Newbie bought a broken Keithley 176 digital multimeter - Page 1
Forum section: electronics / beginners

author: fickit1time
author_rank: Newbie
timestamp: November 12, 2017, 10:02:53 pm
content: Bought a used 4.5 keithley 175 dmm today from an old lady for $35.She didn't know if it worked but it had some parts jiggling around when I shook it and when I got it home I found out why.I've attached photos of the dmm's condition, there is some damage near the battery connection and the both fuse leads are off the board.My goal is to get it up and running and also to do away with the battery component and wire it straight to a 9v adapter ( I have about 20 of them lying around so hopefully one will have the right amperage).Can anyone tell me based on the manual i've attached how I would figure out where to attach the fuse leads onto the board?The red/black wires on the case in the 2nd photo go to the fuse built into the case.Thanks

author: fickit1time
author_rank: Newbie
timestamp: November 12, 2017, 10:07:09 pm
content: Here are some more photos of the board and damage.I've cleaned what looks like corrosion with rubbing alcohol and a brush.Here is a link to the manualhttps://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwiqpa6UhrrXAhUCjlQKHWjrCBsQFggsMAE&url=http%3A%2F%2Fwww.download-service-manuals.com%2Fdownload.php%3Ffile%3DKeithley-3509.pdf%26SID%3D&usg=AOvVaw2HU0_Nuo5KUSaxQf_VeRnA

author: schenkzoola
author_rank: Unknown
timestamp: November 12, 2017, 10:32:37 pm
content: That looks like a fairly easy repair. Your service manual lists most of the components so finding replacements for the damaged ones shouldn't be too difficult. That diode seems to have a broken lead, you may be able to clean the corrosion off of it and bridge the lead with a bit of wire and some solder. Same thing for any traces that have broken, just scrape away the solder mask, and bridge the damaged area with a bit of fine wire. I would get it running on batteries first, then consider using an ac adapter. Just be sure that the adapter you choose is properly isolated from the mains and earth. I don't expect this will use much current, so you shouldn't have difficulty finding one.

author: fickit1time
author_rank: Newbie
timestamp: November 13, 2017, 12:52:37 pm
content: Thanks for the suggestion, I've cleaned the connections to be be sure to get most of the corrosion, then I tested across the diode with continuity and I get .562V when the dmm is off, when I turn power on it goes to OL.I've also tested the test pin 1-7 according to the manual and the voltages all seem fine.I don't have any D cell batteries but I have a power supply i'm using 9V & .5AThe issue I have is there is no display, on the display board there was some corrosion which I cleaned off with rubbing alcohol and wiped down the display glass but still nothing on the display.

author: capt bullshot
author_rank: Super Contributor
timestamp: November 13, 2017, 01:04:46 pm
content: The display is a LCD with normally white background and black segments. Yours looks all black, if so then the LCD glass is fubar. You'll need a replacement.

author: fickit1time
author_rank: Newbie
timestamp: November 13, 2017, 03:01:53 pm
content: You are right, it is black. (attached some photos) I might try repairing but i'll have to do some reading first.Tried finding a replacement but no luck so far.I have found large calculator lcd screens that are about the right size and I believe 44pins just like the display.Would it be possible to use one of those instead and solder the pins to the display board?Here is the lcd driver ICM7224iplhttps://www.intersil.com/content/dam/Intersil/documents/icm7/icm7224.pdf

author: fickit1time
author_rank: Newbie
timestamp: November 13, 2017, 03:43:11 pm
content: O boy! I guess everything is possible if you find a way.Someone has soldered the pins of a newer lcd into the driver board of a keithley 129 dmmhttps://www.schwingen.org/keithley-129-display-repair/

author: capt bullshot
author_rank: Super Contributor
timestamp: November 13, 2017, 04:36:08 pm
content: Quote from: fickit1time on November 13, 2017, 03:43:11 pmYes, someone else here in the forum has done similar

author: KMoffett
author_rank: Regular Contributor
timestamp: November 13, 2017, 06:35:21 pm
content: We had a bunch of those in our Physics student lab. The almost all had failing push-button switch mechanisms. If I were you I would cut my loss and look for a working DMM on ebay or Craig's list.Ken

author: fickit1time
author_rank: Newbie
timestamp: November 14, 2017, 02:26:11 am
content: You may be right but where is the fun in that?? Besides nice bench dmm's are still about $200.Hopefully I can find a cheap suitable lcd screen I can try and I don't mind soldering 40 pins, I just have to figure out which go where, etc.The push button switches can be lubricated with a can of grease? something better than wd40?

author: KMoffett
author_rank: Regular Contributor
timestamp: November 14, 2017, 03:45:57 pm
content: Grease don't help broken mechanisms.Ken