Seems like everyone having nice weekend, and only weird voltnuts are messing with fixing some 25 year old instruments
So I will drop the suspense and show rest of progress on very first meter I got. It is a reason of all this Keithley madness, root of all evils.
As above, this 2001 had analog board revision L, with avalance diodes on front side.
Obviously capacitors leaked after all those years and caused shorts in bootstrap regulator, allowing high current to do some PCB damage around C117.
Pad and connection to negative terminal of cap were completely burnt.
Lucky enough, no low-signal tracing on inner layers was affected. I had quick bodge with some noname fung-hwa-chi capacitors, and had this meter running for
few years in 2012-2013, as it was working for DCV,ohms,current ranges. After that it was collecting dust most of time, as I got other 2001's to play with which were in better shape.
Another issue - was open pin connection on U531 DAC, which was throwing lots of 4xx errors. Previous owner probably was not careful removing analog board, you can see DAC pins bent, like someone pushed poor IC off the PCB.
Fixed by reflowing pins with soldering iron. Easy enough.
As all Rev.L board I saw, this one have more stable Vishay R356,R358,R365 for current ranges, and ULT H5185A 1Meg 1% in little metal can (R360).
Cleaning around them fixed 304.7 issue for nA ohm current source.
Had issue with 309.1, Amps. It was wrong Q305 on board. Replacement to IRFR020 from donor 2001 board fixed this problem.
Replaced fan to ADDA
No reworks for TRMS area.
No reworks on input side either. It's interesting to note, I had 6 units (2001) and none of them had any blown parts on input. Is this a case when sensitive electronics does job to protect 1 dollar protection circuit from failing well?
VREF is alright too, LMx99 with SL40057 IC-527 with datecode 22 week 1993. It have a crack on top cover, since I was curious to see what's inside 5 years ago when got this meter.
Here's recapped board with replaced avalance diodes, CR514,CR513 zeners.
I desoldered U505, U507, U508, U509, cleaned all PCB area, cleaned ICs themselves with KONTAKT60 (not best cleaner, but works OK and not aggressive as acetone)
Then soldered all except U508 back. U508 (74HC00) was replaced to new one 74F00 (just don't have HC ones in drawer). It's jellybean logic part, why not replace it to new?
Notice that I used 1000uF x 50V capacitors for C117,C116. Same diameter, just taller, but still fit nicely. This should reduce wear&tear forcaps, as they are near hot LDO.
I'm not sure why Keithley had 35V rated caps while output on bootstrap circut is +38..+44 and -38 to -44 VDC?
Also this time used general purpose Chemicon KMG 470uF x 63V instead of fattier Low-ESR KZE series Chemicon's.
Here replaced parts marked blue on schematics:
After you gave some capacitor love to analog board, don't forget digital too! There are three lonely 1000uF x 16V, I bet they want to leak and sneeze after all those decades too.
There are no high-power path there, so problem less evident, but it's worth to replace those too. Especially footprint is designed to fit wider 1000uF x 35V.
On my board C630 and C624 had some electrolyte escape already.
Replaced to all three to 1000uF x 35V Chemicon KY's.
Overall photos of digital and analog boards:
How let's slap A/D board on and check this beast..
I used one of A/D boards bought few years ago from our fellow member, free_electron.
Meter power's on. -39 and +39 after bootstrap there, all voltages ok, no smoke or smell elsewhere. Time to run selftest.
And pass, no errors!
Run in continuous loops few dozen times, no problems.
Time to check voltages?Sourced 1.00028VDC from 2400, measured 0.99029 VDC. No good.
Sourced 10V from EDC MV106, measured 9.89829 VDC. No good, definately need calibration, but should be no issues with that.
Bottom 2001 - one fixed last weekend, calibrated using VPG 19K and 1Meg resistors and 2V and 20V from EDC MV106 measured by reference 2001 (it's meter on top right, calibrated by Tektronix in February 2014)
Like always, enlarged photos on clicks
BonusA/D board with probe port to Mictor (which in turn runs into Mictor-P6860 adapter and my TLA714.
What's going on here?
Hope you enjoy reading..