Given my experience, and eventual success in repairing my now indispensable Keithley 199 Multimeter, I recently picked up a non-functional Keithley 197 Microvolt multimeter on eBay for $25. Vintage is about the same for this model and the 199, and after looking at the manual (which includes schematics), I figured it was worth the $25 to give it a go at repair. Best case, it was easy and I could turn around and sell it, or give it to a friend.
The unit arrived in relatively short order, and indeed, in all settings, the meter just read “OL”. So, first thing that told me is, processor is working. EEPROM too, more than likely. So, going by the manual, and logic, I started with the voltage rails. The 197 has a few. Basically a ±15V rails, a +10V, a +5V analog, a +5V digital, a -6.4V, and a +2V reference. All of these are regulated by diodes rather than more modern LM78xx/79xx regulators. So, going through the checklist, it soon turned up that the -15V (-V) rail was dead. At first I thought diode, but testing the diode out of circuit resulted in a working diode. So something was pulling that rail up toward ground. So, trace time.
The schematic showed very little attached to that -V rail. Basically, only some protection (Q127), and the op-amp/PNP pair that converted that -V rail into the -9V and -6.4V rail. After some basic trace finding, I decided to cut pin 4 on the op-amp and see what we got. Bam. Negative rail mostly came back (now to -12V or so). So the op-amp was definitely bad. Thankfully, it was still something in production, a TLC271CP. Mouser order later, and I had a replacement in a socket, and things were better, but not great. -V rail was still at -12V, the -9V rail was at -4.8V, and the -6.4V rail was at +6.2V. Hmm.
After talking some on the eevblog forum, the next step was to remove Q128, and VR101, and drive the -9V rail with my bench power supply. So, removed both, wired in the bench PSU, and fired it up. Normal draw on the -9V rail should be about 20mA, but I had to crank it up a bit to get the -9V stable. At that point, things kind of worked. In the 200V range, I’d get a noisy reading of about 58V with nothing attached to the inputs. But, it was “working”. -6.4V rail, however, was STILL at +6.2V. Hmm. Unfortunately, given how that rail is generated, it wasn’t a simple matter to just try to drive it with me PSU. Thankfully, that rail is only attached to two things: U101 (which seems to drive the guard traces for the input multiplexer), and for some reason, the input further up stream (still not sure on that one). I didn’t want to cut any leads unless I could find a replacement chip for U101, but the model number on the chip was seemingly non-existent, and the manual just listed it as “Integrated Circuit”. The schematic, on the other hand, just showed a dual op-amp.
Thankfully, I came across this. This guy had taken pictures of the inside of his Keithley 197, and showed U101 in A. a socket, and B. different (still available) part number. Which told me it had been replaced before (none of the ICs in mine, except the EEPROM, CPU, and A/D converter are socketed). So, knowing I could replace U101 with a TLC27L2CP, I cut pin 4 on U101, and fired the unit up. Viola! The -6.4V rail was back AND the unit was reading 00.0000 volts in the 20V range. I gave it a 9V, and it read to within the least significant digit of my Keithley 199 and Fluke 189. Cool! So, I removed U101 the rest of the way (and ordered a replacement from Digi-key), installed a socket, and reinstalled Q128, and VR101. Unit still worked!
So, the Digi-key order arrived a few days later, installed, and the unit worked great. Comparing it to my Keithley 199 after warm up, it was extremely accurate down to the least significant digit (even compared to my Fluke 189). My complaints about the unit are only that it doesn’t have an IEC socket (it has a permanently attached cord), and the LCD display (no backlight). The LED display of the 199 is MUCH more readable, even if it does use more power (the purpose of the LCD display was due to the unit being able to be outfitted with a battery). All and all, the unit compares very favorably with the venerable HP 3468 (and family).
I think I’m going to sell it. It’s a great meter, and hopefully someone can put it to good use. I’ll stick with my Keithley 199. =)
UPDATE: I did end up selling this meter over ebay for about 3x what I paid for it, so I’m happy. I heard the buyer is sending it out for calibration, so hopefully they will make good use of it. =)