Dear community,
i am struggeling with my Keithley 6517A that i bought on ebay.
The main problem is offset drift in V-Offset and I-Bias-Offset.
The unit will pass startup tests without reporting faults.
On startup it shows main firmware REV C05 and display firmware REV A02.
The highest date code inside that i could find is 07/45 so the unit should be manufactured in 2007.
The last calibration according to CAL constant was Feb/2008.
The Voltage Source seems to be correct. I checked some cardinal values from -1000V to +1000V and the output is stable and pretty accurate.
My troubleshooting so far:
I checked all power supplies.
All values according table 5-3 of the service manual:
Check point Required value Measured Value
Reference to Common K
Q305, pin 1 +5V, +- 5% +5,03V
U301, pin 1 +12V, +-5% +12,12V
U302, pin 3 -12V, +-5% -12,00V
+120VK (VR301 anode) 110V to 130V +124,0V
-120VK (VR302 cathode) -110V to -130V -125,1V
+1200VK (VR303 anode) 1100V to 1300V N.A.
-1200VK (VR304 cathode) -1100V to -1300V N.A
Reference to Common B
+5VB (U408, pin 3) +5V, +-5% +4,97V
-5VB (Q401, pin 3) -5V, +-5% -5,06V
Reference to Common S
+25VS (C461 +) 22V to 28V +30V
-25VS (C460 -) -22 to -28V -30V
+5VS (U207, pin3) +5V, +-5% +4,94V
+220VS (C401 +) 200V to 240V +250V
-220VS (C430 -) -200 to -240 -249V
+15VS (U206 pin 3) +15V, +-5% +14,90V
-15VS (U205 pin 3) -15V, +-5% -15,23V
These seem to be all good.
The +-25VS and +-220VS are a bit high. But these are unregulated and i saw two other repair projects ( TIN and another guy, i forgot) where these Voltages are similar high.
I also checked the PS of the VFD on the board after the transformer. Above 60 Volts and looks clean on the scope.
This is how the failure looks like:
When i apply a voltage to the input and release Z-CHK ( zero check ) then the displayed value will ramp op ( or down ) slowly with a rate of around 1 mV per second.
If i let it happen then it will settle after a couple of minutes.
The same happens when i short the input with a triax-short. It will not always settle at the exact same value and it somehow depends how often and fast i „trained“ the relays by switching Z-CHK on and off in short sequence.
Because of this behaviour i cannot perform the V-Offset calibration.
It will usually stop after some time with Error 413 -> „Voltage offset not converging“.
Very seldom it will stop with Error 421 -> „Voltage Offset out of spec“.
I managed only one time to succeed with V-Offset but the next step after that is always I-Bias-Offset measurement and that failed.
I-Bias is almost the same but a bit more unstable and it takes longer to settle ( if it settles !).
It usually ends with Error 414 -> „Current Offset not converging“.
I dont think i made it to Error 422 -> „Current Offset out of spec“ ( but not sure ).
Sometimes it is not possible to measure in V mode. The display just shows ——— on the left side.
I found out that it helps to rotate the unit 90 degrees left ro right ( and sometimes knock it a little).
After that i will get displayed measurements again.
Sometimes it is not possible to switch to V mode. (Seems like the relays are not engaging ). Usually here it also helps to rotate the unit left or right.
These problems are only apparent with the Volt mode.
Could it be that the unit was damaged during shipping?
It was very poor packaged, only thin cartboard, no cushioning, no extra foam, nothing!
In fact the rear plastic frame broke completely during the shipping.
I have checked a lot of information repairwise on the internet.
I found TIN´s Video and „teardown“ of the reassembly of his 6517A.
teardown ->
https://xdevs.com/doc/Keithley/6517/img/em_fe.jpgvideo ->
https://www.youtube.com/watch?v=Thread about replacing leaky reed relays in the Keithley 617 ->
https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/The Keithley 617 is an older model but seems to be very similar in the analog part.
I think this guy, Tom Mathews also responded in one of the numerous K617 repair threads.
He exchanged some ( or all? ) of the reed relays because they were „leaky“.
I think this could be my problem too, but i never saw him showing how the failure looked like.
( and i dont want to exchange all the relays and find out that it does not help ).
I am not an electronic engineer, but my skills are getting better fast.
I learned a lot already ( also from many mistakes ! ).
This time i want to take a good systematic approach to this repair.
I dont want to just guess and poke around and change some random parts.
First because the unit is in good condition besides the failure.
Very nice cosmetic condition ( some dust inside on the edges of the analog board) and there is really no sign of someone „troubleshooting“
the board before.
And then i also really want to understand how the input stage works.
Unfortunately there are no schematics in the service manual.
But i could maybe use the schematics of the K617 and try to „reverse engineer“ the input stage.
That all said i will be very thankful for ANY advise!
I dont expect you guys to talk me through this.
If you give me some pointers i will do my „homework“ and try to find the solution myself.
If you need pictures or a video of the ramping offset just tell me.
Thank you very much for helping!
Joerg
Edit : changed link from youtube-video to thread about replacing reed relays in K617