Keithley 199 -- FIXED!!!!

[PedroDaGr8]

I just realized you asked me what the model of the JFETs are. On my non-working one they are ALL 2N part numbers (the working one has the weird custom parts) if you give me a few minutes I'll list the part numbres of all of the TO-92 devices.

Here we are.

Q17 - Harris ITE 2n4392
Q4 - 2n2771
Q9 - NOT POPULATED
Q16 - 2n5818
Q13 - 2n4392
Q3 - 2n2771
Q11 - 2n4392
Q33 - 2n2607
Q34 - 2n4393
Q32 - 2n4392
Q30 - 2n2607
Q35 - 2n2607
Q8 - 2n4392
Q12 - 2n4392
Q7 - 2n4392
Q5 - 2n4392
Q6 - 2n4392

[PedroDaGr8]

Well I pulled U49 and finally got around to testing it. No more constant 5V on the output to clock. So the hex inverter was clearly dead. Time to order some replacements.

[staze]

Well I pulled U49 and finally got around to testing it. No more constant 5V on the output to clock. So the hex inverter was clearly dead. Time to order some replacements.

Fascinating... because yeah, my U49 appears to be failed as well. It's clearly partially working, but not well. I have a replacement one arriving on Monday (NTE74HCT04). For now, I'm just going to put in a socket. Here's hoping.

[staze]

Just posted some pictures of what the IC (U49) does out of circuit... it ain't pretty.

https://www.eevblog.com/forum/chat/logic-inverter-'noise'/msg263583/#msg263583

[staze]

So... it's fixed.

I pulled U49 and it seemed to be busted when I put it on a breadboard... but then I was corrected about being wired backward. doh! So I tested with it wired right, and it was dead. fast forward a couple days, and I test it again on the board. Now it works. huh.

So, after removing U49 from the board, I put in a socket since I don't overly care of soldering ICs directly. I drop U49 into the socket, and viola, my Keithley 199 works flawlessly (after recalibrating the 3V and 30V range). Now, I think I'm still going to replace that chip with the new one that's getting here next week, since I can't be sure if the chip is just playing nice for now, or if, as someone suggested, the gnd or 5V pin was a bad solder. =/

Huh. Stupid logic. Anyway... PedroDaGr8... start with that logic chip. Bet you'll get somewhere. =)

[staze]

One other question, if anyone knows... the selector switch for changing between front and rear inputs, is rather sticky (doesn't return to "front" well when pushed). Think I can clean it somehow, or should I try to buy one somehow?

[staze]

I do want to congratulate alm. Saying to check both sides of the opto-coupler for the A/D counts was almost exactly right. The signal on the optocoupler was fine on both sides, it was one step further at the inverter were things broke down. =P

But still... damn accurate.

[PedroDaGr8]

 great news! I  think I need to get  that blown diode  replaced first. 
As  for theinverter, ut's strange  IIRC they invert the  signal,  push it through  the connector,  through  the optoisolator  then invert it back.  not sure  why they did that

[staze]

yeah, wonder that too... but looking online, it seems it's due to CMOS TTL logic not being able to drive the optocoupler directly... so they use it to sink the pulses instead.

http://sound.westhost.com/project145.htm

Seems like this should be something built into optocouplers.

On the plus side of all of this... I think it's justified my test equipment purchases in my wife's eyes. Pretty sure I couldn't have found the problem without a scope at the least...

[alm]

Glad you were able to fix it, even without me having time to provide measurements from a know-good unit! Yes, a scope is pretty much essential for this. Now to convince your wife that spending money on the scope was good because it enables you to spend more money on electronics junk to fix .

[staze]

Glad you were able to fix it, even without me having time to provide measurements from a know-good unit! Yes, a scope is pretty much essential for this. Now to convince your wife that spending money on the scope was good because it enables you to spend more money on electronics junk to fix .

Oh yeah, that's no problem. I'm just not sure what I want/need next. with the scope, FG, PSU, and now bench and handheld meter...

I wouldn't mind sending out the bench meter for a calibration... but it seems hard to justify the $100+ for it. I got close with my PSU and FG, and calibrated based on my Fluke 189 (which I have no idea how well it's calibrated, and it's a digit lower resolution).

For fixing stuff, one of the blue ring testers would be nice, but that's not something I buy broken.

Any suggestions for something else to buy that would be "cheap" in a busted state, but could be reasonably fixed and provide a good learning experience? =)

[alm]

I would say stuff that it not any newer from this, around the late eighties custom ASICs and poorly documented circuits become common. Stuff like analog function/pulse gens and linear power supplies tends to be fairly doable. In general, I find that used prices for broken units on eBay have gotten way too high. Unless you already know the problem in advance, it's always a gamble.

[staze]

I would say stuff that it not any newer from this, around the late eighties custom ASICs and poorly documented circuits become common. Stuff like analog function/pulse gens and linear power supplies tends to be fairly doable. In general, I find that used prices for broken units on eBay have gotten way too high. Unless you already know the problem in advance, it's always a gamble.

Makes sense. Well, I'll keep an eye out.

Any ideas on the input selector switch?

[alm]

The power switch (which I think is a similar kind of switch) is quite sticky in one of the units I own. I rarely use the input selector switch, and I don't have them in front of me now, but I think it's a fairly common issue. I would try cleaning them if it bothers you. To clean switches, I usually squirt some IPA in, exercise the switch a few times, let it dry, and then optionally add lubrication. I think there are also purpose-made products (eg. the various Caig products) for this. Don't use WD-40 or contact cleaners, they tend to be aggressive and/or leave a sticky mess that attracts dust.

[staze]

The power switch (which I think is a similar kind of switch) is quite sticky in one of the units I own. I rarely use the input selector switch, and I don't have them in front of me now, but I think it's a fairly common issue. I would try cleaning them if it bothers you. To clean switches, I usually squirt some IPA in, exercise the switch a few times, let it dry, and then optionally add lubrication. I think there are also purpose-made products (eg. the various Caig products) for this. Don't use WD-40 or contact cleaners, they tend to be aggressive and/or leave a sticky mess that attracts dust.

Well, I don't have any IPA, but I have a nice Hefeweizen or some nice chocolaty stouts... but I would think this would leave a huge mess. =D

Seriously though, I'll try some isopropanol. And yeah, I've seen a lot of suggestions about using Caig DeoxIt Gold (and I think Red) for stuff like this... but like you, I don't plan to use the switch much, and dropping $25 on a bottle for this one switch is a bit much. =/

Thanks for all the help!

[PedroDaGr8]

Success....kind of.

So the other night before you tried your repair I tried inserting the replacement 74HCT04. No result the system just made a whining noise and I couldn't get a trace to pull up. Tired I gave up for the time being. Seeing yours I decided to go back and revisit it. Sure enough the IC was in backwards  and I could have sworn I triple checked it the other night. Anyways flipped it around, BOOM, I have numbers moving on my display. Unfortuantely, because of the blown zener they make no sense.  Even stranger, I tried the old "bad" HCT04, it works TOO! The only thing I can figure is that there is a bad connection inside the 74HCT04 and the heat of desoldering it reflows that connection. It is the only thing I can think of that makes sense.

Now its time to order that zener and get it installed, found a place selling them for $10. Anyone have some advice to make sure I don't kill another zener?

Lastly, maybe change the name of this to Keithley 199 instead of Bench Multimeter? it will help people trying to repair theirs as this might be a common mode of failure (and one likely unexpected by most people).

[staze]

alm: IPA worked a treat. Just squired a bunch on there, and worked the switch, and it freed up pretty nicely. Guessing I'll pick up some Caig stuff in the future to lubricate, but for now, it works.

PedraDaGr8: LOL! maybe subconsciously we're thinking "it's an inverter, it should be wired backward". =) You're probably right... but hell if I'm not going to replace the inverter anyway. I do wonder if the heat is part of it, because I'll admit, to get that sucker out, I really had to heat the crap out of it. Really wish ICs were in sockets by default, especially since there's room.

So, with the zener, I'd assume you've checked the voltage on it (or at TE1/TE2 in N3 mode) and it doesn't read right? $10 isn't bad for that part... I would assume just solder it quickly, in the right direction, and you shouldn't have any issue... but I'm not an expert.

As for title... will do. I've thought about that. =)

[PedroDaGr8]

alm: IPA worked a treat. Just squired a bunch on there, and worked the switch, and it freed up pretty nicely. Guessing I'll pick up some Caig stuff in the future to lubricate, but for now, it works.

PedraDaGr8: LOL! maybe subconsciously we're thinking "it's an inverter, it should be wired backward". =) You're probably right... but hell if I'm not going to replace the inverter anyway. I do wonder if the heat is part of it, because I'll admit, to get that sucker out, I really had to heat the crap out of it. Really wish ICs were in sockets by default, especially since there's room.

So, with the zener, I'd assume you've checked the voltage on it (or at TE1/TE2 in N3 mode) and it doesn't read right? $10 isn't bad for that part... I would assume just solder it quickly, in the right direction, and you shouldn't have any issue... but I'm not an expert.

As for title... will do. I've thought about that. =)

Haha yes its an inverted inverter; that's my story and I'm sticking to it.  O0

As for the zener: Yeah it reads 1.4V across it in all modes. It's definitely shot.  I was looking at these zeners and its no wonder they are expensive. It has a very nice temp coefficient (0.0005%/oC). Anyways I purchased the zener plus a couple replacement 74HCT04's. Can't wait to get it up and fully functioning. Then maybe off to calibration (a local place does full ISO and NIST calibration with all data for $75, waiting to hear back if they will give a discount if I dont mind waiting up to a month and do both Keithleys and my 6114a at the same time).

[staze]

As for the zener: Yeah it reads 1.4V across it in all modes. It's definitely shot.  I was looking at these zeners and its no wonder they are expensive. It has a very nice temp coefficient (0.0005%/oC). Anyways I purchased the zener plus a couple replacement 74HCT04's. Can't wait to get it up and fully functioning. Then maybe off to calibration (a local place does full ISO and NIST calibration with all data for $75, waiting to hear back if they will give a discount if I dont mind waiting up to a month and do both Keithleys and my 6114a at the same time).

Nice. I need to make friends with someone that has at least a calibrated 5.5digit meter (6.5 or 8.5 would be better). The Keithley's are really easy to calibrate... it's just waiting the 2 hours for them to fully heat up (at least, that's what the manual claims to wait before calibration).

Good luck, and let us know how the replacements go. Where did you source the zener from? While they aren't LM399 good (0.0001%/°C), that's still pretty damn stable.

[mweymarn]

Hi all,

Good to see a few of the Keithley 199s being resurrected.

I recently retrofitted my 199 with an LM399 reference. On my unit the original 1N4579 appears to have drifted significantly, so I decided to replace it. Fairly easy to do, as the Zener voltages are pretty close. I added a 107 kOhm resistor in parallel to R69 to bring the reference voltage back to -2.8V. -10VR and +5VR were adjusted by replacing the following resistors:
R67:   25500 Ohm
R70:   11500 Ohm
R77:   10000 Ohm
R76:   13700 Ohm
The LM399 heater was hooked up to the +15V and -15V regulated supply rails.
While I had the unit open I replaced the NE5534 Opamps with LT1001s (DIP8).

The LM399 was burned in for 4 weeks and the unit then calibrated in a cal lab. I continue to occasionally check the unit against my Geller 10V reference. So far I'm happy with the results.

Appreciate any feedback regarding my approach...

Regards,
Martin

[alm]

Really wish ICs were in sockets by default, especially since there's room.

Careful what you wish for. Some old Tek equipment used sockets for everything, even transistors. Over time, these sockets become intermittent. Multiply this with a few dozen sockets: lots of fun to troubleshoot. Solder connections tend to be much more reliable.

[staze]

Hi all,

Good to see a few of the Keithley 199s being resurrected.

I recently retrofitted my 199 with an LM399 reference. On my unit the original 1N4579 appears to have drifted significantly, so I decided to replace it. Fairly easy to do, as the Zener voltages are pretty close. I added a 107 kOhm resistor in parallel to R69 to bring the reference voltage back to -2.8V. -10VR and +5VR were adjusted by replacing the following resistors:
R67:   25500 Ohm
R70:   11500 Ohm
R77:   10000 Ohm
R76:   13700 Ohm
The LM399 heater was hooked up to the +15V and -15V regulated supply rails.
While I had the unit open I replaced the NE5534 Opamps with LT1001s (DIP8).

The LM399 was burned in for 4 weeks and the unit then calibrated in a cal lab. I continue to occasionally check the unit against my Geller 10V reference. So far I'm happy with the results.

Okay, that's pretty awesome actually. I'll definitely keep this in mind should my ref ever start drifting. =)

For now, I just found a Keithley 197 off ebay for a song that I'll be trying to fix as well. =)

[staze]

Careful what you wish for. Some old Tek equipment used sockets for everything, even transistors. Over time, these sockets become intermittent. Multiply this with a few dozen sockets: lots of fun to troubleshoot. Solder connections tend to be much more reliable.

touché. I guess I just need more experience unsoldering ICs. 

[alm]

Cutting the pins and desoldering each pin individually is usually the best strategy for jellybean parts, since saving the board is far more important than saving the part, and you're not going to reinstall a suspect $0.10 part anyway.

[staze]

Cutting the pins and desoldering each pin individually is usually the best strategy for jellybean parts, since saving the board is far more important than saving the part, and you're not going to reinstall a suspect $0.10 part anyway.

I figured that would be the easiest... though it makes it difficult to test out of circuit. =P