All-Sun DT830B adjustment - yeah, I got that bored on Saturday afternoon :-)

[al777]

Hi everybody,

So, last night I was really bored and was looking for something to do (not  ) - so my eyes landed on the most controversial multimeter of all times - a version of the venerable 830, branded "All-Sun DT-830B". It lives in my car... I don't use it much... There was a time in my life when driving distances were short, and car would sit in the parking lot most of the time, with no way to use a trickle charger to keep the battery up... so I used a dashboard solar battery with a small charge controller, connected via OBD port to backfeed some trickle charging current... and I wanted to see what it does in terms of battery voltage after let's say a full sunny day. So, I needed a MM to live in my car and primary function was to measure voltage, accuracy of 0.1V would be good enough. $7CAD later (shipped!) I got the subject at my door, straight from Ali Baba's cave.

It was not very accurate, showing 2.53V from 2.5V reference, 5.06V from 5V, 10.12V from 10V (all from the AD584 reference module - not calibrated, but still agreed by the Siglent SDM3065X within few mV). I opened it up, saw no adjustment potentiometer, closed it and just lived with it (subtracting ~0.15V from what it would show). The time of short driving is gone (solar setup did help back then), so is the problem of keeping the battery charged. All of that was about 10 years ago :-). The 830 still lives in the glove compartment.

Last night I finally got to look at it closer (provoked by yet another schematic of 830 variant posted someone somewhere on the Net). Sure enough, I could not find schematic matching exactly this variant and it doesn't help that the "epoxy blob" chip seems to be 42 pins vs. 40 or 44 of its likely granddaddy, the ICL7106.

So I decided to just look for the stacked resistor dividers and play with the bottom (grounded) end resistor. In many 830 variants it is ~1K total, comprised of 900 Ohm + 200 Ohm potentiometer. I have found the 1K 0805, put parallel to it an external 10K, readings started to show even bigger +error. Removed the original 1K, replaced it with a multiturn 2K (preset it roughly to 1K position), used 3 points on the PCB (it appears the PCB designer provisioned two parallel 0805 landing spots for this resistor - it came handy as the 3rd soldering spot). Quick test and less than one turn later - I got perfect "2.50", "5.00", "7.50" and "10.00" on the 20V range; ~19V from a bench supply was also perfectly agreeing with SDM3065X - great because under 20V is exactly the range I need for car use. I did a cursory check of other ranges and other measurement types, all within what is expected from a $7 MM . I've then fixed the body of the potentiometer to the PCB surface with two small drops of UV-curing resin.

I'm posting it here just in case someone will be searching for exactly this variant and for exactly the same reason of being bored needing to adjust the All-Sun DT-830B . See the picture, it's pretty self explanatory. The original 1K (actual R=996 Ohm) resistor was in R8 position, the R13 was empty. Oh, and I preserved the originality of the device - I glued the original 1K inside of the case with a piece of scotch tape .

I hope this will help (or entertain!) someone. Thanks for reading and enjoy the weekend (it's a long one in Canada!)!