Tek305 Multimeter Repair: OHMS

[Martin.M]

hello friends,

I have the following problem:
The internal multimeter in my SONY-Tek305DMM (this is a fully seperated board with own DCDC to be full isolated) have a problem when testing OHMs.
AC, DC is allready working.

Description: When testing R, it star like counting, after a longer time it stop at the right resistance.
100k: longer time to be right
10k: short time to be right
zero Ohms: longer time.

Videos to watch that directly, attention large files around 14MB:

testing a 100k.  http://www.wellenkino.de/305/v1.avi 
picture from the stop:

testing a 10k. http://www.wellenkino.de/305/v2.avi
testing with shorted cables: http://www.wellenkino.de/305/v3.avi

the service manual is here: http://www.wellenkino.de/305DMM/305DMM.pdf   (!! 25MB)

I think the current source with the Fet and the OP will make that problem. Good tips are needet.

greetings
Martin