Fluke 87 problem

[LazyJack]

I was given a Fluke 87 (original) as it was not powering up at all. At first it looked easy. Q8 was faulty, once i replaced that from a donor unit it did power up. However, it did only get to the point when all segments of the display is lit, and stayed that way.
I looked at the usual suspect, such as the mode selector thick film contacts, cleaned everything again, etc. but to no avail.
I noticed that if I wiggle the mode selector, in some in-between positions, the unit powers up in AC V mode, which is the first mode next to off.
I could not get any further.
I believe, although I did not yet check it with a scope that it figures out the mode selection by varying the output on AP6 (as it seems to be an analog out port) and getting the input on P03 (digital input), which has a fixed threshold. Depending on where the mode selector is, P03 will flip at different values of AP06. Based on this, I suspect that U4 is faulty, as there doesn not seem to be any other components that can fail to produce this result. This is why the only mode is AC V as p03 will only turn on if the mode selector is in a between setting.
I will need to check if my theory holds and scope on AP06 and P03. The contacts and the divider chain of the 10k resistors all look ok.
Or do you have any idea on what else can be wrong?

[Arhigos]

Are you sure that rotary switch is in correct position?

[LazyJack]

Yes. First of all, I could compare it to a Fluke 85, second it does switch off in the off position, so it must be in the proper position as otherwise the on/off circuitry would not work. It is possible to put the mode selector 180deg off to the range switch, but then it would not be off in the OFF postion.

[CDaniel]

According to the schematic for 87V ( maybe different ? ) that can be found on this forum , the resistive divider is from P3.3 pin68 to the -2,5V supply ( the micro ground ) and the input for analog measurement is P6.6 pin5
The microcontroller is powered from +0,8V to -2,5V ( ~3,3V ) , when the meter is "on" the voltage at pin P3.3 should be "high" at +0,8V . This arrangement is done for low power consumption , in off state te pin P3.3 is also at -2,5V  and that divider is not consuming any power .

[retiredcaps]

The service manual for the original Fluke 80 series is at

http://assets.fluke.com/manuals/83_85_87smeng0500.pdf

Modemhead's blog entry might give some insight?

http://mrmodemhead.com/blog/fluke-83-dmm-repair/

[LazyJack]

Thanks, that is a much more logical description on how it operates. Anyway, the divider is ok, and I can measure the divided voltages at ever segment, and since it basically directly goes to the input of U4, I guess the main IC is faulty, as i can't think of any other problem.
At least I have al the goodies, like fuses, fusible resistor, display as spare from this unit.

[mzacharias]

Make sure the mode select switch on the rear of the main board is not reversed. (Black plastic with feelers to the deposited carbon tracks.

While you're at it, clean the carbon tracks, sometimes loose particles cause erroneous operation.