Keithley 197-8 Battery Option: Re-Engineering an Old Design - Page 1
Forum section: electronics / testgear

author: timb
author_rank: Super Contributor
timestamp: November 12, 2013, 04:21:02 am
content: Collecting (and flipping) vintage test gear has become a recent hobby of mine. I specifically love old Tek and Keithley stuff.One of the first pieces I got was a 197A Microvolt DMM with the GPIB option for $75. I tore it down, measured everything, cleaned the case and put it into service in my lab. It's sense become my goto meter of choice for measuring super low voltage stuff.One of the neat options available for this unit (and several other models of the time) was an internal NiCad battery pack. It recharges at full speed when the meter is off and trickle charges during operation; it has a battery life of 4 to 8 hours and takes just as long to fully charge! I've been searching for a 197-8 module for quite some time and though they do occasionally pop up on eBay, they generally go for well over $100 which is ridiculous for an op-amp, inverting charge pump and a few other passive components.Lucky for us Keithley provided the schematics for the board in the 197 Service Manual! I have two goals for this project:1) Reverse Engineer and Recreate the Original 197-8 Board2) Design a New Board to Support Lithium Battery ChargingThis thread will serve as both a log of the project and to solicit feedback from the community on the new design. I'll be using Upverter to design both boards, which should allow those interested to directly follow along. Below is a link to a "master file" that contains all schematics, text, tables, parts lists and so on that I've carefully organized, cleaned up and enhanced. It will be updated as the project progresses. (It's what I use to keep track of everything relevant with.)PDFReverse Engineering the Keithley 197-8 by timb (Rev. 01)To start things off, here's a few images to get you acquainted with the design. In my next post I'll have a rundown of how it works, but I'm sure a lot of you can figure it out.Fig. 01Picture of a real board with diagram from the Service Manual overlaid on top.Fig. 03Schematics of the 197 Power Supply from the Service Manual.Fig. 02Schematics of the 197-B Board from the Service Manual.

author: idpromnut
author_rank: Unknown
timestamp: February 16, 2014, 02:18:24 pm
content: I have a 175A that I was thinking of building up a battery pack+board for as well. I will look into the schematics tonight and see if they are the same (it would make sense that Keithley would just reuse the design if it worked well).

author: timb
author_rank: Super Contributor
timestamp: February 20, 2014, 01:51:38 am
content: Yeah, I imagine they're very similar! Let me know what you find. I got sort of sidetracked with consulting work, but I'd like to get this built up. I'm working on a battery charging/power solution for client right now, so maybe I'll use some of the left over parts to build a lithium-ion charging board for this.

author: idpromnut
author_rank: Unknown
timestamp: February 20, 2014, 01:54:11 am
content: I looked at this the other day, and whaddya know, exactly the same schematic + board layoutSo I think I will look at the parts list and see if a direct adaptation will work.

author: timb
author_rank: Super Contributor
timestamp: February 20, 2014, 01:59:14 am
content: Quote from: idpromnut on February 20, 2014, 01:54:11 amYup, a direct adaptation should work just fine, so long as you use a NiCad pack (I'd use more modern NiMH). They can be simply charged with a voltage output, which is what this board does. Just don't use Lithium batteries, unless you like explosions!If you want one of my LiPo boards (next month most likely) let me know and I'll send it to you for free.

author: timb
author_rank: Super Contributor
timestamp: February 20, 2014, 02:02:00 am
content: I should mention that the parts used on the board are no longer produced, so you'll need to find alternatives and adapt accordingly. (Or search eBay for the chips!)