DMM Not Accurate after Cleaning it PCB - Page 1
Forum section: electronics / beginners

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 02:41:11 am
content: I got a second hand Keithley 2015 DMM.For a 0.014 OHM short cable (measured by another good DMM -- Keysight 34465), this K2015 showed 0.05 OHM before doing anything.After replacing two 2200uF/35V ECap and RS232 transceiver, I also cleaned the board.Then, for the same short-cable, the K2015 measured resistance is 0.23 OHM.Is that because I do not clean it well?Or, is that because I clean it too good?I dont' what to send it for calibration due to it's too expensive.

author: Nusa
author_rank: Super Contributor
timestamp: October 20, 2017, 05:09:27 am
content: More likely to be because you changed the caps.Both before and after measurements are off by 0.09 ohms. I don't see that it's any more or less accurate than before. Or it may just mean you have more contact resistance on the connection this time.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 20, 2017, 07:09:54 am
content: In two wire mode the resistance measurement is not very accurate due to contact and possible internal relay resistance. To get reasonable accurate readings, there is an internal compensation that can usually be done by the user (2 wire Ohms zero). Usually one does not care that much about this error as the right way to measure low resistance (e.g. < 100 Ohms) is using the 4 wire mode.Dirt on the board is more effecting the opposite end of very high resistors ( > 10 M). So after cleaning the more important points to test would be those large resistors and input bias (e.g. the voltage measured on a 10 M resistor or drift rate with a low leakage capacitor in the 1-100 nF range). If not done right cleaning can make things worse.

author: wraper
author_rank: Unknown
timestamp: October 20, 2017, 07:36:52 am
content: Describe how you cleaned it. Did you clean the whole board?

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2017, 07:47:40 am
content: Trimmer potentiometers won't like being washed, especially with soap and water and it's easy to accidentally alter the pot's setting, when wiping them.

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 09:29:17 am
content: Quote from: Kleinstein on October 20, 2017, 07:09:54 amThanks for explaining.

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 09:36:53 am
content: Quote from: wraper on October 20, 2017, 07:36:52 amAt the beginning I used baby wipes (short-circuit resistance is 0.19 OHM). After noticing the accuracy issue, I bought Alcohol swap (Isopropyl Alcohol 70%) and cleaned the circuit twice (short-circuit resistance is 0.23 OHM).I only cleaned Keithley 2015 top board, which is DMM. The THD board never cleaned.

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 09:40:11 am
content: Quote from: Nusa on October 20, 2017, 05:09:27 amProbably. When I disassemble the board, the front-rear mechanical switch is not easy to deattach from the button. It's bent during disassemble and assemble.

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 09:45:12 am
content: Quote from: silentsnow on October 20, 2017, 09:40:11 amAmazing.I shorted the rear port, and it shows 0.01x OHM (no lonter 0.23 OHM).Does this mean I damaged the rear-front switch? Which has higher impedance (additional 0.2 OHM) for front port?

author: wraper
author_rank: Unknown
timestamp: October 20, 2017, 09:46:12 am
content: Quote from: silentsnow on October 20, 2017, 09:36:53 amPff, those contain a lot of stuff which will contaminate PCB. I guess you'll need to to submerge this board into IPA to somewhat try cleaning that crap from under SMT components.

author: silentsnow
author_rank: Newbie
timestamp: October 20, 2017, 09:55:34 am
content: Thank you very much.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 20, 2017, 11:52:29 am
content: Quote from: wraper on October 20, 2017, 09:46:12 amBefore doing possible more damage, I would check how bad leakage actually is. With the wrong kind of cleaning things can get worse, but there is a chance that nothing bad happened even with the wrong cleaning method.The front rear switch night be one cause of higher 2 wire resistance. Some switches get a little higher resistance if not used for a long time. So it might be a good idea to just use the switch for some 10-50 times - with some switches it helps to reduce surface oxide and remove dirt.

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2017, 12:55:17 pm
content: Quote from: wraper on October 20, 2017, 09:46:12 amThat might not be enough, as the baby wipes may contain substances which are not soluble in IPA but are in water. Try rinsing with deionised water, before IPA.

author: macboy
author_rank: Super Contributor
timestamp: October 20, 2017, 01:21:27 pm
content: Quote from: Hero999 on October 20, 2017, 12:55:17 pmHe used 70% IPA, so it's 30% water. This is actually preferred in many cases instead of 99+% pure stuff. Just makeabsolutely certainthat it doesn't have any aloe vera, vitamin E, moisturizers, etc. in it. It must be just alcohol and water. If unsure, buy 99% stuff and mix some with distilled water. The mixture works great for cleaning cases and front panels too, because it tackles both dirt (due to water) and greasy or sticky stuff (due to alcohol).I'd recommend exercising the front/rear switch several times, like 20 times. Also, make sure that the wires leading to the front panel jacks are nice and secure, tight fit on the terminals. You could also try cleaning the front/rear switch.