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Uni-T UT139c LCD biasing and other mods
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frozenfrogz:
Dear forum,

I am looking for some info on biasing the LCD on the UT139c. The issue in question is quite similar to that of the Brymen BM235 described and fixed in this thread and Daves video.
Forum and web search did not turn up anything useful so far. Has anyone reverse engineered said device or are schematics available?

Thanks and best regards from Germany,

Frederik

Edit: After digging a little deeper I managed to find a solution, see detailed reply below.
frozenfrogz:
From: Kerry Wong's dtm0660l based hack, especially: The translated data sheet, it looks like the LCD Voltage is set by the EEPROM?
I am having difficulties measuring VLCD. I only get 0.5V from VLCD to VSS. What am I doing wrong here? COM to SEG is sort of 3.1 Volt-ish.

Reference from another thread:

Pointers welcome!

Regards,
Frederik

__Edit__

Also: (And maybe I should have asked this in the first place)

How is the biasing on the UT139Cs that you own?

Is this just a one-off problem and I should return the thing to get a new one?
frozenfrogz:
To cut it short:

I found a solution starting from Kerry Wongs article.
The broken out pins on the PCB did somehow not work for me. I bought a SOIC8 programming clip as I thought it might come handy in the future anyway.

Basically this link to STM32duino forums set me up with any additional info I needed.

The EEPROM on the UT139C PCB (full disassembly of the LCD module required) is a T24C02A similar to other meters based on the DTM0660L chip.
I used an Arduino and newly accuired programming clip to dump the EEPROM contents. The sketch of Andy Hulls post over at the STM32duino community is pretty nice and needs just minor adjustments to match an Arduino board of choice.

The factory setting for my meters VLCD was 3.0V (taken from the memory dump). I reprogrammed it to 3.3V and the display is now pretty usable. For my taste, the contrast could still need a little bit more oomph, but I am not complaining. 3.3V is the maximum setting, as the DTM0660L datasheet indicates.

While having the infrastructure set up I am wondering if all UT139 meters are built of the same hardware and just restricted via software. If I happen to find a cheap 139A I will check if I can simply turn it into a 139C as with the Rigol scopes :)
All in all there are a whole lot of hackable parameters to look into!

Anyone wanting more info on the issue, feel free to ask!

Regards,
Frederik

Edit: Added some photos
Kbird:
Thanks for the Post I just opened mine up to try and reset the resolution and change the backlight timeout and discovered that the LCD Backplate (White peice) is now soldered to the PCB which I have not seen in any of the pics about this Hack before .

It appears to be a Rev.4 like yours and I think in your last pic I can see you just carefully bent the white peice (real name ? :) )
up to vertical while you did the mod, is that Correct ?

KB

frozenfrogz:
Yes, two LEDs are glued to the white back plate and you need to bend the legs in order to reach the EEPROM.

Also, I got an email from a reseller regarding the display:

(translated from german)
»Thank you for your inquiry for the UT139C.
Unfortunately the manufacturer told us, that it is not possible to make changes to the display in order to improve readability.
However, the UT139 series will soon be equipped with an HTN display instead of the current simpler LCD model. This will ensure optimal readability.«

HTN are a bit better in regard to contrast and viewing angle, compared to TN as the cheapest option - STN or FSTN would be superior to all of these, but also way more expensive.

As shown, there is a slight reserve to set a higher bias, but moving to a better panel is not the worst decision Uni-Trend could come up with. We will see, how this affects retail prices though.
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