It could just be simple user error, or something more serious, but from the age of the unit, it should be fairly repairable. And thankfully, the manual supplied by Keithley available here , via KO4BB, without registering , has wonderful block diagrams, layout images, and schematics. After receiving the unit, I found it was indeed non-functional, with wildly varying readings. All the buttons worked, and it would switch ranges, but the readings never worked.
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It could just be simple user error, or something more serious, but from the age of the unit, it should be fairly repairable.
And thankfully, the manual supplied by Keithley available here , via KO4BB, without registering , has wonderful block diagrams, layout images, and schematics. After receiving the unit, I found it was indeed non-functional, with wildly varying readings. All the buttons worked, and it would switch ranges, but the readings never worked. Working with some help via the eevblog forums , the general thought was to confirm the input path using the schematics and a highlighter to mark where the signal was found, etc.
Thankfully the Keithley has a diagnostic mode, which allows you to look at the the various inputs to the multiplexer. The meter normally, several times a second, cycles between 0V, 0V, I started looking down that path. But when put in the proper diagnostic mode, I could see the integrator working. I had already checked the voltage references in the unit, and they all checked out. So I started looking down that road. So looking at those, things looked fine coming into the analog side.
Coming OUT of the analog side looked bad. So back-tracking, everything went through a hex-inverter U49 according to the schematics, a 74HCT So, after ordering a replacement, I unsoldered that IC and tested it on a breadboard, and it worked after wiring correctly. I installed a 14 pin socket on the board as well, so I could install a new chip when it came. So I put it back into the board in the socket , and the meter started working properly. Well crap.
Talking it over in the forums it seems likely the heat, and mechanical stress of unsoldering probably made a contact a bit better, but it would likely fail again.
So, once the new chip arrived, and replaced it, and it still works. Check your voltage rails. Check your signal path. Check your glue logic. You might be able to swap 2 and 3, depending on what the symptoms are, but I think I probably wasted a bit too much time looking at signal path issues, and should have trusted what I was seeing earlier when I first looked at the glue logic and saw weirdness.
After all that, someone else on the forums found similar failure in that hex inverter. On a big note, thank you test equipment manufacturers for including schematics with your manuals.
Keithley: DMM 199 Instruction Manual
Cleared when the input is on range. Use uprange or downrange as necessary to select interval, then press NEXT. The time required to change channels is approximately 17msec, which includes bieak-before-make relay time settling time. Conunands sent without the execute character will not be executed at that time, but they will be retained within an internal command buffer for execu- tion at the time the X character is received.
Repairing Keithley 199 DMM/Scanner