Backstory

The Mastech MS8221C was a very popular (and seriously overpriced) multimeter for the local market in the early 2000s. I got mine around that era in an attempt to acquire my first quality multimeter. It was my senior year in college as an electronics student and I didn’t know any better. Besides that, my options were very limited as the retailers I contacted didn’t stock Flukes (or other similar quality meters), so they upsold me what (to their knowledge) was a high quality multimeter.

Inacurate temperature readings

Pretty soon it became obvious that the temperature readings were quite off. For example, room temperature was usually close to 12°C / 53°F, even in the summer. Trust me, I do not live in a refrigerator. This made me question the accuracy of the rest of its functions. So, when I got the chance, I checked against other multimeters in my college and (guess what?) found out that indeed was totally off, way more than the nominal values in the spec sheet.

Professional calibration

I contacted the seller asking for advice on what to do. Can I return it? Is there a calibration service somewhere? Both answers were no. Long story short I was stuck with an overpriced faulty multimeter and I wasn’t willing to give up.

Now I’m not suggesting that you should do what I did. If there is a Professional Calibration Service in your area then definately go with that (although it might cost you more than buying a new meter). If there isn’t, be aware that it’s somewhat impossible to calibrate your meter accurately without using really expensive equipment and without following the right procedure as described in the manufacturer’s service manual. But if you are stuck just like I was, taking measurements that are as much as 10% off what the nominal deviation should be, then go ahead. Most likely you are not gonna make things any worse.

Service manual

Good luck trying to find a service manual on-line. I looked everywhere, professional resources, unauthorized resources, mailing lists, torrents. Even online forums! Nothing. I’m guessing that it doesn’t exist. Or if it does, it might not be in english. So, I decided what every other (un)reasonable person would do: take it apart and try to fix it by trial and error. Did I break it? No. Did I fix it? No. Was it working any better? Somewhat… Readings were not accurate on some functions, but for others were way closer to correct. And that’s what I had to make do with for many years to come.

Schematic diagram

A decade down the road and while being a proud owner of quite a few multimeters, some of which pretty decent, I thought I’d give it another go. I looked online for a service manual but still I couldn’t find any. Then I thought, “maybe there is a schematic”? Et voila, there was! Having that schematic allowed me to skip trying to reverse engineering the damn thing and eventually come up with a method to calibrate it. Hopefully this guide will help you calibrate yours as well.

I’m not sharing the schematic, as I am questioning if that would be legal/ethical. If you are interested though I’m pretty sure that a quick search for Mastech MS8221C Schematic in your favorite search engine should produce a number of positive results (in the images section).

Electrical specifications

Although we are calibrating to a “good enough” standard, attempting to be as close as possible to the ballpark helps with OCD. The following table lists the manufacturer’s specifications needed for the calibration.

If you want to take a look at the full spec sheet, search for Mastech MS8221C Instruction Manual on your favorite search engine.

Preparation

Keep in mind that the values in the table above are valid at 23±5°C and a relative humidity below 75%. Your workplace and meter should ideally be in those conditions, for a decent amount of time. Have four cables with alligator clips handy, a 10nF, a 10μF and a 1000μF capacitor.

Meter disassembly

Removing four screws on the back will enable you to split you meter open. You don’t have to remove the batteries. Don’t worry about parts falling out, everything should be held in its place with either glue or screws.

Meter power

Now that the meter is taken apart, the board has no power; the battery compartment is part of the back cover and normally connects with the main board with spring contacts. The meter has to be powered during calibration so find a way to transfer power from the springs to the respective pads on the main board. I usually solder two thin wires to the pads and connected their free ends using alligator clips.

Calibration procedure

Step 1: Reference voltage adjustment

Set your meter to Volts DC, connect the probes to a known voltage source and adjust VR1 until you get the correct reading. According to the specifications table above, if you are measuring a 1V DC source on the 2V DC range you should get a reading between 0.991 and 1.009.

Step 2: AC buffer adjustment

Set your meter to Volts AC, connect the probes to a known 50Hz or 60Hz alternating voltage source (NOT mains) and adjust VR2 until you get the correct reading. Similarly to step one, if you are measuring a 1V AC rms source on the 2V AC range, adjust until you get a reading between 0.989 and 1.011.

Resistance (is futile)

There is no separate adjustment for resistance. If you’ve followed the steps above, by now you should be able to get good measurements in all the ranges for AC, DC and resistance. If not, then there is something else going wrong with your meter. It could be a bad resistor, diode or op-amp. Troubleshooting such situation is beyond the scope of this guide.

Step 3: Temperature adjustment

VR3 and VR4 are the trimmer pots intended for temperature measurement calibration. Without having a thermocouple connected, adjust VR3 until you read the ambient temperature on the display. When you have that dialed down, connect a thermocouple attached to a known heat source and adjust VR4.

According to the specs, for a 25°C (77°F) room, a reading between 22°C (72°F) and 28°C (82°F) is in the ballpark. Similarly, boiling water at 100°C (212°F) would read between 96°C (206°F) and 104°C (217°F).

If you can’t get it to measure correctly, it might be diode D10’s fault. Replace it with another 1N4148 and start over.

Step 4: Capacitance adjustment

In order to adjust capacitance measurement you will need three known value capacitors. They don’t have to be exacly the values listed in the preparation section, but they have to be of known value and same measurement range.

Set the range switch to 20μF and connect a quality 10μF capacitor. Adjust VR5 until the display reads between 9.3μF and 10.7μF.

Set the range switch to nF and connect a quality ceramic 10nF capacitor. Adjust VR9 until the display reads between 9.50nF and 10.50nF.

Set the range switch to 2000μF and connect a quality 1000μF capacitor. Adjust VR8 until the display reads between 957μF and 1053μF.

Calibration is complete.