Electronics > Repair
Keithly 191 Repair
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at2marty:
A little while back I scored what I think is a decent Ebay auction. I got a Keithly 191 multimeter along with a Keithly 175 multimeter for around $50 US including shipping. I briefly checked them out when I received them, and they both seemed to "work" correctly. They had a few cosmetic defects, which I don't really care about. I was more interested in getting a couple of bench meters to mess around with. The 191 is a nice meter with pretty high precision, and best of all, it's kind of "old school".
I finally got around to messing with the 191 a bit. I noticed that even though it has 6.5 digit accuracy, the least significant 2 digits would never settle down. They constantly would change readings, I also noticed that sometimes when I powered it on, I would just get a "-1. " reading that would just flash, and would not go away no matter which buttons I pressed. If I power cycled it once or twice, eventually it would start to "work".
The troubleshooting part of my mind went to work, and I went back to the fundamentals. CHECK POWER!
I downloaded the manual for it and began testing power from both a cold power-on and a warm power-on. The operating voltages are +5VDC, +15VDC and -15VDC. I noted that the -15VDC was out of spec and the +5VDC was right at the limit. I also noted that during a cold power on the -15VDC was essentially "not there".
Visual inspection revealed that a filter capacitor for the 5V regulator (electrolytic 100000uF 10V) was showing just a slight bulge. The others looked fine. I don't have an ESR meter but on a hunch I decided to replace all of the electrolytics in the power section.
When I removed the old capacitors, I found that the one on the -15V regulator had been oozing crap out of the bottom. It took some time to clean the mess up, but I was able to solder in new capacitors.
After replacing the caps, I went through as much of the calibration that I could with the equipment that I have. All digits are now steady and if I apply 190mV on the 200mV setting I get a reading of 190.001. If I apply 1.9V on the 2V setting I get a reading of 1.90001. If I apply 190V I get a reading of 190.000 on the 200V setting.
For calibration I used an old Eico resistance decade box (un-calibrated) and my Fluke 3330B voltage standard (un-calibrated). However, the readings that I get are consistent with the other meters that I have. I really don't care or don't need accuracy down to the nanoVolt range.
I'll post some photos when I get the chance to do so. I really have a love for the "old school" equipment.
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