Keithley 2001 "No Comm Link"

[C0nr]

Hello there, I have here a Keithley 2001 witch show powered on just „no comm link“. I replaced all the old capacitors, erased and reflashed the EPROMS but the problem is still alive.
With the help of the service manual form xdevs.com I checked the voltages of the power supply and recognize a strange behaviour at U103. Pin 2 should have 8 volts but I measure about 1.2 volts.
Should I replace U103(LM317T)?

Also, on the measure points the -/+ 8 VF are not there. At the beginning +8VF is about 7.5 v and -8 is about -5.5v. The longer the DMM is powered on, the voltages change. After 10 minutes it is about at -2,4V and +11V. I tried to understand this behaviour with the schematic but I have no idea where to look exactly or what it could be.

I also discovered a spot which looked like an arc to me. R303 is one resistor before the TLPs. If it was an arc could that crack the TLP U311? I measured the diod of the TLP U311 (Pin 1 and 2) which looks normal compared to the other optocouplers.

TiN wrote in thishttps://www.eevblog.com/forum/repair/keithley-2001-meter-went-for-cal-working-arrived-dead/ post what potentially could cause that comm error.
 

Sometimes, esp on old meters the cable between boards get dodgy, resulting in your comms errors.
Other times DC/AC/DC converter (ERG potted brick or little PCBA on digital board) that powers VFD glass crap out, making CPU to go sick and resulting same comm errors, that happen to units I serviced (more than one) .
Or ADC board went loose, that will result comm error. Or the TLP opto on analog board went poo, also comm error.
Or the analog board caps leaked their goo, and ruined everything. 

I checked the cable between the boards witch seems ok. ADC board is also not lose.
Can someone tell me how I can check the other compontents?

[coromonadalix]

Seems an arc to me,  be sure to toroughly clean the board, and measure the components, the resistor and the TLP

Did you measure voltages over 300vdc ??  there was talk over Keysight meters who had component (s) who could not stand the full meter specs 1kv at the input ...

If the voltages drop, is there any anormal regulator (s) dissipation after your 10 minutes ??  do you have an flir or any electronic thermometer ?? 

You may have something who consumate more current and may affect another voltage  Ex:  if you have some virtual ground, or symetrical supply (ies) ...

Is the pcb  was "unseated" ??   https://www.eevblog.com/forum/repair/keithley-2001-meter-went-for-cal-working-arrived-dead/

And finally if you use the comm port, is it isolated ??  Normally you use usb and or serial isolation between pc and usb related equipment, i always do.

Service manuals and repairs and firmwares dumps  :       i'm on my cell phone can't see it for the moment.
https://xdevs.com/fix/kei2001/#internal

[TiN]

Well, you gotta repair power supply and replace all capacitors on both analog and digital boards. That is your number one job BEFORE turning on anything with this 2001's units. Once you have that done, then you can worry about digital issues. Old TLP optos may go bad too, I had to replace few before. Cable between boards may also "look" ok but actually missing connections on connection points, because it's just crappy crimp flat cable.