly noticeable. It again makes me think that there is something to do with the test pin, but I already checked Q2 as stated on the schematic, but on the circuit of my device is Q5.

 

• ... re: ON OFF switch
• Just check to see that there is continuity between the COM terminal input and the ground trace on the circuit board. ... Where it goes to the diode/resistor junction. ...d4/R17 ... seems like a bad connection there would cause a major problem.

 

Hello adventum,
I have same multimeter problem. When I've extracted LCD display, up left corner of display was broken. First contact of LCD display is Backplane. So display with out Backplane show all elements, or may shown invert elements.