Multimeter UNIT-T UT70D display 63% of Voltage value on 8.000V range only.

[FredCailloux]

I’ve had this UT70D meter for years. I love it. It is sturdy with a great resolution. Last month it started showing wrong values on the 8.000 DCV range.
I cannot find anything wrong on the PCboard. Everything seems immaculate. The meter was not dropped or anything.
It simply started showing wrong values on that range. All other ranges are OK.

I started taking some measurements just trying to understand the problem.
I found that the inside ADC chip FS9704B is taking the input voltage on an approximate 1meg ohms input impedance for that range.
The input voltage range on the chip TENM pin 18 is from ±0.8V but that is approximation since I cannot calculate the real value, the meter being defective.
I found that when going to other ranges like 80.00 or 800.0 or 8000 ranges the meter will change the input impedance by a factor of 10 for each range.
I found that this procedure end up dividing the input voltage to suit the range. Ex. At range 800.0 the input impedance is lowered to around 10k.
The input path has 2x 5meg ohms resistors. So, an 800V dc input would be necessary to raise the voltage to full range, probably 0.8V.
Following this logic it appear that the FS9704 ADC activate the correct input impedance on the 8.000V range.
The problem seems to be inside the chip. Something like a bit that does take the proper value for the 8.000 range to be functioning properly.
At first I suspected some kind of corroded selector switch contacts but that is not the problem since no resistor dividing scheme is involved in the range selection. In all evidence, it is done inside the ADC chip.

Does anyone have any idea how I can investigate and or fix this weird problem ?
Cheers

[hexreader]

Probably a long shot, but I would carefully examine the soldering for D5 through D10

Maybe it is a trick of the light, but the amount of solder looks a little mean to me.

I had a similar problem with a diode on my UT70A. All looked good, but there was a bad solder joint on one SMD diode. A quick re-heat and all was good.

[FredCailloux]

Well, thanks for the suggestion.
I just re-solder them. No luck. Same behavior.
Anyway, these diodes seems to only be involved with anything other than Volt reading.
It was worth the try.

The more I cogitate about this problem, the more I think the problem must reside in the FS9704 ADC that got de-programmed.
I cannot think of how that could happen but all signs point toward the ADC not behaving properly on the 8.000 range.

I guess that meter will end-up on the Obsolete Apparatus shelf.
I just cannot resolve myself to ship it in the landfill.

[Hamelec]

i think the 2nd resistor of the voltage divider is R10..(or R9) ?
check both..

[FredCailloux]

Those 4 resistors seems OK.    I measured them. 1M, 100K, 10K, 1K

Which appear to be correct. The ADC act as an Amp meter.
Depending on the ADC input impedance, which is determined by R7,R8,R9,R10 selection, the meter must read 0.8V for a full scale display.
It would appear that the selections of one of those resistor is internally connected in parallel with the input pin TENM, making a voltage divider down to the 0.8V range.
I'm getting out of testing options.