Wow, only 4 euros for an MX56 ?!
![Shocked :o](https://www.eevblog.com/forum/Smileys/default/shocked.gif)
that's pretty much free then... normal / average going price here in Frogland, its home, is about 120/150 Euros.
At 4 Euros it sure is worth spending some time trying to fix it !
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif)
Plus, as I understand it, only the current ranges are kaput, so the meter is still 100% usable on the DC voltage range, which is of course where its high accuracy makes this meter valuable.
Current measurement you can always do with another meter.
Anyway, good job on the reverse engineering !
![ThumbsUp :-+](https://www.eevblog.com/forum/Smileys/default/icon_smile_thumbsup.gif)
The table in the schematic looks nice, as does the reverse engineered ARC1 pinout in the lower left corner... nice job.
Looks to like you have it all already figured out, so not sure how useful it would be for people to take measurements on their meter. But if you need a specific measurement done then yes I can probably do that, when I find time.
After a few tests, everything is quite clear - the switch for the 500 mA range is shorted and still open.
Shorted...ior open, which is it ?!
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/grin.gif)
Is there any reviewers for my schematic design ? (logic errors and etc..)
As for reviewing your schematic, according to the cartouche, looks like "Jack Daniel" has already done it, no ?!
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/grin.gif)
I am no expert / engineer, but a few things caught my eye. I will probably make a fool of myself but well, I like to participate anyway !
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/grin.gif)
1) T1A and T1B. I guess you put them there to do some voltage translation to accommodate the 5V inputs of IC3, the TTL logic gates / inverters that they feed ?
If so, I don't understand how it can work. The way they are wired, the inputs of the logic chip will always be at logic level high, it will never change : the base of the transistors are fixed at 5V permanently, so the output voltage, which you tap at the emitter, will always be 5V - Vbe = 4.3Volts or something.
2) The dual MOSFET that switches the 500mA range. I don't understand how the two MOSFETS are wired together. In series ?
Shouldn't they be wired in // so as to build a bi-directionnal switch and/or lower the Rdson value ?
Even if you meant to wire them in series, why are they wired front to back ?
Looks like you are 95% there... if the PCB's are two expensive to make, maybe you can make a prototype board by hand to ensure that it works (and if not, modify it 'til it does) before ordering nice PCB's.