Mastech MS8040 True RMS DMM Teardown

[enut11]

I needed a new DMM that I could leave on all day without worrying about battery life. I also wanted at least 5 digits and reasonable accuracy. Other wants (not needs) included temperature logging and and PC interface for data logging. Price budget was around $200AUD.

The MS8040 that I ended up buying has the following specs:
LCD Count: 22000
DC range: 200mV to 1000V, 0.05%
AC range: 200mV to 750V, 0.5% TRMS
Ohms range: 200 to 200M, 0.1% up to 2Mohm
Temperature: -30C to 1300C!, 1%
Other functions include DCA, ACA, Capacitance, Frequency and the ability to be used with an AC/DC clamp meter.

Bundled items include test leads, power cord, RS232 cable, Temp leads and Cap/Temp adapter.

This unit has a large back-lit LCD display which clearly shows which function you have selected. It also includes a useful analog display.

Because it has dual power (9v battery or 6 AAA cells or 240VAC) the back-light only stays on for less than 1 min. I never intend to use it as a portable DMM so I wired a permanent fix for the back-light.
https://www.eevblog.com/forum/testgear/mastech-ms8040-bench-dmm-back-light-problem/msg953142/#msg953142

[enut11]

It did not take long for my curiosity to take over and I had it apart on my bench. Below are some shots of the internals...

[enut11]

More pictures including power supply. At the rear of the box is where the 9v or 6x AAA cells go if you want portable power. I simply power mine with 240VAC.

[tautech]

I needed a new DMM that........

AC range: 200mV to 7500V, 0.5% TRMS

Really? 

[enut11]

Oops, I meant to type 750 VAC...

[enut11]

The Mastech MS8040 is easy to use. All functions are clearly labeled and easy to interpret. My only complaint is that the 12 position rotary knob is not smooth to operate. Internally, it is simply a nylon sprung "ball" locking into a V groove with no lubrication.

The PC interface relies on RS232 connectivity although I have yet to try it. I am told that it works with an XP system. I have Win 10 so will try a RS232 to USB adapter.

[Hydrawerk]

It is CAT II 1000V but has only 250V fuses. 

[enut11]

Good point on the fuse rating but not a problem for me. In 40 years of electronics the highest voltages I have measured were around 100vDC and 250vAC.

However, if an instrument is rated for 1000VDC and 750VAC, it should be properly designed.
Thanks, enut11

[grantb5]

Dumb question ... I've only ever used a handheld DMM.  Is the COM banana plug electrically connected to the AC power input? Pretty sure my soldering iron tip and oscilloscope ground are all connected.  Just worried about what I can and cannot probe without investing in an isolation transformer or something.

[Vgkid]

Dumb question ... I've only ever used a handheld DMM.  Is the COM banana plug electrically connected to the AC power input? Pretty sure my soldering iron tip and oscilloscope ground are all connected.  Just worried about what I can and cannot probe without investing in an isolation transformer or something.

The positive and common jack are connected to each other through a 10Megohm resistor. You can probe voltages until the max ratings.
Edit.
Multimeters are differential , you won't hurt anything with the ground lead.

[grantb5]

I meant between the COM banana jack and the 3rd prong on the mains AC cable in.  I can buzz it out if I can find a portable meter around here someplace.  With a portable DMM I can measure a voltage across an arbitrary resistor in a live circuit without worrying that I'm grounding one end of said resistor. That's my concern I guess. Or maybe I didn't comprehend your answer...

[Vgkid]

Ground(earth) is not connected to the meters common input terminal.