Keithley 2602a wrong Voltage after a couple of minutes

[Aheld]

Hello, 
My Keithley 2602a seems to have an issue with output voltage and current measurement.
After powering up, it works fine. But after a couple of minutes, the values are complete unrealistic.
Tested with 4R7 resistor, with output voltage setting at 500mV, it messures a negativ current of a couple of hundret mA. The 5W resitor started with overheating. Does any one has any idea where to start with searching?
Is there any source to download or buy schematics?

Thanks a lot.
Aheld

[MathWizard]

here's the manual with lots of user interface help, funtions, etc. But no schematics is given, IDK, it's probably not given out if it's built in early 2000's
https://download.tek.com/manual/2600AS-900-01_B-Sep2008_User.pdf

could be some bad part or solder joint heating up and not working, or some chip not happy and shutting down something

[Aheld]

Thank you for your reply, but the user manual can‘t help on this topic. I just opened the device and tried to find some test points to check the voltages. But t here are only a few testpoints visible and labeled with voltage levels.
In my mind the defect needs to be located in power supply or other central position, because both channels are affected.
If someone has information about the colour coding of the supply cables and the voltages in channel switched off and switched on mode? That would be helpful to find a solid starting point.

[MegaVolt]

Photograph was found on this forum.

[Aheld]

Thank you very much. It seems that used wrong words for searching in this forum.

[MegaVolt]

There is one good person who has collected everything that can be found on the Internet: TiN.
Here is its repository: https://xdevs.com/doc/Keithley/2601_2602_2604/
Here is the file that I shared. And if you go a little higher in the catalog, there is more information about 26XX where XX differs from 02

[Aheld]

I had first success. The first device 2612 is up and running again.
Some main voltages on the power connectors haven‘t been there. The effect was that the output voltage was wrong and only negative.
I changes all Elkos to new one and found a couple of defect elkos.

I‘ll post the voltages at the connectors soon. Maybe it helps for other repair trails 
Thanks MegaVolt for the tip on xdevs.com