Brief Teardown: Keithley 193 System DMM

[nidlaX]

I purchased this one used off of eBay sometime back. Exterior is pretty crusty, rubber handles have long broken off, and that LIMITED sticker on the range control section led me to believe that something is amiss. Thanks to TiN for hosting a copy of the Instruction Manual on xDevs.

Images follow, but first: does anyone know what the differences between the 193 and 193A models are? I did a quick search online and saw that the listed stability specs for the 193A seem to be significantly better, even though the two models appear to be functionally identical. If anyone has information, please post!

Full resolution album here.



When I saw the back, I assumed that neither the ACV nor current measurement options were installed...


Surprise! They're both installed. Digital section somewhat reminds me of that of the 196.


Back side.


Model 1930 TrueRMS ACV Option


Model 1931 Current Option




That's actually black colored silastic underneath the capacitor.


30+ year old NiCd battery? Not confidence inspiring.


This GPIB controller looks to have been replaced at some point. Date code doesn't match surrounding components either.


Nice soldering job.


Peering down into the great divide. I guess opto-isolators weren't prevalent back then.


Analog section. Shield is held (firmly) in place by four metal clips. I see resistor networks everywhere, hope they're built to last.


Classic LM399.


Two Caddock networks.




I like the look of these Potter & Brumfield relays.


Some quick sanity checks using an U8001A power supply. I dislike the auto-ranging behavior for DCV: if no input is connected, it will start rapidly cycling the relay (2-3 times per sec) to no end. I don't know whether or not this is normal, but it seems like an unnecessary burden on the relay's lifespan.






Pardon the crappy camera work, hope you enjoyed it!

[crispy_tofu]

I don't have any info about the differences of the two meters, but thanks for sharing, it looks like a fantastic meter. 

Pardon the crappy camera work, hope you enjoyed it!

Crappy??? 

[ruffy91]

AFAIK the cycling is due to leakage currents. The input impedance is 10GOhm up to 2V. Higher ranges have 10MOhm. So the input gets charged to 2V, changes to 10V range and gets discharged.

[wiss]

Some quick sanity checks using an U8001A power supply. I dislike the auto-ranging behavior for DCV: if no input is connected, it will start rapidly cycling the relay (2-3 times per sec) to no end. I don't know whether or not this is normal, but it seems like an unnecessary burden on the relay's lifespan.

It has very high input impedance, any amount of bias current will make it drift off and when the voltage reaches the first high range (20V or 200V) the resistive divider will zero the input and it starts over again. Don't leave it like that but short the input

[nidlaX]

Thanks for the explanations!

[Vgkid]

Thanks for the teardown. No teardown of the ac/current section though  .

[TiN]

Big bulky units which I'd never get due to exorbitant shipping .
Thanks for photos, they are not bad at all. I'd like to steal your pics for my archive 

I don't think relay autoranging is normal, perhaps something leaking somewhere, causing incorrect range switching. Check impedance on input.

[wiss]

Keithley 192, prema 6001, solatron 7075 all behave this way, perfectly normal.

[nidlaX]

Thanks for the teardown. No teardown of the ac/current section though  .

Sorry guy, I knew some people would be disappointed.  I think I will revisit it at some point in the future. The modules should be easy to uninstall/reinstall, but I don't want to deal with re-calibrating the unit until I've finished functionality testing.

Big bulky units which I'd never get due to exorbitant shipping .
Thanks for photos, they are not bad at all. I'd like to steal your pics for my archive 

I don't think relay autoranging is normal, perhaps something leaking somewhere, causing incorrect range switching. Check impedance on input.

Feel free to use those pics TiN! For now, I'll trust what wiss said unless I find other measurement problems. There's still the question of why someone put that "LIMITED" sticker on the range section, it worries me slightly.

[PedroDaGr8]

Nice teardown!!! Uses the same case as the Keithley 236 I just tore down (will be posting the teardown in a few days). These large Keithleys are REALLY impressively built inside, very nicely laid out.

[nidlaX]

Nice teardown!!! Uses the same case as the Keithley 236 I just tore down (will be posting the teardown in a few days). These large Keithleys are REALLY impressively built inside, very nicely laid out.

Ooooh, nice! [emoji2]

[sorenkir]

My Keithley 199 System DMM does the same relay clicking in DCV autorange with no input connected.
Michel

[nidlaX]

I got an answer from the Keithley support team regarding the differences between the 193 and 193A. They say that the specs were tightened up without any changes to the design or hardware, and the 193A should use later revision firmware.

Incidentally, my 193 is supposed to have the most up to date firmware available (revision C11), and the power-on behavior matches that described in the 193A manual as opposed to that described in the 193 manual. Specifically, the 193 manual says the instrument will print the current IEEE address and software revision after the power on self-test, but the 193A does not, and my instrument goes straight into default operation. I'm not sure whether or not the firmware revision affects the instrument's performance.

Here's a link to the 193A manual.

And a link to the specsheet with improved listed accuracy.

[saturnin]

I have been repairing Keithley 193. I managed to fix a couple of bugs in its digital section: U306 - 74LS04 and U316 - GPIB controller were faulty), so the unit is able to boot up now.

Since my unit has both DC AMPS (1931) and TRMS AC (1930) options, I decided to share their photos (sorry, for their quality - I don't have better camera now):

[saturnin]

And TRMS AC option photos:

[nidlaX]

Nice! Now I don't have to bother messing with mine.  O0

By they way, which firmware revision do you have? I've dumped my C11 ROM in case anybody needs it. What do you think you'll use to replace the backup battery?

[saturnin]

Since it displayed "UNCALIBRATED" message after the first successful boot-up, I didn't have a bad feeling when I was removing options from it   

My unit has FW C9. I had dumped it before I started with the repair (it is attached below, I had to change their extension to .txt since .bin files were rejected by the forum engine). Would you share your firmware version?

Surprisingly enough, the backup battery is still alive ! The multimeter is now turned off for one day. I just measured battery voltage and it has approx. 4V ! I think it will not replace it since it only powers internal RAM memory, when multimeter is turned off.

[nidlaX]

Sure, here's my C11 ROM. Perhaps TiN could host these on xDevs?

[saturnin]

Thanks! I will try to flash it to my ROMs. I wonder what they changed...

As concerns rehosting of files, I have absolutely no problem with it   

[saturnin]

Finally, I found some time to update the firmware.

At first glance, everything works fine. The multimeter passes Self Test (which checks ROMs integrity too). I've made brief tests and seems calibration done with FW C9 is compatible with FW C11.

@nidlaX: Thanks again for providing your FW!

[nidlaX]

Finally, I found some time to update the firmware.

At first glance, everything works fine. The multimeter passes Self Test (which checks ROMs integrity too). I've made brief tests and seems calibration done with FW C9 is compatible with FW C11.

@nidlaX: Thanks again for providing your FW!

Nice. Your 193 looks like it's in great condition!

Sent from my LG-E970 using Tapatalk

[dom0]

Keithley 192, prema 6001, solatron 7075 all behave this way, perfectly normal.

Every few seconds or once per second - okay (input current integrates a couple dozen input capacitance). But rapidly ("2-3 times per sec")? I'd check the input current, maybe it's leaky. Or this model has very low input capacitance...

[Ordinaryman1971]

Anybody maybe having broken down Fluke 193 or 193A with the current options 1930 and 1931, maybe willing to trade or sale them, they are missing in the one I have. Or maybe know the source to buy both/either boards.

[Jelcke]

Hi,
My Keithley 193 does not settle when measuring / dc or ohms . The last two digits keep climbing up (slowly).
Could this be due to aging / leaking caps?
Any suggestions are welcome.
Best
Jelcke