Keithley 616 electrometer problems - Page 1
Forum section: electronics / repair

author: Ahax
author_rank: Newbie
timestamp: October 09, 2025, 09:35:58 pm
content: HelloSome time ago I acquired a Keithley 616 electrometer. It was powering on but the display was in constant over-range. I fixed that problem by replacing an op-amp (LM301AN with NE5534P), now it seems to be working. I can perform some measurements. The unit was very dirty inside so I cleaned it with 100 percent IPA. I used a toothbrush and some q-tips. The problem is it appears to be very noisy (1000fA with open inputs) on current range after 2 day warm up time, zero check works ok. Also when I put it in voltage mode it goes up to 260mV and stops (before I cleaned it it went to 5V with open inputs). If I apply a larger voltage and then disconnect the probe it just drops down to 260mV. The number 260 is also frequently displayed no mater the range or what I'm measuring (not always, just sometimes, after I put it in zero check and back it usually goes away). I was careful to not leave any contaminants on the boards, and I wore gloves and a ESD strap when cleaning (I also checked for any strand left behind). The PCBs were coated in some sort of clear coating that was just flaking away, is it possible that I messed the unit up by cleaning off this coating? Any idea on how I should proceed?The LM301AN I replaced is marked in the datasheet as ITS6214, one is on the differential amplifier board (Q101), and one is on the ranging board (Q301), they both seem to be original factory components, both LM301AN. I also tried replacing Q301 with AD797ANZ, but I got constantly rising offset. NE5534P appears to work fine.

author: jwet
author_rank: Frequent Contributor
timestamp: October 10, 2025, 03:30:29 am
content: A couple of things that might help.You "probably" didn't do any lasting harm to the unit by cleaning it up. These things are really fine instruments and are extremely sensitive to contamination. Picoammeters are as close to real magic as electronics gets. 100% IPA should be residue free, most IPA's available commonly here in the US are 91% or in rare cases 99%- the rest being distilled water. I don't have a source for 100% Isopropanol. Inspect things very carefully and get them really clean. No fingerprints, etc.The coating you saw is humiseal or conformal coating. Its a low leakage clear finish to keep air and moisture out. After you get things completely fixed and working, it can be bought and reapplied. Don't do this until things are completely working and things are really clean otherwise you're just locking in dirt. Google Humiseal and find a grade that is suitable for electrometers.I also wouldn't make ANY substitutions in this instrument in any way. I would not only get exact replacement for the devices but would even try to match the manufacturer and the general date codes of the parts you're replacing, parts are tweaked over time and these instruments are extremely sensitive with lots of real analog magic built in.The input connection to these things is usually a triaxial connector with a shield and guard signal. Don't use adapters, etc or if the instrument has been modified for BNC, etc, it should be restored.There was a good teardown of a Keithley 616 in 2014 here on the EEVBlog by a user named Muvideo. You can search for it, pretty comprehensive.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 10, 2025, 08:50:08 am
content: The NE5534 is quite a bit faster than the LM301 and this can cause oscillation in some places. A compensation capacitor to slow it down can fix the speed problem, but it still has more current noise than the LM301.In many cases the old 741 is a much better match to the LM301.There are quite some OP-amps in the meter and a suitable replacement depends on the position. From a first glimps the OP-amps are generally not relevant for the noise performance and thus nearly nothing gained from using lower noise types.Even of seamingly clean, dirt can still be an issue. High humidity can also make dirt more critical.

author: Ahax
author_rank: Newbie
timestamp: October 10, 2025, 12:02:37 pm
content: After some head scratching I found that the only relay in the meter was leaky. 80M ohm with open connections leaky. Desoldered one end and now the meter behaves like it should. The relay is comprised of a separate coil and a reed swich. I have some new reed swiches at hand, do I need to get a special low leakage one or will just any do?