Products > Test Equipment
Keithley 6485/6487 teardown
plesa:
6487 does not have guard at all.
Input stage of 6485 and 485 it different, 6485 use the opamps and 485 use discrete FET.
Razberik, did you tried LMC replacement? I do not think there is any matching nor preselection of them for this entry level meter.
razberik:
Hello there.
A big credit goes to SKE. After a few talks he came with a theory.
Lets analyze the most simplified loop for nanoAmpere range. I omit capacities and use most simple opamp model.
Firstly we apply Vsource+resistance to give about 2nA.
U26 creates about 1V across 500Meg resistor. This is the way it should be. When I place 1TOhm at the input to GND, it simulates open input, the U26 output settles aprox. at zero.
We can now believe that U26 and U50 are OK. So the wrong part can be propably U25 or U27.
If we assume that the instrument died after crazy relay clicking and U27 works probably as an input protection (I was told that the only input FETs are used, it is easier to buy this cheap opamp than low-leakage diode itself).
So If I bring some leakage into input, I receive the exact behavior like on real instrument.
Shortly ... this was true. When U27 LMC662 was desoldered, instrument started to work. I attach pictures to prove it. ;D
One thing interesting. I soldered new LMC662 and washed the soldering area using IPA in few cycles, but there is still some residue remaining at the pins (tiny flux ring around pin). This doesnt affect the input current.
plesa:
Congratz to successful repair. In your box is lot of space for input protection so I will consider to add it.
TiN:
Very nice job documenting and getting down to a rabbit hole, thank you!
Can I use your info to add in my 6485 article, with link to this thread posts?
Smith:
The 6485 is a nice device, just like the older 485. I use both, but the 485 has two main advantages: the connector is positioned on the front (just like me) and the autorange doesn't tend to keep switching between two ranges in particular cases. The latter can be solved on the 6485 by lowering the reading speed.
For everyday use I mostly use the 485, it's just so easy to use and fast and stable. I think it was build somewhere in the 80's, but it's still spot on.
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