Mastech MS8268 calibration + BL hack

[BiOzZ]

i got as a gift a cheapo Mastech MS8268 ... its a very nice meter! it has allot of functions and its nice! ... and one of the best (tho short running ... must hack this) backlight i have seen

BUT its out of calabration

i ordered as reccommended by someone on this form a current, voltage and resistance reference that has been  precalabrated (bargan!)

but for now i needed to trace out what did what ... the datasheet for the controller IC http://www.silan.com.cn/english/product/pdf/SC9711AY.pdf was fairly useless for this but i was able to figure out what did what

This is what i came up with



Im still working out 10A range and capacitance


The uA & mA pot is odd ... when the multimeter is set to that range it shorts that pot with a 9K resistor to the above voltage pot that has a 10k resistor ...  like this



i point this out because i found this an interesting solution ...

im going to calibrate it to the best i can soon
if nessary i will replace the reference resistors with better ones or put pots on them
and POSSIBLY put in a more stable voltage reference

i do plan on installing a flipflop on the display and shorting a few things out to get the display switched

NOTE: the datasheet is in chinglish but possibly the capacitor marked on the multimeter as C8 is the reference capasitor but also C6 might be it ... datasheet calles that one "The capacitor of Resistor voltage" >_> ... R8 measured in circuit is 39.03nf and C6 is measured as 39.40nf ... ... im not sure how caps are measured so this might all be moot ...

hope i helped!


EDIT:
Current measurement in the 10A range just sticks the output of the shunt threw the same 990k resistor it sends the voltage threw in to the voltage input of the IC ... i can only think that if it overshoots you can put a 10K divider threw that line ... if it undershoots and you dont want to break out the low voltage opamp you can probally trim the shunt or just live with the fact that your $30 DMM wont take spot on 10 amp measurements XP


EDIT 2: BL HACK

I was shocked to find out the BL does not use the 2 secound BL counter in the IC but a dual transistor one shot occolator as shown here from reverse engineering the circuit in to a circuit simulator (NOTE the cap is by default 10u NOT 100u AND the 1K going from the cap is 1M not 1K ... this was just a simulation setup ... there are also 2 30 ohm resistors for the BL LEDs not shown)



replacing the cap (marked C24 on the PCB) from 10u to 33u the blacklight stays on for about 8 seconds! (yes i did this and tested it ... i dont think its really worth a youtubing ... )
far more useful!

[jebcom]

Nice work Biozz, thanks.

When mine came, a quick check also showed that it was out of cal by several percent.

When I investigated mine, I connected a 330 ohm resistor and I did not find that any of the pots affected the reading.
But I did find that VR3 affects capacitance reading when measuring 10 microfarads. It's a slow update; you have to wait a few seconds after you turn the pot.
I also found that both DC mA and AC V are affected by VR1 as well.
This means we should calibrate DC Volts first using VR1, and then calibrate DC mA and AC V by adjusting VR4 and VR2.
I'll try to do the calibration this weekend using an in-cal Fluke.

Depending on how the calibration comes out, this might be OK for a secondary multimeter.

BTW, take a close look at the shunt and see if you can tell me how they calibrated the 10A range!

[BiOzZ]

Nice work Biozz, thanks.

When mine came, a quick check also showed that it was out of cal by several percent.

When I investigated mine, I connected a 330 ohm resistor and I did not find that any of the pots affected the reading.
But I did find that VR3 affects capacitance reading when measuring 10 microfarads. It's a slow update; you have to wait a few seconds after you turn the pot.
I also found that both DC mA and AC V are affected by VR1 as well.
This means we should calibrate DC Volts first using VR1, and then calibrate DC mA and AC V by adjusting VR4 and VR2.
I'll try to do the calibration this weekend using an in-cal Fluke.

Depending on how the calibration comes out, this might be OK for a secondary multimeter.

BTW, take a close look at the shunt and see if you can tell me how they calibrated the 10A range!

thats why i do it! XP

Mine was fairly well in cal but fairly well wont do it for me

i can verify that for me at least VR3 adjusts resistances between 0 and 1k although by only a small ammount

VR1 is the whole system reference voltage and will effect just about everything so be sure to change that first

VR4 gets put in parallel with VR1 when in miliamps so when in miliamp mode both VR1 and VR4 will effect it but you should only adjust VR4

i dont have any AC references so i cant verify VR2 and wont touch it

i did trace out the shunt and it simply takes the shunt lead threw the same place on the IC as the voltage input via a 990k resistor shared with the voltage input

you can cut the trace (its fairly long) and divide the voltage to ground with a trimpot to cal it DOWN but you cant cal it UP unless you stick in an opamp ... the shunt is 0.01 ohm so st 10 amp its just 0.1 volt so you have to keep that in mind ... to cal it up you can try taking a pair of wire snips and pinching the shunt ... but the risk is there that you cut it and TOTALLY throw it off or you trim it toooo much

im waiting for my voltage and resistance and current standard comes in the mail before i cal it but the resistors look nice ... the PCB looks nicely traced and the chipset is reliable so i can see it holding its cal nicely but worse case you can swap out the internal voltage reference (pin 90) with a nice low PPM one

im not trying to get this to simulate a fluke just be more useful XP

[jebcom]

The shunt in mine had several wedges chewed out by a Dremel (or similar) tool; I presume to pad the R up a bit.

im not trying to get this to simulate a fluke just be more useful

I agree completely. I'm very happy with the calibration results. I'll check it again in a few months to see how it holds up.

[spencer2004]

Hello, I found this thread pretty interesting so I thought that I'd expand on it and share what I discovered for anyone who has this model of multimeter and wants it to be as accurate as possible.

All I have done to mine is replaced the voltage reference with a much more stable one and replaced the pots for the voltage reference and current measurement with multi-turn ones. I didn't bother replacing the reference resistors as they are already within a 0.25% tolerance so it would be too expensive to replace them with better ones for the little improvement it would give.

The voltage reference first comes out of pin 90 of the main IC as a voltage between 1.1v and 1.4v, it goes through some voltage divider circuitry then back in to the IC through pin 89 as 0.44v, this reference has a voltage coefficient of 2000 ppm/V and temperature coefficient is not stated in the datasheet so it's a fairly unstable reference. I have replaced the voltage reference with a MAX6138 (1.2205v) which is a shunt reference with a temperate coefficient of 25ppm/C max (probably overkill but oh well).

All you have to do is cut the trace coming from pin 90 (reference out) and wire up the external reference of your choice connecting the ground connection to analogue ground (not battery ground!), VIN to the positive battery terminal and VOUT to a point after where the reference out trace was cut.

[henryx7]

@spencer2004

Hey Spencer2004 can you explain more about what you did? Or make a short tutorial for it? I just ordered one of these  multimeters because of its functions and price, but as I have read many reviews on Amazon, people say their devices begin to malfunction after a few months. So instead of buying one less appealing to me I went ahead and bought this one anyways, but in the event it does break I would want to try to do what ever I needed to to fix it.

You mentioned replacing the reference with a MAX6138, but how was it connected to the multimeter? I'm a MechE student so I don't know too much about circuitry.

[suntop]

It's a shame that the LOW BATTERY WARNING shows at 3.9 Volt on MS8268 Multimeter ! What's the point of operating with 3 AAA Battery then ?
Low battery at 1.3 Volt/Cell !

So, I replaced the reverse polarity protection diode ( which has 0.6 Volt Forward drop) with LOW forward drop diode 1N5819 (0.2 F/W drop), Now I get low Battery warning at 3.6 Volts !!


Not Bad , Ha ?

By the way, this meter consumes about 5-6 mA current ! So, we can expect 400+ hrs of operation with Alkaline AAA batteries !

One more thing, To disable the Sleep mode, hold down the SELECT key while turning the meter on.

[Svetoklik]

 My first post so Hi to everyone.
I am from Serbia and I occasionally mess with electronic related to audio and video.
I have this DMM with one trace of sparkle on board that starting from mA input so I
am suspicious about that  PIC16F54 processor, because it has like from the heat mark on it.
There is in electronic store here PIC16F54 IC but how to flash it?
Is it possible to by pass it if it has only function for making usefull lght show on terminals?
The DMM is dead on all possitions but it draws very low current with  dial on and  4 Ohm resistance between battery inputs and discontinuity with dial on off possition.
There is 0.2V between com and V Ohm terminal.
There is one picture with that trace. Is it line under just broken?
Thanks!