[fixed] Keithley 2400 in reset loop

[miele]

Hi,
I have been reading this forum for quite a while, so here comes my first contribution:

One of my current repair projects is a Keithley 2400 SMU with the follwing symptom:
1)Power on
2)Display test with beep
3)Display of model and firmware for less than a second
4)reset, start form 1)

My first thougt was, that the 5V/12V power supply is suffering dry alu caps resulting in a breakdown of the supply voltage.
After disassembly and measurement, I found out that both supply voltages were in deed breaking down with every reset.
And when I disconnected the ribbon cables to the analogue board, the rebooting could be stopped.
To be more precise, only the connector opposite to the heatsink (which carrying the power supply) seemed make the supply break down. Leaving this connector open, the unit booted and reacted normally (without beeing able to source and measure of course)

To further verify my assumption, I next tested the 5V/12V power supply separately. This is a small supply, not integrated in the rest of the SMU.
Surprisingly my assumption turned out to be wrong. I stressed the supply with a power resistor, and it could easily deliver current beyond its specification without showing any sign of resetting.

As a next step, I will disassemble further and check the state of the power stage. I read in this forum, that the power stage of the Keithley 24xx series frequently gets damaged and can easily be repaired. But I did not find any report, where the power stage was damaged so badly, that it was shorting the whole device similar to my observations.

I am curious what comes next. I will keep you updated.

Ralph.

[Le_Bassiste]

the power supply connection running from P1002 on the digital board to the analog board  is also carrying the auxillary supplies for the ouput stage pre-driver. if my memory doesn't fail me, they should be +/-30V for the 2400 and +/-70V for a 2410, measured at TP506, TP509  withe respect to TP500 FCOM
these voltages are generated by voltage multipliers around J2, J4 on the digital board. the jumpers determine the multiplier settings, thus the same digital board can supply both 2400 and 2410 versions.
you may want to check as to whether there is a short on the aux supplies, which could indicate a fault in the  pre-driver and its associated circuitry.

[miele]

Hello Le_Bassiste,
thanks for your input. Currently, the problem seems to be related to the output stage for the follwing reason:

I started with checking the transistors of the output stage and found out that Q521, Q523 and Q525 had a short between drain and source or collector and emitter respectively.
The good news is, that after removing these three components, the power supply stoped breaking down and the 2400 finally started up with the analog connected. 

I am not shure, how the unit should behave without these components. I am currently measureing 240V on the output, even with the output swiched off. I belive, this is not correct (even with half of the output stage removed) and I have to look for further damaged components.

I try to understand the (partial) schematics of the output stage shown in the service manual. Does someone know actually, where the output is connected to this circuit?

[z01z]

I try to understand the (partial) schematics of the output stage shown in the service manual. Does someone know actually, where the output is connected to this circuit?

If I remember well, it was mentioned in some topic that the output stage is similar to the previous generation (Keithley 236), for which schematics are available. It might worth to take a look.

[Le_Bassiste]

I started with checking the transistors of the output stage and found out that Q521, Q523 and Q525 had a short between drain and source or collector and emitter respectively.
The good news is, that after removing these three components, the power supply stoped breaking down and the 2400 finally started up with the analog connected. 

I am not shure, how the unit should behave without these components. I am currently measureing 240V on the output, even with the output swiched off. I belive, this is not correct (even with half of the output stage removed) and I have to look for further damaged components.

I try to understand the (partial) schematics of the output stage shown in the service manual. Does someone know actually, where the output is connected to this circuit?

the partial schematic in the service manual is not error-free, but it gives you a picture 
the output of the power stage is the triangle marked "F". it is then routed through the current sensing resistor bank (the blue ones like R451, R452, and so on), then passes a semiconductor disconnect switch (made up by Q415, Q414, Q412 and a PV optocoupler), going through the disconnector relay K206 and the FRONT/REAR selector relays K207, K208, to finally arrive at FRONT or REAR outputs. so, when setting the instrument to OUTPUT OFF, there should always be approx. zero volts across the output terminals.
note that the bipolar supply of the push-pull driver OPAMP U500 is critical for proper operation. the supply is quite "whimpy". so, if the opamps's supply voltages are less than expected (V+ and V- referred to TP501 OCOM TP500 FCOM!!), then you may want to replace U500,  and expect a hole in your pocket.  i can only assume that VR500 and VR501 may be part of the supply circuitry for that particular opamp, because that is exactly the case for my 2410. 
your 2400 is slightly different in that respect, but TiN 's website https://xdevs.com/fix/kei2400/ may give more clarity in this case.

EDIT: forgot to mention, make sure that your instrument is in 2-WIRE mode, and _not_ in 4-WIRE  mode. if you operate it in 4-WIRE mode, without having the sense inputs connected, the ouput voltage can indeed go up to high voltages, no matter what the output voltage was programmed for:
try [CONFIG], [V], SENSE-MODE, [ENTER], 2-WIRE, [ENTER],  [EXIT]
 

EDIT: corrected OCOM and FCOM

[miele]

If I remember well, it was mentioned in some topic that the output stage is similar to the previous generation (Keithley 236), for which schematics are available. It might worth to take a look.

Thanks, I also read about that, but it took me a while to find the right service manual for 236 WITH the schematics included.

note that the bipolar supply of the push-pull driver OPAMP U500 is critical for proper operation. the supply is quite "whimpy". so, if the opamps's supply voltages are less than expected (V+ and V- referred to TP501 OCOM !!), then you may want to replace U500,  and expect a hole in your pocket.

Good hint. I measured the supply of the OpAmp U500 and .. I see approx 240V vs TP501. I am not sure, how that can be, it looks like the whole output stage is lifted by 240V. I guess, there is still something really wrong and I have to measure the way down until I find it. Next step is to check, how the 236 schematics will help me.

i can only assume that VR500 and VR501 may be part of the supply circuitry for that particular opamp, because that is exactly the case for my 2410. 

Yes, they are nearby. Do you know, how they fit in the circuit? Do you know, where the U500 actually get its supply from?
I checked all major test points, the supply voltages seem to be ok vs their respecitive COM. But the shift of OCOM vs FCOM of 240V apprears to much. Could that be?

[Le_Bassiste]

 yep, the shift of opamp supply is indeed ok. the opamp's negative input basically "rides" on the output voltage at TP500 FCOM, and therefore, the supply must actually reference to FCOM as well ( not TP501 OCOM, sorry, my bad  )
note, however, that bypass capacitors of the opamp's supplies may be referenced to OCOM!
have a look at the attachment. it shows the circuitry around U500 of the KEI2410, as i have pulled it of from the pc.b. layout. as i said before, your 2400 may differ,take that schematic with a grain of salt. if you look closely, you can see a schematics error around D513, which seems to have slipped through on early 2410 production units. 

[miele]

Thanks a lot for the schematics. Together with 236 schematics, it gets at least a little bit clearer.

The more I think about it, the more I belive that it make no sense to continue measuring without the missing BJTs and MOSFETs.
I can't figure out, how the circuit behaves with only the positive leg present, but I would not exclude, that the output will swing to the 235V rail.

So I will wait, until the parts arrive and then continue analysis.

[miele]

I replaced now the damaged transistors Q521, Q523 and Q525.

Unfortunately, I still have the +240V of uncontrolled output.

As a next step, I checked the opamp. The supply here is not correct: V+ = 2.42V and V- = 0.43V, so I expect the the supply circuitry has been damaged as well.

While this needs to be checked further, there might be another root cause for the uncontrolled 240V output, as it was also there without Q521 and Q518 present.

Still big question marks...

[miele]

Victory!!! 

Thank you, Le_Bassiste, you brought me on the right track.
While compairing your schematics with a photo of my KEI 2400 output stage to understand, how the supply for the OpAmp works, I suddenly realized that I have overseen the burn mark on resistor R519.

This resistor is part of the current source to bias the zener diode, which supplies the negative rail of U500. That of cource explains, why the supply voltage was not right.

A quick replacement using a wired resistor verified the solution. Everything works as it should. I only need a SMD resistor, to close the case.

BTW: Compaired to your schematic for KEI 2410, the circuit was not changed a lot. The little issue around R559 was corrected. Some component names changed, a lot stayed identical.

[Le_Bassiste]

congrats and thank you for verifying the schematics around U500