Help Repair Multimeter Uni-T M890C+ - Page 1
Forum section: electronics / repair

author: adventum
author_rank: Newbie
timestamp: May 15, 2017, 08:08:43 pm
content: HiI 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 shows 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 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 is for Mastech's instruments not for Uni-T.I cleaned everything possible even before I post anywhere 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 shows 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 15, 2017, 09:25:15 pm
content: check the rotary switch for contaminants, the pcb's too around the capacitor tests blades and the temp probes, the added red wire has been smashedcheck the connections it may be at the wrong place ??Lucky you have the schematics, maybe the icl7106 has gone bad, i hope its a dip 40 pins, not a blob

author: ModemHead
author_rank: Frequent Contributor
timestamp: May 16, 2017, 02:26:21 am
content: Measure TEST with respect to the COM jack at all dial positions. If it remains at -2.0VDC, it is OK. If not, keep looking for faults on the TEST fan-out.Do you have a scope? Any kind, new or old, will do. If so, check the LCD backplane signal at pin 21 of the 7106. In addition to the LCD itself, this signal fans out to any logic that controls LCD icons and decimal points. If it gets pulled to some DC value by a fault, all segments might appear to come on. (This would not be a good situation for the LCD or the 7106 to remain in.)BP (p21) should be a 50% square wave, about 4.8V P-P at around 50Hz, at all dial positions. If a scope is not available, a DMM should read about 2.4VAC with respect to the COM jack on the backplane signal (or higher with a non-TRMS meter.)Due to the accidental drop, I would be looking for shock damage. Physically broken components.