Keithley 190 Nixie DMM resto... waxy "Tropical Fish" caps & crusty switch help - Page 1
Forum section: electronics / repair

author: eurekaiv
author_rank: Contributor
timestamp: July 21, 2024, 05:36:03 pm
content: I'm working on repairing/restoring a Keithley 190 nixie DMM and I'm not sure what's going on with all the .1uf tropical fish caps on the board. They have this dusty and flaky white crap on them, which has built up around the legs and solder pads in most cases, in addition to having made its way around the PCB in the form of some kind of white dust that's collected in some spots. I haven't pulled one of them yet as I kind of want to know what this is before I start trying to desolder. It does seem non-corrosive, does not respond to vinegar and does not clean up with ISO. It's almost like a waxy substance. I don't think I've encountered something quite like this in my rather modest time messing about repairing electronics.Also, the switches on this thing were basically frozen when I got it, but are open and easy to access. I have them all moving after two or three flushes on contact cleaner and plenty of actuations in between each flush, yet they still feel very stiff. I'm not sure if this is normal for this meter or not. Is there any recommendation for lubricant of these switches or should I run them dry?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 21, 2024, 06:16:13 pm
content: Philips/Mullard caps. I used a lot in the 1970s. I think that was some kind of coating to increase moisture resistance. Doubt it has had any effect on the capacitor itself.(Sometimes called liquorice allsorts because the stripes resemble one of the varieties in a pack of Bassett's liquorice allsorts here in the UK.)

author: Gyro
author_rank: Super Contributor
timestamp: July 21, 2024, 06:26:30 pm
content: As far as the tropical fish capacitor goes, they did have a waxy coating*that would melt if the capacitor got hot during soldering. It could well be that some was deposited on the pads during flow soldering. If you're concerned, it's just a standard 100nF 250V Polyester. It does add a nice colorful period look to the board though. Go easy with that vinegar though, it's useful for neutralizing alkaline battery spills, but can be just as corrosive on it's own, make sure you've cleared any residue with distilled water as well as the IPA (ISO).For the switches, if they are standard open wafer switches, they probably have some dry corrosion between the shaft and front panel bush. Contact cleaner will probably have washed out any remaining lubricant. Ideally you would disassemble them, clean, and add a little grease. As an alternative add a drop of clock or sewing machine oil to the knob end of the bush (after removing the knobs) and exercise the switch. Lubricating from that end makes it less likely that the oil will migrate. Greasing the detent mechanisms might help too.Edit:*As wasedadoc says.

author: eurekaiv
author_rank: Contributor
timestamp: July 21, 2024, 08:11:34 pm
content: I just put a tiny bit of vinegar on a pointy swab and touched the buildup at the base of the capacitor to see if it would effervesce, so very little was used. I then went in with the IPA which hopefully cleaned any residue up.The switches appear to be a molded body with an open end (looking at them in circuit) so I don't think I could disassemble them to clean. I'll post some more pics in a bit.

author: eurekaiv
author_rank: Contributor
timestamp: July 22, 2024, 02:31:54 pm
content: Thought I'd update. The switches are good after a few flushes of contact cleaner and the tiniest amount of lubrication on the bushings. Thanks for the suggestion. I also had a bad 200uf cap in the power supply, so I replaced all 3 electrolytics, and it's working fine now. Just seems to be a bit out of spec maybe, when comparing to my old Fluke 45, which itself is probably a bit out of spec.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 22, 2024, 04:56:30 pm
content: thoses switches can be dismantled but you need to fully isolate them mechanically, dismantling any metal braces who connect them where the spring is located, and you may have a small formed metal part who act as the lock and release pin ....lots of work, i would not do that, use contact cleaners / deoxit .....