Keithley 155 restoration project - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2024, 05:12:55 pm
content: I've recently grabbed this null meter - I wanted one for quite a while, as in my opinion, it may be the best of any options available. This is due to the SS chopper, form factor, reliability, options. I like just about everything about this instrument.This one's in just about pristine physical condition - if I didn't know anything about it, I'd think it's NIB and just came out of the box. Even the batteries inside are original, in perfect shape (though... we'll see, maybe there's surprises at the underside of the holder?...), and they only fail marginally (one polarity is a bit under 8V...).One thing I need to sort out is the mode switch at the back of the 1554 power supply module. The switch is very reluctant to move. I'm also reluctant to use contact cleaner or even heavier lubricants (I think the switch is moving on dry in a way that makes me suspicious it will fail soon), from the perspective of leakage. The unit is advertised to not present any degradation of its floating characteristics and any leakage to AC line. Any thoughts would be appreciated on how to handle this.Also, I'd greatly appreciate any specific files or info on the 1554 power supply module. Particularly a schematic would be nice. I'd like to both rebuild it, but also sort out the switch smooth operation issue. The schematic would help assess the impact of treating that switch with contact cleaner, I wager.I'm currently printing a 3D holder from this forum - thank you lowimpedance!! - and will replace the batteries with 9V lithium batteries, which I assume will hold forever and, even more importantly, not leak!Thanks a bunch!

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2024, 05:13:45 pm
content: A couple more pics.

author: andrewtaylor
author_rank: Contributor
timestamp: November 15, 2024, 06:55:56 pm
content: Congratulation to your new meter -- really in great shape.I am shure you will enjoy to work with it, it is a very reliable instrument.

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2024, 07:34:44 pm
content: Quote from: andrewtaylor on November 15, 2024, 06:55:56 pmThank you very much - that's where my efforts are currently focusing - making sure it remains reliable and I extend its life even further. I may examine a selective recapping, 3D printing (currently underway) a housing for modern 9V batteries, maybe a couple other things.It'd be great if I can get a hold of some materials on the 1554 module though.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 15, 2024, 07:51:26 pm
content: Your 155 looks very niceThis was my projecthttps://www.eevblog.com/forum/repair/keithley-155-null-detector-repair-and-restauration-attempt/

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2024, 08:04:39 pm
content: Quote from: HighVoltage on November 15, 2024, 07:51:26 pmGreat, thanks a bunch HighVoltage!I assume if you had any materials on the 1554 module, you probably would have posted it on the old thread and/or already mentioned it here, right?I'm currently printing the battery holder - quite a chunky thing, probably about 12 hours long when all said and done and I can unstick it from the hot plate - and will post pics once it's installed.

author: Rax
author_rank: Super Contributor
timestamp: November 15, 2024, 08:19:35 pm
content: Quote from: Rax on November 15, 2024, 08:04:39 pmWell, as soon as I said that, this happened - see pic - !!!Darn, I was about 7 hours into the print. It looks like my 3D printer jumped a certain offset to the right. Anyway, OT, I'll seek to figure what happened maybe on the 3D group.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 16, 2024, 05:01:43 am
content: Would be interesting to see pic's from inside the 1554 moduleAs for the battery compartment it's the dissolving foam and it's adhesive I had to remove from mine , and from your pictures that is what you have waiting on your unit. Not much fun.The rest of your K155 looks in nice condition, good testing needed before bothering with any thoughts of selective re capping .

author: guenthert
author_rank: Frequent Contributor
timestamp: November 16, 2024, 10:20:17 am
content: Quote from: Rax on November 15, 2024, 05:12:55 pmI haven't studied the Keithley's manual and I wouldn't trust it to describe a decades old specimen, but one feature which makes null meters still interesting is their high impedance to ground. Once you got more pressing issues resolved, you might want to measure that; on battery it should easily be >100GOhm (iirc HP419A is specified for 100GOhm, Fluke's 845A for 1TOhm), not sure what to expect when on mains.And yes, congrats to the lovely and still (or soon again) useful instrument.

author: Rax
author_rank: Super Contributor
timestamp: November 16, 2024, 04:17:51 pm
content: Quote from: guenthert on November 16, 2024, 10:20:17 amAny thoughts on what not to do with that switch, given the lack of documentation/schematic? I may carve out some time to document that 1554 module with pictures, and if things go extremely well, maybe draw up a schematic, but I really have no inkling when I'll have that opportunity given the other current projects.I have a few options, including MG 801B, certainly Deoxit (D5), maybe even some grease. Leaving it alone is also a possibility - I assume once I have some lithium batteries in there, I may never need to run this off of the mains - but if I used it it's just a matter of time before it finally fails, I feel.Quote from: guenthert on November 16, 2024, 10:20:17 amThanks, guenthert!

author: Rax
author_rank: Super Contributor
timestamp: November 17, 2024, 03:05:00 pm
content: Quote from: lowimpedance on November 16, 2024, 05:01:43 amHere you go!

author: Rax
author_rank: Super Contributor
timestamp: November 17, 2024, 03:22:30 pm
content: Quote from: lowimpedance on November 16, 2024, 05:01:43 amSo true. I've spent a good part of yesterday digging into the mess in there. I put a sheet of paper between the messiness and the PCB - I hope I managed to protect the electronics.

author: Rax
author_rank: Super Contributor
timestamp: November 17, 2024, 07:55:11 pm
content: Finished project (the batteries part, that is).

author: Oldtestgear
author_rank: Regular Contributor
timestamp: January 01, 2025, 10:24:53 am
content: Keithley 1554 PSU schematic as requested. Hope this is what you need.My mains PSU arrived with a dud transformer. After a bit of trepidation I replaced it with a cheap transformer without the nice screening & was surprised that it was stable down to the microvolt ranges.CheersPhil

author: EC8010
author_rank: Frequent Contributor
timestamp: January 01, 2025, 10:38:49 am
content: I note that guenthert mentioned that null meters have a high impedance to ground and your power supply diagram shows a mains transformer having E/S screens around both primary and secondary. That typically reduces CPSto <5fF. The significance being that it keeps that high impedance high to mains frequency and harmonics. An off-the-shelf transformer will be >20pF. For sensitive work, you'll want to use battery power. It would be interesting to measure CPSon that dud transformer.