Electronics > Repair

Keithley 6517A Electrometer: dead voltage source; Replacment for 2SK1412 MOSFETs

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Kleinstein:
Except for CR303 the positive side now makes some sense. They may be something the other way around or so.

VR305 and likely also C322 should be more to the left as part of the gate control. C322 is however a bit odd.

The negative side could in theory would like a constant current source and thus the amplifier like a class A type.
For this the link from R558 to VR307 should however be on the other side or the diode.

the still open end of C339 is likely going to one of the gate controlls.

DB4UCH:

--- Quote from: Kleinstein on November 01, 2024, 07:34:47 pm ---Except for CR303 the positive side now makes some sense. They may be something the other way around or so.

VR305 and likely also C322 should be more to the left as part of the gate control. C322 is however a bit odd.

The negative side could in theory would like a constant current source and thus the amplifier like a class A type.
For this the link from R558 to VR307 should however be on the other side or the diode.

the still open end of C339 is likely going to one of the gate controlls.

--- End quote ---

You are right, I drew the link between R558 and VR307 wrong, in reality it connects to the cathode of VR307. I triple checked the circuitry connected to the gate of Q311 and to CR303 and it seems to be the way I drew it in the diagram. C339 actually only connects to the anode of the LED in U310.

Greetings,

Simon

PS.
During reverse engineering I found that one of the solid-state relays (K302) was faulty. (ca. 25 Ohms between Pin 4 and 6). Interestingly, all of those solid-state relays (K302,303,304) are a different model from what is specified in the service manual. (LH1540 vs the specified LH1056AT). Judging by the date codes and soldering, those are most likely original to the instrument. And those can still be ordered from Digikey and the likes! So, I will replace all of them. (Another one is suspect as well ca. 500kOhm vs. the specified at least 500MOhm Roff)

DB4UCH:
An update from my repair attempt:
After finding another broken part, this time a SMD resistor (open circuit) & installing all the new active components, the source now lives! It accurately outputs stable DC voltages in both ranges, so between ±100V and ±1000V.  ;D
(Neither the source nor the meter is calibrated at this moment (or were in the last 20 years), so I'm not really able to accurately judge, if it meets its specs)
I used the 2SK1412 MOSFETs from the link voltsandjolts had sent and they seem to be genuine. (Behave similar in DC and regarding their capacitances)
Attached is an image of the Electrometer measuring the output from its own voltage source.
I will now run the source for some period of time to "burn it in" and to see if it fails again.
Thanks for all of your help!

Greetings,
Simon

P.S.
What are your opinions on recapping the whole unit. (I think Keithley 2000s from this era suffer from leaking caps) Does anyone have experience with leaky caps in those electrometers or are they generally not that problematic?

voltsandjolts:
Thanks for the update on your successful repair. Very nice unit, with perfect looking VFD display too.

IMHO recapping a 20 year old, yet valuable unit like this would be sensible, for both electrolytics and tantalums.

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