Fluke 8508A 8½ digit reference dvm repair/recapping

[mzzj]

Symptoms of the patient: 8508A gone bonkers, fails to zero on resistance ranges (error during input zero 2004) 
Being switched off for 30 minutes and it fails to start up properly, one of the lcd displays remains black and entire unit is nonresponsive.

There is a signs of earlier butchered capacitor replacement on the power supply board, blue 50v 470uF electrolytics have been replaced before. Power supply seem to run rather warm (8508A doesn't have any sort of cooling fan.)
On closer inspection the blue 50v 470uF caps measure fine on ESR but the 3300uF 35V snap-on electrolytics have 10 ohm ESR or about 500x higher than expected!
One of the  small 10uF 50v/63v electrolytics next to  R514 also measures 1.5kOhm ESR 

https://xdevs.com/doc/Fluke/8508A/img_cm/Guts_3.jpg

3300uF 35v snap-on electrolytics are some ultra-miniature series and I didn't have any in "stock".
Quick&dirty repair with taller 2200uF 35V caps and the unit powers up fine, clears all self tests and runs through the zero routine just fine.

Hopefully this is helpfull with someone working with these dinosaurs. I'd assume that these have come to mature age and failures are more of rule than exeption.

Also question if someone has replaced or measured the electrolytic caps on the "monster relay board"?

https://xdevs.com/doc/Fluke/8508A/img_cm/Guts_7.jpg
They also look like recipe for disaster being next to heatsinked components inside a rather hot running equipment.

[Andreas]

Hmm,
there seems to be more trouble left of the reference board
with best regards
Andreas

[mzzj]

Hmm,
there seems to be more trouble left of the reference board
with best regards
Andreas

Pics in first post are not mine but the unit I was working on had exactly same bodge as shown in this xdevs pic:
https://xdevs.com/doc/Fluke/8508A/img_cm/Guts_15.jpg

DIP-8 opamp with different pinout  mounted to SO-8 smd footprint 

[Ugur]

Hello,
I tested it when I did a similar repair and it passed all the tests without any problems, but during the adjustment, the AC 200mV 60 kHz adjustment step could not pass.

[mzzj]

Turns out most of the errors were not directly related to cooked caps. Unit started to misbehave once it warmed up and threw again dozen of ohms function related errors.

Ohm range current supply was bit off(97...90uA) for 100uA 20kOhm current supply and it showed considerable drift.
TIP32C transistors on A3 resistance measurement PCB run rather hot and PCB is slightly toasted in the general area of the transistors.
Replacement of TIP32C transistors and nearest BC856A (SMD marking 3A) seemed to fix the the resistance function.