Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617

[balage]

So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So when you probe between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminal (triax) of the 617 no change is seen on the display. This still would indicate something is probably wrong with Q308... But than the "zero check" should not have any effect.

Okay, then I should start looking this site: http://www.linearsystems.com/product.html 

[_Wim_]

So I have only applied voltage to the input of the instrument. The reading on the display changed only because of the probing impedance I guess.

So when you probe between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminal (triax) of the 617 no change is seen on the display. This still would indicate something is probably wrong with Q308... But than the "zero check" should not have any effect.

Okay, then I should start looking this site: http://www.linearsystems.com/product.html 

 , that was indeed done in a crude way!

No,I would not look for a replacement fet, but I would do the excellent modification described by user Alex Nikitin: https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/msg759354/#msg759354

I had a good original Q308 and also did the modification, and the LMC662A is readily available.

[_Wim_]

Had another look at your picture, it is Q311 that is replaced, Q308 is the smaller FET that still seems original. Q311 is used as a protective diode. I would start by removing this one, maybe it is shorting to ground. The 617 works perfectly without, it is just less protected at its input.

Q311 is not needed any more when doing the mod described by Alex Nikitin.

[_Wim_]

Please also read the beginning of this thread thoroughly about the cleanliness required of this pcb. Wear gloves at all this because fingerprints can ruin already the performance of this unit. It does seem however the previous owner did not really care about this, to some extensive cleaning might be required, but would still very much avoid additional contamination!

[balage]

Had another look at your picture, it is Q311 that is replaced, Q308 is the smaller FET that still seems original. Q311 is used as a protective diode. I would start by removing this one, maybe it is shorting to ground. The 617 works perfectly without, it is just less protected at its input.

Q311 is not needed any more when doing the mod described by Alex Nikitin.

At the bottom of the PCB there are soldering residuals at Q308, that is why I think it is also replaced. On the top it seems clear around.

I have removed Q311, but it has no effect, everything is the same. Actually the reading become a few milivolts more. So the source of this 90mV is one question, and the other is that why the input nis not reacting for input signals. Where should I proceed? Maybe I buy a new LT1012.

[_Wim_]

I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.

[MiDi]

I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.

First thing to blame is Q308 if voltage NOT slowly rising to overload (leaky Q308).
If you are brave you can replace whole frontend with ADA4530-1.
Than you can get rid of TC and bias offset compensation (cut R332), transient protection (cut Q311) and voltage offset compensation (cut R351 & R352).

[balage]

I would start with the replacing Q308 by an LMC662A as described by Alex Nikitin. If you have to order parts, I would order an LT1012 also as it possibly also took a hit when Q308 died.

First thing to blame is Q308 if voltage NOT slowly rising to overload (leaky Q308).
If you are brave you can replace whole frontend with ADA4530-1.
Than you can get rid of TC and bias offset compensation (cut R332), transient protection (cut Q311) and voltage offset compensation (cut R351 & R352).

Thanks for the tip. After I have repaired the device and still have spirit than i'll try. Your 0,000pA reading looked nice.

But anyway, components for Alex Nikitin style modification are on their way.

[balage]

Components arrived, old JFET removed, LMC662 is put in, LT1012 replaced. But now the 617 displays overload.

So I have measured the voltage going to the AD from the EM board, and now between pin 1 and 10 there is -225V. For sure, -225V!. 

How the f..ing hell can this can happen? Previously it was this:

...
I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4.
...

Supply of LT1012 is still stable +/-5V. Maybe this bootstrapped bridge puts the negative supply to the central line that is going to K301. This must have damaged the AD. Great.

[_Wim_]

Components arrived, old JFET removed, LMC662 is put in, LT1012 replaced. But now the 617 displays overload.

So I have measured the voltage going to the AD from the EM board, and now between pin 1 and 10 there is -225V. For sure, -225V!. 

How the f..ing hell can this can happen? Previously it was this:

...
I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4.
...

Supply of LT1012 is still stable +/-5V. Maybe this bootstrapped bridge puts the negative supply to the central line that is going to K301. This must have damaged the AD. Great.

225 volt can go to pin 10 (same voltage as preamp out), as the preamp out still has the full voltage swing.

[_Wim_]

I think you best should try and read at least chapter 6 of the service manual (theory of operation), and also chapter 2 (operation). This should give you a better idea what to expect, otherwise debugging will be very difficult. The service manual is very well written, but there are some complex parts. After those 2, start from chapter 7.6 (trouble shooting) and follow it step by step. Report back which step failed, this way it might be easier to follow along.

[balage]

I think you best should try and read at least chapter 6 of the service manual (theory of operation), and also chapter 2 (operation). This should give you a better idea what to expect, otherwise debugging will be very difficult. The service manual is very well written, but there are some complex parts. After those 2, start from chapter 7.6 (trouble shooting) and follow it step by step. Report back which step failed, this way it might be easier to follow along.

Yes, you are right! It is now not a simple transistor replace-type of repair. Sorry for exaggerated questions. 

[MiDi]

Do you get overload on zero check?
Is the overload present on highest amp range?
If bias current compensation still present, it needs adjustment.

[analogNewbie]

As far as I understand, the IV converter circuit should work like this
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However, K617 connects the Rf to the In+ of LT1012. Why?
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[_Wim_]

Yes, you are right! It is now not a simple transistor replace-type of repair. Sorry for exaggerated questions. 

You can ask many questions, that is no problem. But when you fully understand the background, you will have much more fun doing the repair, and also learn quite a bit from it. And especially when a good service manual is available, it is the ideal opportunity to really learn how the unit is working. For many of the test gears I have, the most fun I had with them, was during the repair, not once it was repaired, and that includes my 617...

[_Wim_]

As far as I understand, the IV converter circuit should work like this
(Attachment Link) " alt="" class="bbc_img" />

However, K617 connects the Rf to the In+ of LT1012. Why?
(Attachment Link) " alt="" class="bbc_img" />

That is because LT1012 is only functioning as a part of the amplifier (gain stage), and is not the complete amplifier. If you include Q308 (a differential input pair, which causes inversion), the figure is correct.

See also here https://www.allaboutcircuits.com/technical-articles/the-basic-mosfet-differential-pair/ for a nice description.

[analogNewbie]

Thanks.

But Q308 is connected as follower not common source amplifier, right? the phase is not inverted in this stage

[TurboTom]

That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).

[_Wim_]

That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).

Thanks for the detailed explanation, I missed that completely before. 

[MiDi]

There are a lot of mistakes all over the place - not only in the documentation, but also in design - at least the first revisions.

[analogNewbie]

That's the result of the ingenious concept of the 617's output stage that keithley enabled to reach such an extremely wide output voltage range while still using "relatively" common input / gain stage components.

It's obvious, from a common point of view, the tranconductance amplifier stage appears to be connected the wrong way round (inputs reversed). And it's also correct, the input buffer (Q308) doesn't cause the polarity reversal. Instead, it's very important to have a close look at the power supply and especially the "grounding" scheme. The PSU for the output stage (+- 210V) isn't referenced to signal ground. Instead, the output terminal of the output stage that usually drives the load, is grounded, thus pulling the whole output voltage PSU "around", providing an inverted output signal at the output stage PSU's common terminal. And that's how Keithley's transconductance amplifier works, and also, why they use such funny (sophisticated...), staged and screened supply voltage transformers. This concept allows Keithley to drive the output stage with the low output voltage range available from the LT1012 by basically converting a voltage follower output into a true (common emitter) voltage gain output. Yet, this concept still keeps the voltage follower advantage from a biasing / driving point of view, eliminating all the voltage-shifting otherwise required.

The schematic is somewhat confusing since the multitude of supplies and ground symbols aren't always in direct view. It may be recommendable to print the page with the PSU schematic and have it always as a reference when working with the other schematics.

Edit: For a better understanding of the explanation, please see pages 6-5 and 6-6 of the manual. Observe where "Gain Stage Common" is connected to.
Edit2: Only now I noticed that Keithley made a mistake in the schema of the current output configuration: Both the "lower" supply and the transistor are of the wrong polarity (is NPN, should be PNP).

Thanks for the details.  I have tried in the LTSpice.

[snik]

I bought recently a defective K617 and the Input Dual-Fet was destroyed in this unit also.
Because of restoring it for original look inside, i don't want to make the OpAmp mod.
Now i managed to get 5 Transistors for replacement (I also found an old original Micro Power Systems inc. Datasheet).

Since they arrived, i asked me, for which values Keithley have selected the FETs for the Electrometer and if i can do a selection too ...

The 2N5906 is the one in this series who has the best values, maybe a selection is not necessary and i should dice for one. 

Do anybody know how and for what they selected the Input Dual-FETs ?

[r6502]

Hello all,

I also got now my 617 from EBAY.

When it came, it showed in all ranges Overflow - ok, for Ohm's this would be fine if connections are open - ;-) ...

A check of all pats of the device showed, that U307, Q303, Q305 and Q307 on the electrometer board where blown.

U307 is easy to fix, it is an LM337. But the transistors Q303, Q305 and Q307 are MPS-U60 types from good old Motorola.

I have looked at different mfg sites of discrete bipolar transistors and found out, that MJD340 (NPN),
MJD350 (PNP) will fit mostly to the parameters of the original MPS-U10 / MPS-U60 types, but the parameter "Emitter−Base Voltage" is only 3V with the Motrola  type. The Transistors specified with 6V here.

Does anybody have an Idea, if MJD340 and MJD350 will be fine replacement for MPS-U10 / MPS-U60 - yes, thy are SMD, , but I think, could be adapted ...


Kind regards

Guido

[MiDi]

Since they arrived, i asked me, for which values Keithley have selected the FETs for the Electrometer and if i can do a selection too ...

The 2N5906 is the one in this series who has the best values, maybe a selection is not necessary and i should dice for one. 

Do anybody know how and for what they selected the Input Dual-FETs ?

As this instrument has input current in range of couple of fA, at least the input FET has to be in that order.
Input current has to be canceled anyhow (+-20fA adjustment range), but the lower the input FET bias current is, the better.
One way is to put them into the unit, disconnect cancelation circuit (R332) & all reed relays at the input node and get the reading as mentioned in performance verification for input bias current after ~1day settling.
K307 has to be bridged manually for zero check & zero correct while operating the unit (! dangerous, high voltage present).
Very good cleaning is mandatory when dealing with fA, especially the teflon standoffs and between legs of FET.

Alternative is to meaure FETs with another EM

Chances are that some of the critical reed relays show significant leakage/DA, which would degrade input current - see previous posts for details.

I have looked at different mfg sites of discrete bipolar transistors and found out, that MJD340 (NPN),
MJD350 (PNP) will fit mostly to the parameters of the original MPS-U10 / MPS-U60 types, but the parameter "Emitter−Base Voltage" is only 3V with the Motrola  type. The Transistors specified with 6V here.

Does anybody have an Idea, if MJD340 and MJD350 will be fine replacement for MPS-U10 / MPS-U60 - yes, thy are SMD, , but I think, could be adapted ...

MJE340/350 are the THT versions of MJD340/350, but have different pinout than MPS-U10/U60, either should work as a replacement.
Veb is not relevant for this (and most) circuit, in general the emitter base breakdown voltage has wide spread and DS numbers are given for safe operation w/o breakdown in reverse bias only.

[r6502]

@MiDi:
Thanks for the reply. Theese I can get really easy I think.

@snik:
As MiDi wrote, one parameter would be the input current of the JFETs and how equal both JFets fit together. Another thing ist the thermal drift. Also the chacteristic from input voltage to drain current will be cheked I think. The 2 JFETs are already good matched, but some will be better than other, and these are beeing selected to use for the instruments.

Question would be, how to measure all these parameters  - for the input current you will need an electrometer ...

I'm going to order some transistors, and I will also replace all electrolytic caps - some look not so well.  Then I will continue to check the rest of the instrument. Calibration will be easy for me, especual the current range - own a 263 current source ...

Does it make sense, to replace the internal reference of the device or is it just fine for the application?

Kind regards

Guido