Keithley 197 Repair

[SeanB]

Is the display one with part no MD400F640BDT8409 as I have one that came out of an old Panasonic Executive Partner.  If you need one i will post it to you.

[codeboy2k]

*defibrillating this old thread*
smear gel....
clear!!!.....
zap!!!

I just got a 197a from a seller marked dead.. does not power on.  Paid $40 on the 'bay and received it today.  I like buying things that are sold as "does not power on" since it's usually just a fuse or voltage regulator problem or something else equally simple.

So I opened it up, fuse was blown.  Replaced the fuse... works like a charm! All functions test normal and it appears to be in conformance with my Tek TX3.

However, its EL backlight is very dim.  So, did anyone get the schematic for the display and backlight, and/or manage a replacement mod?  I like LEDs on the bench too

Thanks!

[rastro]

Hello codeboy2k,

The patient is resuscitated...beeeeep

I went the "EL" route.  I found and EL-panel that could be trimmed down to fit.  Although it was an improvement I probably would not make the effort in retrospect.  It would be nice to use some smd LED's.  However I really don't see clearance behind the LCD.

The "EL" panel I used:
http://www.ebay.com/itm/300935081746?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2Fi.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D300935081746%26_rdc%3D1
The power module was functional so it's just trim and solder.

The available 197A manual as you may have discovered is missing schematics so the next closest is the 197 manual with schematics.  If you do locate 197A schematics I would like to get a copy.

Also what kind replacement fuse did you use?  The original main-fulse from the factory is a glass fuse with a series resistor inside. 

The 197's are great meters, enjoy!

Regards
rastro

[codeboy2k]

Thanks @rastro.. I'll take a look at the EL panel you linked.  I would really like LEDs here too, but then you have the problem of the symbols like V, A, DC, AC, omega, mu, etc. One would have to decode the protocol on the display cable and also create a new display with all those symbols too as backlit individual leds. It all seems like more trouble than it's worth.

For the fuse, it's just an old style 1/8A / 250V  slow-blow fuse. The resistor takes some time to heat up. If there was an in-rush current it won't overheat, but if it's a prolonged over-current condition, the resistor heats up and melts the solder at its end.  The spring pulls it back and away, breaking  the circuit. 

I just replaced it with a modern MDL 1/8A slow-blow, as I had them in my parts drawers.

[codeboy2k]

Oops.. I just realized you meant SMD LEDs for back lighting , not SMD 7-segment LEDS as I was thinking but I do like 7-segment LED  meters.

Anyways, does the EL panel you linked to work off the original back light inverter on the display board? I presume it will.

Did you pick a different color ?

[idpromnut]

I actually recently replaced the EL backlight on my 175A without any issues. The inverter measures 75VAC, so pretty much anything on flabby should work a treat. I choose electric blue as the replacement, but red was a runner choice

A 7-seg LED might be doable. The supply to the inverter for the EL strip is 5VDC if I remember correctly on the 175A (I can't imagine the 197 is much different, I suspect they used a similar PSU). I didn't check, but I seem to remember that the LCD is driven by a controller on the PCB. I think I might need to pull apart my 175A and sniff the data going to the display

Keep in mind that there are a few other display items that would need to be replicated as well (dB, Recall, Store, Cal, etc) but those could be done with a few SMD LEDs.  Sounds like a neat project!