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Keithley 197 Repair

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Keithley 197Given 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.

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Blue ESR Meter Kit

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Blue ESR MeterEver since I started doing LCD monitor, and G5 iMac repair, I’ve wanted/needed to pick up an ESR (Equivalent Series Resistance) meter. Electrolytic capacitors are the bain of pretty much all modern power supplies. Pretty much everything else in a modern power supply is invincible compared to the caps… and while I’ve been extremely lucky that every iMac and LCD monitor I’ve repaired had obviously bad capacitors (bulging), that isn’t always the case… which is where an ESR meter comes in.

I won’t get into ESR in general, but you can find info on wikipedia here, or on this site. But basically, as Electrolytic caps age, their ESR increases, and generally once it rises above it set point, the device it’s in will stop working. In many cases, you can tell when this happens due to bulging/leaking. But that isn’t always the case, and having a meter is a good idea. I purchased the Blue ESR kit made by AnaTek, based off a design by Bob Parker for $73 off eBay. The kit arrived in a standard box, and was well organized. In about an hour and a half of soldering, I had the unit up and going. Really all you need is a soldering iron, some flush cutters, and if you want to calibrate the battery low indicator, you’ll want a variable bench power supply. Other than that, it was a piece of cake. While hourly, it’s not worth the $20 saved to buy the kit, it was worth it to get the satisfaction of building a kit and having it work. The instructions are easy to follow, and the unit fired right up when I was done.

I’d highly recommend it if you spend time doing repairs. At some point I’ll buy the Blue Ring Tester kit as well, as it makes diagnosing bad transformers easier, but for now, I’ve probably spent enough on test gear for the time being.

[xrr rating=5/5]

Repairing Keithley 199 DMM/Scanner

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Keithley 199About a month ago, I purchased a broken Keithley 199 multimeter off ebay for about $70 including shipping. From everything I’d read, it was a pretty accurate 5.5 digit, 310,000 count multimeter with a nice bright LED display (as opposed to the Fluke VFD, or HP non-backlit LCD displays). Having bought sight unseen, I wasn’t sure what exactly was broken. 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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