Keithley 2000 “Repair” Tip

I won an auction to buy a Keithley 2000 6 1/2 digit multimeter on Ebay for ~$250, a pretty good price. The seller said it had some scuffs, but was “tested and ready for work.”

When I received it, it was clear that it was a little worse for wear than claimed. It had a cracked rear bracket, and a yellowish/brown tint, rather than the shades of grey of a new machine. At first I thought it might be yellowing due to sun exposure, but the yellowing seemed to afflict the painted metal case as well as the plastics.

It did power up, and when I tested it with a voltage reference I have, its readings came in pretty close to the expected value, so I didn’t worry too much about the physical condition beyond trying to wipe the outside down well with cleaner and isopropyl alcohol.

Once I’d done that, I decided to look inside, to see if I could get an idea of the manufacture date. As soon as I removed the case, it was obvious where the yellowing had come from. It stank of old tobacco smoke on the inside, though fortunately, there wasn’t an obvious film. As I looked around, inside I could see a number of components with date codes for mid 1995, which matched well with the date of the first and only calibration, November 1995. It was almost 20 years old.

In the process of looking at the insides, I noticed that he input wires seemed a little close to some metal projections from the input selection switch, which seemed a little sloppy. Then I realized the board seemed a little slanty, and was out of its mounting tracks, possibly it had been jarred loose in an impact. On the opposite side of the chassis, I saw that some wires to the front panel and the power transformer weren’t routed through a retaining clip. Someone had taken this thing apart, and done a poor job of putting it back together. I loosened some screws so I could slide the board back into position and noticed a gouge in the PCB when it had been forced into place during a previous reassembly. Fortunately, it only damaged some solder mask, and not the trace underneath.

Once I had it back together, I ran the self-tests and was discouraged when it reported a number of faults for tests 100.1, 101.2, 101.3, 200.1, 200.2, 201.1, 201.2, 300.1, 301.1, 301.2, 302.1, 302.2, 303.1, 303.2, 304.1, 400.2, 401.2, 402.2, 403.2, 500.1, 500.2, 600.1, 600.2 and 601.2. That’s most of the self tests.

I tried to work through the troubleshooting in the repair guide, but it was dismal, it didn’t even describe the signals on the half-dozen or so test points on the board. I found a reverse engineered schematic and dove in.

I was working my way around the A/D chip trying to orient myself to the various signals, when I noticed…something. At first I thought it was a stray line on the silkscreen, but on a closer look, it seemed to be a fiber, cat hair? Dog hair? I flicked it away with my gloved hand, and then blew the area clear with some canned air.

I reran the tests and they all passed! One tiny fiber in the wrong place was enough enough to through the A/D converter out of spec cause a cascading failure.

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