HP 3465A DMM - zero ohm problem... - Page 1
Forum section: electronics / repair

author: e-doc
author_rank: Contributor
timestamp: April 03, 2016, 08:10:15 am
content: My HP 3465A has a problem to display zero in the lower ohm ranges (10, 100, 1k and 10k).The offset is about 1-2 ohms. At the beginning it was up to 10 ohms, but changing on moving the gang switches.So I tried to clean the switch contacts with a few drops IPA per switch. After moving each switch a few times the offset reduced to 0.3 ohms.I also cleaned the 4mm input sockets using IPA and Q-tips. The first result was not too bad: 0.17 ohm.But over night the problem came back (1- 2.5 ohm offset).BTW: what's the specification on shorted input?What can I do now?I don't want to contaminate the whole gang switch area with IPA residue by using more than a few drops directly to the contacts.I also dont' want to remove the lubrication of the contacts completely...(Edit: picture borrowed from IntelJoe)

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2016, 08:55:38 am
content: Often these sort of problems in CRO's are remedied with switch "exercise".Tarnished contacts result in "noisy" changes of range selection and the contact lubrication is best totally removed with either IPA or as many recommend Deoxit along with exercise.Of course the contacts are gold plated and they need be protected from wear with an appropriate lubricant.

author: e-doc
author_rank: Contributor
timestamp: April 03, 2016, 09:46:13 am
content: I forgot to say it is a DMM not an oscilloscope, but you are right, the contacts often causes problems...I think DMM switches are more problematic because isolation usually must be much better and contact resistance usually must be lower than in oscillosope switches.What is the best (easiest) way to remove the lubricant and clean the contacts without complete disassembling the switches or instrument?Is using a few drops of cleaner and moving the switch enough? I just did it this way with not too much success...How to "relubricate"?

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2016, 10:06:11 am
content: See the posts in this thread, there should be all the info you need:https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/

author: e-doc
author_rank: Contributor
timestamp: April 03, 2016, 10:20:33 am
content: Thanks for the link.Is "Deoxid" cleaner and lubricant worth the money?Can I buy it in Germany?

author: tautech
author_rank: Super Contributor
timestamp: April 03, 2016, 10:49:45 am
content: Quote from: e-doc on April 03, 2016, 10:20:33 amDon't know.It is the most quoted contact cleaner on the forum AFAIK.Here's the manufacturers page:http://store.caig.com/s.nl/sc.2/category.188/.f

author: The Soulman
author_rank: Super Contributor
timestamp: April 03, 2016, 11:48:13 pm
content: How do you short the inputs? 0,17 ohms doesn't sound that wrong to me for a 2 wire ohms meter.

author: Vgkid
author_rank: Super Contributor
timestamp: April 04, 2016, 01:34:24 am
content: That is fine for a 2 wire measurement. Might even be acceptable for some 4-wire multimeters.

author: e-doc
author_rank: Contributor
timestamp: April 04, 2016, 12:02:34 pm
content: Quote from: The Soulman on April 03, 2016, 11:48:13 pmI use one of these:0.17 ohms is OK, but now I have up to 80 ohms (fluctuating).

author: babysitter
author_rank: Frequent Contributor
timestamp: April 04, 2016, 03:33:27 pm
content: Teslanol Oszillin T6 bekommst Du recht leicht... Finger weg von CRC Kontakt 60 ohne Nachbehandlung, das Zeug macht zwar schön sauber, aber nach dem reinigen versucht es weiterzureinigen, greift also Zeug an. Mit einer Nachbehandlung wie ausspülen aber verwendbar.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 04, 2016, 03:56:03 pm
content: Any resistance in the ohms "path" will cause this. I had an issue with my Fluke 8840A where (apparently) dissimilar metals may have caused this where the front input jacks attach at their rear side. The small metal plugs which attached to the rear of the jacks appeared to have a "zinc-oxide" powdery appearance. Cleaning them, tightening and reconnecting fixed the issue. Working flawlessly for three years plus since then. Don't rule out any relay, switch, etc in that path. The fact that it is somewhat intermittent works in your favor. You can push, poke, tap around the area to help localize the problem. Using a 'scope will help you "see" any noisy connections.

author: e-doc
author_rank: Contributor
timestamp: April 04, 2016, 04:53:57 pm
content: Quote from: babysitter on April 04, 2016, 03:33:27 pmDanke!400ml only 5€https://www.buerklin.com/de/katalog/Kontakt-und-Tunersprays-Oszillin-Typ-Teslanol-t6-L636100.htmlHört sich gut an:QuoteHoffentlich Tatsache und kein Voodoo..."Kontakt 61/60"Eines davon war aggressiv, hat sich nach ein paar Jahren durch die Spraydose "gefressen".Weiß bloß nicht mehr ob es das "60" oder das "61" war. Habe seitdem lieber die Finger davon gelassen (von beiden).PS: English translation:"Kontakt 60/61"One of these two was so aggressive, the can was eaten through from the inside after a few years of storage.Don't know if it was "60" or "61". Kept my hands off it since.

author: babysitter
author_rank: Frequent Contributor
timestamp: April 04, 2016, 07:06:54 pm
content: Habe mich schon seit 13 Jahren nicht mehr so recht drum gekümmert: Das Aggresive ist das 60, ist auch ein bischen legitim für ein Reinigungsmittel. 61 ist ein öliges, könnte konservieren. Kontakt WL war meines Wissens das richtige um das 60 nach Einsatz wegzumachen. Habe das Teslanol schon eine Weile und bin dabei geblieben.

author: The Soulman
author_rank: Super Contributor
timestamp: April 04, 2016, 07:31:45 pm
content: Allo, bitte auf englisch weiter gehen oder anderes sollten die andere neugierig werden!Did you resolder all the switches and contacts jet? It could easily be a bad solder joint..

author: e-doc
author_rank: Contributor
timestamp: April 04, 2016, 08:36:09 pm
content: Quote from: The Soulman on April 04, 2016, 07:31:45 pmYes, Sir!Quote from: The Soulman on April 04, 2016, 07:31:45 pmNo, I did not resolder yet. I first have more to disassemble...