math_indy: I see some problems in your approach. It is not very clear in your video, but it seems that you are touching these very sensitive areas with your fingers. Perhaps you know what are you doing and you also say that in video.
But anyway for everybody: Never touch picoamp parts with bare fingers. Of course I break this rule sometimes.
But I touch only leads, since there is no change in resistivity. Sensitive are insulation areas, like hi-ohm resistor bodies. When I manipulate with these parts, I use tweezers. When risk of touching, gloves are must.
I don't think methanol cleaning is the best idea when I am not absolutely sure what exactly the problem is.
Please read this topic, there are valuable information:
https://www.eevblog.com/forum/testgear/keithley-64856487-teardown/There is also a schematic of input of this instruments. Since it is descent of K617, the principle is the same, ZeroCheck included.
Also that 1240-0197 COTO relay is a special shielded relay ! You definitely cannot replace it with 7301-05-1000. It can perhaps work in DC area since it has good insulation, but transient response would suffer. That guard shield helps condition material between contacts and outer world to zero voltage difference, so it wouldnt suffer stabilizing time.
Explained here:
https://www.embedded.com/print/4375459Modern replacement should be 1203-0147. Unfortunately no datasheet nor information on COTO website. I emailed czech and german representative yesterday about this relay since I am also interested in it! No answer yet.
1203-0147 is used in Keithley 6514 and 6517A.
I would use MadTux approach. Pull out old reed and replace it with new one. Dont screw guard foil. Perhaps Standex-Meder 1A66 reed should work too:
http://cz.mouser.com/ProductDetail/MEDER-electronic-Standex/KSK-1A66-1015/?qs=KFo7JewZbUHhbHIVxk%2FaJw%3D%3DIt seems that reed is covered with polyetylene (third best insulator after sapphire and teflon) and wrapped in shield foil. Therefore I would wrap new reed with foil and then try to insert it into coil package.
AD) Also, It is a luck this bad relay was discovered since it is really bad. But if relay has some small leakage problem, it wouldnt be discovered in nanoAmp range and unshielded arrangement.