Metex MS-2000.

[jstanchev]

I found 2 type of MAX134CMH
1. MAX134CMH+D
2. MAX134CMH+TD
Which is difference?

[retiredcaps]

I found 2 type of MAX134CMH
1. MAX134CMH+D
2. MAX134CMH+TD
Which is difference?

Packaging?  See mouser and digikey screenshots.

[Fraser]

I have a complete MS-2000 if you would like to buy a spare parts donor. It has a dead DC-DC converter and I just have not got around to repairing it. If you are interested, let me know what it is worth to you.

Fraser

[jstanchev]

Who DC-DC converter doesn't work on Your METEX?

[Fraser]

I'll fix it and let you know ! 😆

[jstanchev]

Today replace MAX134CMH.
Result is the same.   

[retiredcaps]

Today replace MAX134CMH.
Result is the same.   

The only thing I can think of is that there is still some contamination somewhere on the pcb.

In this recent Fluke 83 post, it wasn't very obvious at first glance there was a problem ...

https://www.eevblog.com/forum/repair/fluke-83-multimeter-repair-attempt/

[Stefano267]

Hi all,
I found this thread because I had the same issue with my multimeter, mine is a Tenma 72-6640, another re-labelled Metex MS-2000.
I followed all the suggestions given, but I also had a blown resistor, the 1Kohm resistor labelled R221 in the DG Scope schematic and R52 on the DMM PCB, thanks to jstanchev for confirming me its value. I reaaly have no idea how I blew that resistor, but it helped me in finding the solution.
Thanks to the schematic I traced the signal path in the input stage and I found the protection transistors not shorted, but somehow damaged, they are Q59, Q60, Q61 and Q62 in the schematic, Q17, Q18, Q19 and Q20 on the PCB. To test the possible solution, I removed Q17 and Q19, in testing the DMM I had to be careful because there was no protection against either to over voltages (especially in the 400mV range), or to DC/AC voltages while measuring resistances. I did not use the auto range feature to be sure not to damage the MAX134.
The schematic diagram lists two possible types for these transistors, in my case they are KTC3875S Y-RTK. I replaced all four of them and of course the blown 1K resistor.
After replacement I cleaned with the ultrasonic cleaner both PCBs (not the one with the keyboard and the one with the display) with a 30 minutes bath in IPA at 50°C, then I "baked" the PCBs in the oven at 50°C for 10 minutes to let the alcohol evaporate completely.
Now everything works perfectly, input impedances are in specification, about 10MOhm in all ranges. I also measured resistors, capacitors (down to 115pF), DC and AC voltages, I tested also the frequency meter. Great!

I considered also the contamination issue. This may be possible because of the semi-sealed design of the DMM case and because of the rechargeable NiMh batteries inside. My issue started when the battery pack began losing its capacity, probably some exhalations from it caused loss of insulation between the traces and/or the components, considering also that I work with vacuum tubes and high voltages, this combination might have caused the resistor to burn and the subsequent damage to one ore more protection transistors. Anyway, now it is fixed but I will not use the autoranging feature for measuring high AC or DC voltages, the switch between 400mV and the higher ranges is done by a relais that might not be sufficiently fast.

Now I'm trying to restore the rechargeable battery pack, unfortunately I lost the old one after moving to the new house, if someone can post pictures of it and its schematic it would be very helpful. I do not remember if it has 2 wires (in this case no problem, one is the positive, the other one is the negative pole), or if they are 4 and this would be an issue, what are the other 2 for??? 

Thank you all for the suggestions.

[yuraol]

Hello colleagues! Please share a circuit diagram for Metex MS-2000 or its equivalent Tenma 72-6640

[Stefano267]

Hello yuraol, I did bot find any schematic of this meter, in this thread you can find the METEX DG SCOPE 20Mhz schematic whose DMM part is the same as the MS2000.

http://www.produktinfo.conrad.com/datenblaetter/125000-149999/126560-sp-01-en-DG_SCOPE_20MHZ.pdf

If anyone can post the complete MS2000 schematic it would be helpful considering that MS2000 PCB labels are, of course, not the same as the DG SCOPE ones.

Thank you
Stefano