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Tenma DMM repair

PostPosted: Mon Aug 24, 2015 1:17 pm
by Dr Wobble
The first half decent DMM I bought was a Tenma 72 - 7750. I chose as at the time i wanted a DMM with a good capacitance testing range. I'd chose differently now and get a Fluke which is my main meter now.

Anyway, foolishly I shoved 300v DC up it's pipe whilst on the ohm's range so it's not in the best of fettle.I took it apart and found a blown thermistor a WMZ11 - not the best protection. Havn't got one so have replaced with a jumper for now. It looks like two S8050 transistor have fried too. E-B reads a short. What I'm puzzled about is that theyre connected with the base and collector shorted, there's a pair like this so it isn't a soldering fault. Why would this be, why not just use a diode?

The Tenma uses a ES51986 chip for everything and apart from a few discrete and surface mount components that's it. I'm hoping it's not fried too. the other fault is hard to fix namely some traces on the PCB have vaporised, this is part of the range switch. this switch is crap and caused me lot's of hassle - see pic. Two of the silly tiny contacts flew into the air when I took it apart. I found one after crawling round the carpet and the other 3 months later near where Molly sits; looked down and there it was. The problem then was trying to figure out where they sat. I sussed it in the end using a magnifying glass to see if any tiny scratches or marks were left when originally seated, that and a bit of trial and error.


Re: Tenma DMM repair

PostPosted: Mon Aug 24, 2015 2:00 pm
by Refugee
Those transistors were most likely diode wired due to them being no more expensive than diodes so that they save fitting a reel of diodes to the assembly machine.
That PCB under the switch looks very badly burned.
Perhaps you need another cheap meter for quick indication only work.
Save your Fluke for real measurements like I do with my Fluke.

Re: Tenma DMM repair

PostPosted: Mon Aug 24, 2015 6:17 pm
by Dr Wobble
Re transistors, could be mate but there's diodes fitted on the left. The S8050 is an odd choice and annoying as it doesnt have many equivelents that I have to hand.

I have other meters, just don't like chucking ote away especially as it was £70 if I remember right.

I contacted Tenma some time back for a schematic to no avail.


Re: Tenma DMM repair

PostPosted: Mon Aug 24, 2015 7:17 pm
by Mark Hennessy
Using a transistor as a diode results in something with much less leakage than a typical signal diode, for much less money than a low-leakage diode.

It's an old trick - the oldest reference to it that I have seen was from Fluke in the '70s, but no-doubt others knew about it before then...


PS: Sorry to say, but that meter looks like toast. If you do get it going, it would be hard to trust long-term - those carbon deposits could cause leakage and breakdown at high voltages... Tenma don't make good meters. In fact, Tenma don't make them at all - you probably have a badge-engineered Uni-T. We had a batch at work, and most started playing up after just a couple of years of light use. I - chief skip diver than I am - managed to through them away with no pangs of guilt, after doing a "tear-down" for the course that was in, to demonstrate why you should buy Fluke instead :bba

As I say, sorry to give bad news, but remember the risks here :thumb

Re: Tenma DMM repair

PostPosted: Mon Aug 24, 2015 9:38 pm
by johntheboffin
The original post reminded me of an incident when I worked at Marconi.

The inspection department was checking out a batch of mains transformers that had just arrived from the supplier. This was in the thermionic valve era, of course, so we were looking at a multi-tapped primary and secondaries of 5 V, 6.3 V and 350-0-350 V.

The first test was winding insulation using a Megger. Next was winding resistances using an Avo 8. The Avo was also used for the next test, secondary voltages with mains applied to the primary.

Unfortunately the inspector left the Avo set to ohms when doing the 350 V secondary voltage test. The mechanical cut-out didn't stand a chance!

The pointer went across the scale so fast that it was already bent by about 30 degrees (due to air resistance) when it hit the end stop. The stop left a large kink in the pointer. But it was repairable and no other components had been damaged.


Re: Tenma DMM repair

PostPosted: Tue Aug 25, 2015 12:00 am
by Pamphonica
I am still amazed how most modern DMMs can happily deal with mains voltage accidentally applied on an ohms range. They just politely say "overload" or equivalent until you remove the volts. A good feature for numpties like me!
If you want to see the excitable Davey Jones from EEVblog torturing DMMs, look here...

Or if you want to see Dave Jones at his most squeakily excited, look at this rant on a DMM that fails the "Mains on Ohms range" test. Gave me a laugh but is actually quite a serious indictment of a poor modern design.

Please note - I say "most" modern DMMs are oK. Don't just assume yours is safe in this area - check the specs first!!! I think the major manufacturers are good in this respect (Fluke, Keysight/Agilent, Keithgley etc) but the £5 chinese meters may well not survive the experience.

I think you should look for a "Cat III" input rating if you want to be sure a DMM will fail safe with mains inputs on non-volts ranges.


Re: Tenma DMM repair

PostPosted: Tue Aug 25, 2015 12:55 am
by johntheboffin
The Fluke 845AB null detector specifications said that it could take a 1000 V overload on the 1 microvolt range.

I'm confident (well, reasonably!) that they are correct but, despite the temptation, I never dared to try it. It's a bit like seeing a "wet paint" sign. You really want to touch the paint to see if they are right!


Re: Tenma DMM repair

PostPosted: Tue Aug 25, 2015 1:24 am
by Refugee
That was pretty stupid to have an interlock that can fail to fail safe.
I also like the wet paint analogy.
Once my boss was showing a rep around work and the heating was broken so I had a dummy load plugged into the power to keep warm and it was hot with live terminals.
I spotted a hand heading for it and shouted do not touch that and in a split second there was a squeak :elc:
I never did find out if it was a burn or a shock.
It did have a big notice that said live mains on all terminals but they just don't read :zx:

Re: Tenma DMM repair

PostPosted: Tue Aug 25, 2015 7:24 am
by Dr Wobble
Have to agree with you, it's not really worth fixing. Like I said it was my first attempt to buy a "decent" DMM. I know quite a bit more now and would'nt buy one now. I'll see if I can fix it to use on capacitance and temperature range though.

My Fluke 25 has survived similar daft mistakes ( it's easily done ) as well as being dropped off the bench. I think it's also survived being damp/under water as when I got it secondhand it had rust stains in the fuse/battery compartment. Bullet proof.

your right there Jeremy, Dave Jones does get excited, his passion is one of things I like about his video's. I'll re watch those video's while I have my breakfast- better than TV. He know's his stuff too. I've learned a lot from him.


Re: Tenma DMM repair

PostPosted: Wed Sep 02, 2015 9:34 pm
by Doz
A wise person many years ago, when I was an apprentice, gave me some advice.

There are three manufacturers who make decent meters:

1) Fluke
2) Fluke
3) Fluke

I took his advise and spent two weeks money on a Fluke 77. I still have it today. The switch is a little worn. I purchased it in 1988, and it has been used more or less everyday since. I do have a more modern fluke now as well, useful to have two...