Fluke 83 III Ohms Fault

[stoz]

I have a fluke 83 III with a fault on Ohms. It will read correctly under 400K \ ohms in auto select. Anything greater and the displays is showing numbers constantly changing. I have cleaned the rotary switch with no change to the fault. If I go to manual Range select in Ohms Meg Ohms *4 and Meg Ohms *40 give readings that are incorrect.
I measured the OP voltage of the meter in manual Range select and got the following.
Range 400 Ohms        1.242 volts
Range K ohms 4          1.24 volts
Range K ohms 40        1.23 volts
Range K ohms 400       1.128 volts
Range M ohms 4           0 volts
Range M ohms 40         0 volts
Has anybody got any ideas  as to the fault
Also if anybody can point us to where I can find a schematic for this meter would be appreciated.
Thanks John

[bdunham7]

No available schematic AFAIK, not in the service manual.  It is similar enough to the earlier 83/85/87 that you can usually get by.

Do ALL of the voltage ranges work properly?  The switching is all done within the main IC, U4.  Your problem is likely there or in the divider resistor network Z1.  If Z1 is bad, at least some voltage ranges won't read correctly.

[stoz]

It's only the Ohms that is giving faulty readings. The voltages I posted are voltages measured at the terminals by another meter when in the Ohms position and in Manual Range mode selected.
Thanks John

[bdunham7]

I understand what your open circuit voltages are.  Those are determined by analog switches within U4 in conjunction with the various elements of Z1.  Z1 is also used in a different way to divide the input for the voltage ranges.  So if you have tested ALL your voltage ranges, mV to 1000V,  and they all work, Z1 is good and your ohms problem would appear to be within U4.  The usual disclaimers apply, like I might be wrong, strange things happen, we don't have the exact schematic, this advice is worth every penny you paid for it, etc. 

[stoz]

OK now I understand where you are coming from. I will test all the voltage ranges and I located 83 85 87 file. Will let you know how I go. Thanks for your help.
John

[Wallace Gasiewicz]

I had a problem with a different Fluke that would not read over a certain ohm value.
Try disconnecting the MOV.

Wally

[stoz]

Wally I disconnected the MOVs fault still the same. I think  bdunham7 might be right and after looking at the cct dia for the 83 85 87 manual I will check Z1 notably the 9.996Mohm resis. Will let you know how I go. Once again thanks to both of you.
John

[stoz]

bdunham7 would you know where I could find a pinout of Z1. I am quite confident the 9.996MOhm resis is open. I couldn't find a pinout in the manual you directed us to. Thanks again for your help.
John

[mendip_discovery]

Service Manual and other stuff can be found,

https://xdevs.com/doc/Fluke/83_85_87smeng0500.pdf
https://xdevs.com/doc/Fluke/83_85_87.pdf
https://xdevs.com/doc/Fluke/83_1000.pdf
https://xdevs.com/doc/Fluke/83_1000_sm.pdf
https://xdevs.com/doc/Fluke/83_1000_smeng0000.pdf

[stoz]

Worked out the pinout of Z1. Unfortunately fault appears to be something else.

[stoz]

Fault ended up being one of the wiper contacts. I waited till new ones arrived and checked if meter now operates OK. Thanks to those that helped.