Part 2: Teardown/Evaluation Mastech MS8040 Bench DMM

[med6753]

Reference Part 1 here:

https://www.eevblog.com/forum/testgear/part-1-teardown-mastech-ms8040-bench-dmm/

In Part 1 I did a teardown of the Mastech MS8040 Bench DMM. Part 2 is a quick evaluation of it's performance. This is by no means a compare to lab standards nor an in-depth analysis. But I think it's enough to determine if it's suitable for a hobbyist's bench.

DC volts check: right out of the box the DMM indicated +9.995V when checked with my +10.001V reference. Close, but I think it can be better than that. Some quick tweaking of the DC cal and in minutes it was reading +10.001V consistently. Once that was done I compared it with some of my other DMM's with readings ranging from 100.0mV up to 190.0V and it tracked accurately. So this seems to be a case of sloppy factory calibration.


Ohms: Did a check of resistances ranging from 1 ohm to 10M  and it's accurate.

AC volts: Sample of some AC voltages and again it's OK. 

Capacitor test: 4.7uf tantalum cap.



Continuity test is instantaneous. Diode test fine. Amps and ma: tracked accurately when compared with other DMM's.

Sample frequency measurement. 3.0 v p-p Sine@1.0KHz


Sample ambient room temperature measurement


That's about the extent of the functional tests I've done so far with the exception of the PC Link software which I'll describe below. In daily use there are 2 annoyances which surface very quickly. One of them I think I can fix with a hack but the other might not be so easy to address.

While the LCD is large and clear it is still sometimes difficult to see some of the small symbols without the backlight. The backlight works well but it only stays on for 30 seconds. And that's typical of battery powered DMM's. But this is a bench unit and can also be mains powered. I'd like to have the ability to keep the backlight on as long as I want. It appears that would be an easy hack.

The other annoyance is that after 15 minutes to DMM goes to sleep if there's been no new activity. Again, this is typical of battery powered DMM's. Since this will be powered almost exclusively on the mains I'd like to be able to turn it on and have it stay on. According to the ES51922 data sheet there are 2 ways to bypass the sleep function. Method 1 is to turn on the RS232 interface. That does work however if you change the selector switch say from volts to mV it turns it off. You have to remember to turn it back on. The other method, which is not mentioned in the operating manual, is to hold down any key, other than the “Hold” key, while turning on the power. That does not work which must be why it's not mentioned in the manual.

The DMM came with a DB9 cable. The Circuit Specialist site said it's a DB9 to USB cable. That's not what came with mine. So I purchased one so I could connect and run the PC Link program.

   
The CD that came with the DMM is divided into 2 parts. The DMM program and RS232 drivers. The program would load under WIN7 but would not run. It wouldn't even load under WIN10. The drivers failed for both WIN7 and WIN10. I went to the Mastech website to see if any updated program/drivers were available but they don't offer any downloads at all. Surprisingly I found program/driver downloads on the Circuit Specialists website. I tried it and the DMM program loaded and ran under both WIN7 and WIN10. The drivers still failed but it turns out they weren't needed. WIN10 found the proper USB to RS232 drivers all on it's own. WIN7 needed some manual help to go find the drivers but it eventually did. But after all that effort the program is a joke. There is no interactive control. All it does is output a screen shot at a fixed rate that can't be changed and graph it. I suppose if you're looking for a trend it would be useful. And you can output it to a spreadsheet. But other than that I can't see much use for it. 
 


Conclusion....I give this Mastech MS8040 a provisional thumbs up. As a basic bench DMM it does the job accurately. It's easy to use and I like the layout and styling. Seems to be rugged enough to withstand some banging around. The Chinese filter caps in the power supply are a little worrisome as the power supply is on at all times if it's plugged into the mains and the rear switch left on. And it has it's other faults as mentioned which I think can be fixed. The PC Link program is essentially useless. That's the bottom line and I hope someone finds this useful.       

[med6753]

Reviving this thread to document a modification I made to the DMM. 

Successfully modified the LCD back light circuit so you have the option for the back light to remain on permanently as long as the DMM is mains powered. Prior to the mod it would go off after 30 seconds.

The back light is powered by 6VDC. Since I was only interested in having the back light on all the time when the DMM is mains powered I decided to find a suitable source in the section of the power supply that is active only when mains powered. That turned out to be a 12 volt source. So the circuit I put together was a LM317T with a 560 ohm and 22 ohm resistor in series on the adjust pin to set the output to approx 6.1 volts. The LM317T is overkill but I didn't have the 100ma version in my stock. I wasn't sure if the back light was an LED assembly or an incandescent bulb and resistance measurements were inconclusive. So I thought it would be a good idea to put a current limiting resistor between the 317 output and the back light. I started experimenting with 270 ohm and worked down until suitable brightness of the back light was obtained. That turned out to be 5.6 ohms. This mod has been in operation now for many hours and no issues such as overheating, etc.

When the switch is in NORMAL position the back light will go off after 30 seconds regardless of battery or mains operation. When the switch is in BYPASS the back light will stay on continuously when mains powered but will not operate when on battery.   

The schematic.


The modification installed. The LM317T and associated components are enclosed in the white case which is installed in the accessory compartment. The bypass switch to the left.


View with the top installed and the accessory compartment open.


Next potential mod is to try to defeat the auto power off feature without having to turn on the RS232 interface. I have the data sheet for the ES51922 DMM chip but I think I need a full schematic which I suspect may be unobtanium. I'm going to e-mail Circuit Specialists and see if a service manual is available.

Finally, been using this DMM for a few months now and I'm quite happy with it usability and accuracy. I would recommend it to anyone looking for a budget bench DMM.   

[pmsr]

Have this same DMM for a couple years, and it is indeed quite accurate for the price. Used the DMMCheck r3 unit to check and tracked very well, no need for calibration. The backlight thing was one of my complaints, so will build your mod circuit for it. Thank you very much for this!

/Pedro

[The Soulman]

The PeakTech 3430 uses the same ES51922 chip so this meter could possible also work with PeakTech DMM-Tool.

 http://www.peaktech.de/productdetail/kategorie/software/produkt/dmm-tool-basic.1034.html

[pmsr]

Can confirm it works with the PeakTech DMM-Tool in 3430 mode, at least in V & ohm modes. In temperature mode it complained about receiving bad serial data.

EDIT: Saw that the 3430 has no temperature measurement so, yes, it makes sense it doesn't work.   



/Pedro

[pmsr]

Started putting together the backlight mod and have no 2.2uf capacitors left. Damn ... 

/Pedro

[colorado.rob]

A bench meter that won't stay on?  That's like the #1 requirement of a bench meter.

Why would anyone choose this over a decent hand-held?

[The Soulman]

Started putting together the backlight mod and have no 2.2uf capacitors left. Damn ... 

/Pedro

That 2.2uf on the output of the lm317 is not that critical choose a higher value if you have one.

Nice to see that software (partially) working. 

[pmsr]

Started putting together the backlight mod and have no 2.2uf capacitors left. Damn ... 

/Pedro

That 2.2uf on the output of the lm317 is not that critical choose a higher value if you have one.

Nice to see that software (partially) working. 

Thanks for the tip. Have plenty of 4.7uf, that would work. Still leaves me with one off. Just ordered a set of 2.2uf capacitors, anyway.

/Pedro

[med6753]

A bench meter that won't stay on?  That's like the #1 requirement of a bench meter.

Why would anyone choose this over a decent hand-held?

It will stay on if you turn on the RS232 interface. But agreed, there should be a better way to defeat the auto power off when connected to mains.

[med6753]

The PeakTech 3430 uses the same ES51922 chip so this meter could possible also work with PeakTech DMM-Tool.

 http://www.peaktech.de/productdetail/kategorie/software/produkt/dmm-tool-basic.1034.html

I tried that software and the results were poor. In 3430 mode it did connect but the readings were all over the place with a steady state voltage input. I tried all the other modes and it refused to connect.

[pmsr]

@med6753, I can see the pictures you posted were on Photobucket so now don't show up. Fortunately I have downloaded them. A quick question for you since I am wiring your mod up in place (which gives a tight 6.11V in my case, btw, as you designed it for). Presume you used the negative wire that goes to the LCD backlight as the common ground, am I correct in this? But where did you source the 12V from? It's not clear from the photo or description. Was it at one of the power supply board terminals that go to the control boards?

/Pedro

[med6753]

@med6753, I can see the pictures you posted were on Photobucket so now don't show up. Fortunately I have downloaded them.

Yep, Photobucket has screwed me over big time with their changes and demand for ransom. Greedy bastards.

A quick question for you since I am wiring your mod up in place (which gives a tight 6.11V in my case, btw, as you designed it for). Presume you used the negative wire that goes to the LCD backlight as the common ground, am I correct in this?

Correct. The backlight ground and power supply ground are one in the same.

But where did you source the 12V from? It's not clear from the photo or description. Was it at one of the power supply board terminals that go to the control boards?

/Pedro

On the main power supply by those large filter caps. +12 VDC is available there when plugged into the mains.

[pmsr]

In retrospective, pretty obvious there would be 12V out of that 7812 voltage regulator. Thanks.

/Pedro

[legacy]

The Chinese filter caps in the power supply are a little worrisome as the power supply is on at all times if it's plugged into the mains and the rear switch left on. And it has it's other faults as mentioned which I think can be fixed.

how worrisome is it? I need to keep the unit power-on for two weeks, 24h/day in order to acquire a set of measures.

If that capacitor is really bad, how can it be sorted out?

The PC Link program is essentially useless. That's the bottom line and I hope someone finds this useful.     

the protocol is simple, and can be readdressed to a Linux SBC. In my intentions, I am willing to connect the multimeter to a RB532 board, with a custom program to log data on a microdrive, an then using GNUplot or GTKwave to rappresent them in graphical form.

[med6753]

The Chinese filter caps in the power supply are a little worrisome as the power supply is on at all times if it's plugged into the mains and the rear switch left on. And it has it's other faults as mentioned which I think can be fixed.

how worrisome is it? I need to keep the unit power-on for two weeks, 24h/day in order to acquire a set of measures.

If that capacitor is really bad, how can it be sorted out?

Hard to say. Never heard of that brand. If you're worried about it I would just pro activity replace them with high quality caps such as Panasonic or Nichicon.   
 

[macguyver]

Overriding the Auto Backlight Off "feature" is actually very simple:  Q8 (located underneath the LCD display) is a simple transistor switch for the backlight controlled by the uProcessor.  Simply bridge the emitter and collector and voila! To defeat the  Auto Power Off "feature" push the PC-LINK Button- "RS232" will appear on the display, otherwise functionality is not altered. 

[cos]

I solved the backlighting issue while on mains power as shown below.

Benefits:
- when the unit is on mains power and switched on, display lighting is always on.
- when the unit is on battery power, display lighting functions as original saving battery power.
- display lighting current is not affected by the mod.

Steps:

1) Using a bench power supply, measured the backlighting current as a function of voltage, see the attached spreadsheet.
From the results it appeared to be LED lighting.
Note: If it had been incandescent lighting, in the lower voltage range current would have risen fast (almost linearly) and at higher voltages flattened off to a much lower di/dv.

2) Reconstructed part of the lighting circuit, which is shown black in the attached circuit diagram (also see Remarks 1 below).
Note: from the component values and sizes, it must be low current lighting.

3) Defined a modification using 3 resistors (R1001 thru R1003) and one transistor (Q1001) as shown in red in the diagram.
An option might have been to delete R1003 and connect the collector of Q1001 to the collector of Q13 but I prefer the mod as shown.

4) Mounted the mod on a small strip of PCB connecting it to the MS8040 circuitry with a 3 pin PCB header and associated plug (the wire to the power supply via 1 turn through the toroid).
See IMG1 thru IMG3.

5) Melted a small opening in the LH-bottom side of the display housing for the 2 wires to the base of Q8 and 0V on the display board. See IMG4 (deburred it afterwards). 

6) Used the middle 2 display mounting holes to mount the PCB on 2 plastic spacers with 2 longer M2 screws (securing the display as well). See IMG3.

Remarks:

1) Be careful: there may be variants under model number MS8040, e.g. mine has 2 ceramic 1000V HV fuses (HV110PV 10A resp. HV620 250 mA) which is not inaccordance with the user manual (Fast 250V types 10A and 300mA resp.). Also according to the user manual, there should be a separate fuse cover in the tool case which is not there. It does however exist on Mastech Model M9803R which somewhat resembles MS8040.

2) The user manual bears the title "4.5 Digit Autoranging Bench Top Multimeter" but fails both in text and pictures to show the applicable model(s) and applicable differences.

3) The auto power off function can be defeated by pressing PC-Link to ON (APO symbol in the top LH-corner of the display disappears).

4) Although there is no separate symbol to show the LPF function on the display it is indicated by flashing of the AC symbol and the wave symbol below it.

[cos]

Follow-up to my previous posting.

WARNING!
The design of the MS8040 display lighting circuit appears to overload the LEDs used, aging them rapidly resulting in reduced brightness (and unnecessary high drain from batteries when activated). Continuous use while on mains power significantly accellerates this aging because dissipation is increased. This posting describes my fix.

UPDATE
Having used the MS8040 on mains power for half a year (incidently, not continuously), I noted that display backlighting brightness had degraded significantly.
Note: IMG numbering continued from my previous posting.

The modification I had made earlier to keep display lighting turned on while on mains was still ok, so something else had to be wrong. Series resistor R1 was still 47 ohms. I measured 3V across it, so the current was in excess of 60 mA.
Removed the display module, removed its front bezel and the LCD display to get access to the light plate assy.
Opened the lightplate assy which exposed a small pcb strip with two - each size 0603 - LEDs (1.6 x 0.8 mm) connected in series via a 0 ohm resistor.
The LEDs had seriously discoloured by overheating (see IMG-5), which is no surprise when they are operated @ 60+ mA using mains voltage.
Connected the strip via a 47 ohm resistor to a variable power supply and recorded the u/i characteristic, see LED Module YDGHY7448H original.

Looked for new high brightness @ low current size 0603 LEDs and chose SLOAN P/N: SLO-230W-00007 (old P/N SMD-W0603-02), Peak Wavelenght: 6000-7000K, intensity min: 460 mcd, typ: 600 mcd, angle: 130 deg, Voltage min: 2.7, max: 3.3, lens diffused, If typical: 20 mA.
The repaired strip is shown on IMG-6, dimensions in [cm].

Driving it again with 60+ mA was no option, so some sort of current limiting was required.
From visual evaluation a LED current of approx. 15 mA would be more than adequate. At this current the voltage drop across the strip was 6V.
The supply voltage for a.o. the lighting circuit is either derived from mains (12V) or from battery (9V). These voltages are ORed via diodes.
For the available voltage drop of the current limiter, the battery voltage where the low battery indicator turned on was determined to be 7.6V.
As the various source voltages are diode ORed (0.7V drop) the the minimum available voltage would be 6.9V, so the series part of the limiter had to operate down to 0.9V.
The updated circuit diagram is shown in MS8040_Backlighting_mod_R1.
With the components shown, the LED current is 14.2mA, slightly temperature dependent, decreasing with temperature.

In order to keep MS8040 circuit changes to a minimum, the current limiter was inserted in place of R1 (the original current limiting resistor). On its soldering pads two new wires (white and yellow) were soldered, see IMG-7.
The piece of circuitboard containing my original modification (3 rows of conductors) was replaced by a slightly wider piece (7 rows), see IMG-8.
The connectors changed from 3 to 5 wires/pins (one wire to the power supply board, 4 wires to the PCB area under the display module). Near the solder points, the wires were fixated with a drop of glue. The opening I made in the side of the display module for the 2 wires of the previous modification was adequate for the additional 2 wires. See IMG_4 Previous posting.

The results are shown on IMG-9 and IMG-10.
Display brightness is more than adequate at significantly less current, saving both LEDs and batteries.