UNI-T UT58C DMM Repair.

[Zenith]

I've had this DMM for about 20 years. They're still sold and cost £40. So a decent enough DMM, although not considered top flight.

The other day I tried to use it and it wouldn't power on. I changed the battery and tried it a few times. Sometimes it would power on and sometimes it wouldn't. Increasingly it wouldn't.

I thought it was worth looking at rather than just throwing it away. There's a circuit diagram avaliable on the web in pdf form, but no service manual.

https://elektrotanya.com/multimeter_ut5 ... nload.html


I removed the PCB from the case and checked the power supply including the power switch.

UT58C_PSU.png

The switch seemed an odd arrangement from the circuit diagram. I didn't understand what capacitor C1 was doing. Eventually I removed the switch and washed it in IPA then tested it. It's a twin single pole double throw switch. What appears to be the cross connection in the switch shown in the cct diagram doesn't exist. I resoldered the switch.

I powered up the PCB using a bench power supply and began to investigate. The power supply was covered by the glass LCD display, which had become glued to the Zebra Strip which formed an electrial connection. The Zebra switch was also glued to the PCB. I don't like messing with Zebra strips but there was no choice, so I peeled it off the PCB. 9.2 volts was going into the PSU but a fraction of a volt was coming out. I checked voltages and buzzed out connections to check for dry joints and bad circuit tracks. Then I pondered what C1 was doing, permanently connected to the battery. It's a kickstart. I bodged in a 47µF cap on leads and connected it across C1 and the DMM powered up every time. There was no display, so I set the DMM to the continuity range and checked the buzzer output with a scope. It was fine. C1 was bad and I replaced it with a 33µF surface mount electrolytic, and that was it.

DSCN4018.JPG

Now for the dreaded Zebra strip. It had the impression of the contacts on the PCB. I cleaned the metal contacts with a pencil eraser and washed them off with IPA. The Zebra Strip was still attached to the LCD display and I rubbed it on a piece of 1000 grade wet and dry on a flat surface, then cleaned it with IPA on a lint free cloth. People report all sorts of problems with Zebra strips, but in the factory they my have cleaned them up with a solvent but I couldn't imagine there would be much more to it than putting the parts in place and snapping it all together. So that's what I did. To my surprise it worked. No problems with the display at all.

A minor success story and a bit of e-waste saved from landfill. It did cross my mind a couple of times to just throw it away, but I do use it and like it.

DSCN4020.JPG

[AVGresponding]

I suspect the crossbar is just an inappropriate attempt to show the switch is ganged. It does seem an unnecessarily complicated power on control circuit.

[Zenith]

I really can't see the point of that PSU circuit. It seems to be referenced to a 1N4148 diode. However, the DMM's been in production for years, so I assume it's been satisfactory. There's a battery low indicator symbol on my UT58C but I can't see it on that schematic, which I believe is for the slightly different UT58A.

Most DMMs have a battery low indication. This uses an Intersil 7106 which is a standard 3½ digit DMM chip which operates the digits. The other functions on the LCD are operated by other parts of the circuit.

The circuit diagram is unclear about what some of the sub circuits with op amps are for, and I'm not motivated to work that out, but it's a lot better than nothing.

The meter was fixed. It's always satisfying to repair these things rather than chuck them away. The account was offered partly in the hope it might help someone else. I suppose few people consider repairing a cheapish DMM.

[vk6zgo]

I really can't see the point of that PSU circuit. It seems to be referenced to a 1N4148 diode. However, the DMM's been in production for years, so I assume it's been satisfactory. There's a battery low indicator symbol on my UT58C but I can't see it on that schematic, which I believe is for the slightly different UT58A.

Most DMMs have a battery low indication. This uses an Intersil 7106 which is a standard 3½ digit DMM chip which operates the digits. The other functions on the LCD are operated by other parts of the circuit.

The circuit diagram is unclear about what some of the sub circuits with op amps are for, and I'm not motivated to work that out, but it's a lot better than nothing.

The meter was fixed. It's always satisfying to repair these things rather than chuck them away. The account was offered partly in the hope it might help someone else. I suppose few people consider repairing a cheapish DMM.

The only UNI-T I have ever used started to lie to the user when the battery voltage dropped a bit----well before the "low battery" alarm showed.
The "clever chaps" sneered when I double checked a resistor value with my "antique" Fluke 77.
The "77" was right!!

[Zenith]

A low end meter brand which has been around for years and is sold by hobbyist and maker suppliers. For ages they've been sold by Maplin (Dick Smith equivalent) who still sell this particular model.

https://pro.maplin.co.uk/pages/search-r ... multimeter

I bought it because the leads on the DMM I'd had for years had started to break up, and I couldn't get replacements to fit. I didn't want to spend any more than about £30 at the time. At one time it was rebranded as Tenma and sold by Farnell. In recent years UNI-T have started with other TE.

This one needed slight adjustment after 15 years or so, which is normal. This is the first time it's really played up and it's had a fair amount of use.

I like the separate on-off switch which means it can be given a setting, say continuity, and just turned on, without turning the dial. It turns itself off after about half an hour. I used to find that irritating, but it must have saved a fortune in batteries over the years.