Need a schematic for Digitech QM1320 DMM

[TerraHertz]

Yes, I know, fat chance. It's a cheapo meter I was given, among that recent pile of stuff: https://www.eevblog.com/forum/chat/unexpected-treasure-find/

I was told this meter had a 'water encounter of the wet kind' along with a motorcycle and pack that ended up in the drink.
On opening it up, there was a little corrosion on a few PCB areas, but not too bad. I thought I'd give it a chance.
Water and detergent, a good scrubbing with stiff brushes, rinsing, blow off with compressed air, repeat rinse with distilled water, blow dry again, then some scrubbing with IPA and a final blow dry. The board looks clean now.

And it works, or at least DCV and Ohms seem to behave accurately enough. See pic.
The Capacitance range seems to be dead, or maybe I'm doing it wrong.

But.... there is a piezo beeper, and it screams ALL THE TIME the meter is on. This is not very practical.
Searching for a schematic for Digitech QM1320 finds lots of those vile web-scrapping sites, but nothing useful.
Does anyone have an idea where I might find one?

It's a cheap, crappy meter. So this is more an exercise for the principle of not throwing stuff out. Also it has a nice big, clear LCD.

[SeanB]

There is a little corrosion on the small transistor controlling the beeper, clean the board again, and pay attention to the top section by the transistor test sockets and the capacitance test area. The display IIRC is tilt able, and this uses a standard 7106 blob on the board for display, using discrete parts for the rest.

[TerraHertz]

Yeah, it's coming apart again for a second look. A schematic would save time, plus there are some cal adjustments.

The beeper is just a piezo/brass disk, not a self-oscillating one. So not so simple as a single transistor getting biased on and sending power to the beeper. I'd like to avoid tracing circuits, since I have too many things to do atm.

[elex_enthusiast]

It reminds me of my very first DMM with a tilting screen. After a few years it became honky dory. I repaired it and gave to a friend after I got a better DMM. I've searching for its schematic but I also had no luck. Chinese manufacturers of such meter seems pretty clever in releasing its schematic.

[ModemHead]

Searching for a schematic for Digitech QM1320 finds lots of those vile web-scrapping sites, but nothing useful.
Does anyone have an idea where I might find one?

Try searching for a Mastech MY-64 schematic.  There is a current MY64 model, but there was also an older version that looks just like yours.

[retiredcaps]

Try searching for a Mastech MY-64 schematic.

Yes, ModemHead is correct.

[SeanB]

So beeper is done by IC2, pin 7 and 8 are shorted. That solves the beeper. Capacitance the opamps or the 4 protection diodes are shorted or dirty still.

[TerraHertz]

Try searching for a Mastech MY-64 schematic.  There is a current MY64 model, but there was also an older version that looks just like yours.

Ah ha! Thanks for that. And now I feel stupid, because it actually says Mastech MY64 on the PCB. But I didn't recognize the name and ignored it.

RobertoLG, thank you too for the manual.

Readable hi-res schematic here: http://www.eserviceinfo.com/index.php?what=search2&searchstring=mastech+my-64+schematic.bmp
Why would anyone still use BMP? That's 1778 KB, and simply converting it to PNG makes it 114KB, with zero data loss.

Did I say I didn't want to waste time tracing circuits on a multimeter that probably cost less than $50?
Ha ha, what's a good way to make someone trace circuits when they don't want to?
A: Give them a schematic that is subtly wrong. So they think it will be easy, but then get bogged down and confused. Grrrr...

Anyway, it's fixed now. SeanB, you're just trying to silently remind me about those GaAs bits, aren't you? Don't worry, I haven't forgotten. And it wasn't anything *near* as simple as your "IC2 pins 7&8 shorted."

IC3 is the standard CMOS nand gate astable oscillator, driving both sides of the piezo buzzer. At about 9KHz, which is right on the boundary of what my stuffed ears can hear. Making it super annoying.
IC3 pin 1 is the enable, and sure enough IC2 pin 7 was sort-of high all the time, hence the permanent piezo enable. But it was lower than IC2's +ve supply pin 8, so not shorted.

Looking closely around IC 2 & 3 and under C25 I could see a little bit of corrosion crust I'd missed. So removed all three parts and cleaned them and the board underneath them more carefully. Also noticed that the piezo brass body (which has signal from IC3) is hard against other tracks, relying only on the board resist for insulation. So I removed it too, and bodged insulation with capton tape.

After soldering all the parts back in place, there was no difference whatsoever. Piezo still shrieking all the time.
At this point I decided it would be a good idea to shift the frequency down to something less fingernails-on-blackboard, so I'd still be sane by the time this thing was fixed. So pulled R24, used a decade box to get a less painful note, and soldered in an 820K resistor.

Still thinking it was sure to be easy, I started poking around with the scope.

Right away things got messy. Firstly IC2 pin 8 is NOT the V+ rail the schematic says it is. But it is around 6V, so the op amp should be working. But pin 5 (+input) is way too high for the schematic - it's about half a volt above pin 8. How...?
Pulling pin5 hard down to V- didn't disable the buzzer either. What?

A bit of probing and track tracing discovered that the 'down bar' power rail symbol (eg on R23, near IC2 pin 5) I'd assumed was the same as V-, is not. It's actually Vcom, from the COM jack.
Or it's *supposed* to be, but at that end of R23 it's floating up higher than the +ve supply to IC2.

Much track tracing and confusion later, I'd become suspicious the problem wasn't unwanted connectivity, it's lack of connectivity somewhere. Which, considering there seem to be multiple differences between the PCB and schematic, was hard to zero in on.
But I did have a suspicion. See the pic IMG_3015_spot_missing_track.jpg
Can you see it?

After repairing that, it works. Silently, except when it's supposed to make a noise. DCV, Ohms, Capacitance ranges work; I didn't try the others. As for calibration, as best I can work out the trimmer functions are:
VR1  7106 Ref (pin 36) Adj DCV
VR2  AC V adj
VR3  Capacitance adj
VR4  Temp adj
VR5  7106 'in LO' (zero?) adj. Not sure what this does. Seems to be hard at end stop.
VR6  Frequency span adj.

What's conspicuously missing is a Resistance span adjustment. The meter is not far off on resistance but could be better. In the last pic the "99.6" should be reading 100 K.

[ModemHead]

What's conspicuously missing is a Resistance span adjustment. The meter is not far off on resistance but could be better. In the last pic the "99.6" should be reading 100 K.

As with most DMMs with dual-slope AD convertors, the resistance measurement is ratio-metric and thus does not depend on the voltage reference, so not easily tweakable.  It depends instead almost solely on the precision and stability of the reference resistors, which in this case appear to be dual-purposed with the voltage divider (R5-R10).

Nice work on the repairs. Even if it's a cheap thing the experience can be invaluable sometimes.

[SeanB]

Told you there would be corrosion by those sockets..... Nice work there, and in the schematic the ground you refer to is analogue ground, not batt minus as you thought. Interesting on the 7106 is that the test input is also the analogue ground point, and will do as a pretty reasonable reference as well as it maintains 6V between it and the battery positive. that is the 6V you measured, you were probably using common on the meter as a reference.