Keithley 2010 Repair

[Cyclotron]

Does anybody know where Q156 is located? It appears to be another proprietary chip.

Q156: TRANS, DUAL N-CHAN JFET, SNJ450H99 Keithley Part #: TG-326

I can't find the bastard anywhere though. Maybe it doesn't exist on my meter, but I do wear glasses. 🤷

Thanks,
Josh

I'll take a look.

As a side note, I've printed the board layout from the service manual and I gave to the wife to play where's waldo when I can't find something because here close up vision is a lot better than mine. 

[KungFuJosh]

As a side note, I've printed the board layout from the service manual and I gave to the wife to play where's waldo when I can't find something because here close up vision is a lot better than mine. 

I usually lose a small screw, or something like that, on my office floor and my wife finds them for me. 

I took high-res photos of my board, and looked at it with a zoomed in digital microscope, and I couldn't find the bastard. Of course, I didn't take off the metal shield plate, so it could be in there.

[KungFuJosh]

the board layout from the service manual

I totally overlooked that since it wasn't searchable. 

You can zoom in on the PDF on computer screen. Why isn't the bastard searchable??

[KungFuJosh]

Here's a vector zoomable copy of just the main PCB layout.

[Cyclotron]

Here's a vector zoomable copy of just the main PCB layout.

I was just thinking about taking the image and running it through OCR to see if it would figure out the numbers.

I have found one part on the service manual board layout that isn't on my board. U196, which is depicted right at the rear of the front/rear switch near U153/U169.  There's no pads or any other hint that the part should be there.  I assume board rev differences.

Also, my wife finds the crap I drop, too. She'll hear me start cursing, and she pops in and says, "What did you lose?"

[Cyclotron]

Just realized there are board change notes on that image too. U196 is on that list.

[KungFuJosh]

I tried Acrobat's OCR, and it sucked. It worked on the obvious outside text, but ignored the component labels on the board.

ETA: I tried it again a couple times, including after rotating the page, and it did a little better. It still can't handle two text directions well though.

BTW- Q156 is a dual n-channel JFET, so we should expect at least 6 pins, right? So it should be even harder to hide. 

[Cyclotron]

I've also tried Acrobat and several tricks I've done before, and it really sucks.

I have another schematic I really want to OCR, so this is a side quest for me now or later.

I have my meter warming up. Been running for 3 hours, no problem. It will fail eventually. 
Side quest today is to plan my DC rail around my bench to supply LEDs and get rid of the flipping awful EMI I have in my lab.
I think I've settled on an LM317 current sink for each strip, since they vary in length and number of LEDs.

My guess is Q156 is a replacement part that may also appear in other board revs.  I can't find it on the images of my board or the service manual layout.
Maybe it was deleted from early boards?

[KungFuJosh]

The only thing near U177 that could possibly replace Q156 is maybe Q160. Q160 is TRANS, NPN PAIR, LM394, Keithley Part #: TG-142.

I dunno if this is true, or total bullshit from google's AI, but it said this:

Finding an LM394 labeled as Q160 confirms you have one of the original, early-generation production revisions of the Keithley 2010.

Here is exactly what is happening on your PCB and why Q156 is missing:
The LM394 / Q160 Swap
The LM394 is not a JFET; it is an ultra-matched Dual NPN Bipolar Transistor pair. In the very first design iterations of the Model 2010, Keithley engineered the low-noise composite pre-amplifier stage for U177 using this super-matched NPN pair (Q160) instead of a dual JFET.

...and looking at picburner's schematic, Q160 is tied to U177, so it's plausible.

[KungFuJosh]

To rule it out, I replaced U176. The problem in the 10V range persists. Not surprising.

Anyway, it's probably worse now. 

Before it would go to -11.9V in the 10V range stay there. Now it goes out of range. Of course, the problem only seems to exist in the 10V range, so switching ranges isn't helpful. Never mind. It's stopping at -11.9V again after warming up for a bit.

I suppose this verifies that I need to chase down some stupid switches.

Is there an simple way to test Q104, Q105, Q108, and Q113?

Does anybody know the correct replacement part numbers for them? The manual says SNJ132199. ETA: maybe MMBF4393LT1G?

One other question- is it possible I'm not cleaning the board well enough, and that's responsible for the leakage?

Thanks,
Josh

[Kleinstein]

The MMBF4393 is a reasonable choice. It is kind of the standard switch.

For checking the switches it is worth to look at the gate voltages. The LM339 comparators are known to be a little failure prone and this may cause some switches to be stuck open.
For testing the switching one look at the voltage after the buffer or main amplifier. AFAIR the issue was high hum for the DCV input. This would be Q104 not turning on.
Another test would be switching the meter to non AZ mode and have a signal generator with a low frequency (like 100 Hz) signal at the input and follow the signal path with the scope.


The schematics around U174 is definitely wrong: The circuit with TL431 and current source is at U190 - the plan somehow combined U190 and U174 to one. One can see the difference on the photos.

U174 should be wired as a buffer. A common block found in such meters is a buffer for feedback voltage (gate of Q193) as a kind of guard and maybe bootstrapping some protection / use for JFET gates as switches.

[KungFuJosh]

Here's a bunch of corrections. I did not test the source voltages yet on U190, I assume for now they're fine. U184 was really U185. Some device pin corrections, device ID corrections, etc. etc.

Thanks,
Josh

PS. Yes, I will share the updated schematic project after I've gone through some more corrections/additions.

[Kleinstein]

There should not be a link from Q184 to U174. That link is very likely toward U190 instead.
Instead there should be a resistor from the TL431 to ground.

[KungFuJosh]

There should not be a link from Q184 to U174. That link is very likely toward U190 instead.
Instead there should be a resistor from the TL431 to ground.

There is no link between Q184 and U174 anymore. Did you mean something different?

I think you meant the opposite? See attached.

Thanks,
Josh

[KungFuJosh]

A few more updates.

[picburner]

I've tweaked the schematic a bit and fixed the errors you found.
It's nice to see someone continuing what I started.
Did you happen to find where the two Q160 (LM394) bases connect?

[KungFuJosh]

I've tweaked the schematic a bit and fixed the errors you found.
It's nice to see someone continuing what I started.
Did you happen to find where the two Q160 (LM394) bases connect?

I certainly wouldn't have wanted to start from scratch, so thank you very much for your hard work that made any of this possible.

I will track Q160 down further when I get back to this. I made a new rack system for my workbench, and it's taking all day to reinstall and wire everything up. 🤦‍♂️ ...my wife painted it. She's awesome at that, and I'm not. 😉

Thanks,
Josh

[Cyclotron]

I've tweaked the schematic a bit and fixed the errors you found.
It's nice to see someone continuing what I started.
Did you happen to find where the two Q160 (LM394) bases connect?

I'd also like to thank you. The schematic has been great. I'd like to contribute changes and updates but I'm not sure what's the best method.

I have reached a point in my repair where I believe the PLCC is the problem, and I need to locate one to test that theory. All of the chips in the Analog integrator controlled by the PLCC have been replaced on the PLCC page, except the PLCC, and all on the Integrator page of the schematic. I've disconnected as many other pathways as possible to eliminate external influences, and I've confirmed that at least the voltages on the TF245 are correct. I believe that U138 is the beginning of the chain for the saw tooth wave form on the integrator and that may be a current drive, and I'm not measuring it right. The signal there is very, very small.

I'm going to work my SMU for a bit and see I can get further along on it while I look around for a PLCC. Josh, if you hear they sell them at Keithley/Tektronix, let me know. I'm curious what they go for.

[KungFuJosh]

while I look around for a PLCC. Josh, if you hear they sell them at Keithley/Tektronix, let me know. I'm curious what they go for.

Which part number specifically? U402/U403? Or something else?

[Cyclotron]

while I look around for a PLCC. Josh, if you hear they sell them at Keithley/Tektronix, let me know. I'm curious what they go for.

Which part number specifically? U402/U403? Or something else?

Sorry, it would be the Orbit, U165.

[KungFuJosh]

Sorry, it would be the Orbit, U165.

Ah, ok. I emailed my contact. I'll let you know what he says.

[KungFuJosh]

I've tweaked the schematic a bit and fixed the errors you found.
It's nice to see someone continuing what I started.
Did you happen to find where the two Q160 (LM394) bases connect?

It looks like they connect to one end of C270. I'm not taking the PCB out right now, but you can see the traces pretty clearly. The other side of C270 connects directly to pin 1 of VR121.

I measured both ends in circuit with my ST42 tweezers, and it read 9.277nF. 10nF is correct, as listed in the service manual: CAP, .01UF, 10%, 50V, CERAMIC.

Thanks,
Josh

[Kleinstein]

The old schmeatics has a C2670 in parallel to VR121.  Chances are this is a typo in the capacitor number and this could be C270.

With the corrected U190 part the amplifier itself looks sensible with the Q160 bases connected to C270/VR121.

For the connections around Q184 , I had show a sensible circuit with the parts in an earlier post:
https://www.eevblog.com/forum/repair/keithley-2010-repair/msg6268114/#msg6268114
It is still missing the Q160 base connection however.

[KungFuJosh]

The old schmeatics has a C2670 in parallel to VR121.  Chances are this is a typo in the capacitor number and this could be C270.

With the corrected U190 part the amplifier itself looks sensible with the Q160 bases connected to C270/VR121.

That seems likely. I will confirm the position of C270 and get rid of C2670.

For the connections around Q184 , I had show a sensible circuit with the parts in an earlier post:
https://www.eevblog.com/forum/repair/keithley-2010-repair/msg6268114/#msg6268114
It is still missing the Q160 base connection however.

I completely missed that correction suggestion. I will investigate.

Thanks,
Josh

[KungFuJosh]

This is where I'm at currently. Pins 2+6 of U174 are harder to trace out. I think they hit a resistor on the back before coming to the front again (no, not R430, which is on the front and 100K). For the moment, I'm avoiding removing the PCB, so I won't be able to trace that out yet.

There's 1kΩ between U174 p2,p6 and U188 p1. I assume that's the path, but I won't update that part of the schematic until reality is checked.

Thanks,
Josh