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

[Noopy]

With different illumination that thing at the side of the metal structure looks always different than "normal dirt".
Perhaps that´s a production problem leading to a leaky JFET?

Degraded performance caused by an ESD strike?    It presumably tested fine at manufacture.

I´m no expert regarding semiconductor damages but I´m pretty sure ESD causes little holes. Here it looks like there is too much metal.
I could imagine that the part was good but some manufaturing residue has degraded the performance over the years...

P.S.

Couldn´t assign the "M" logo...   

Motorola maybe?

Possible but I haven´t found a Motorola-Logo looking like the M on the Q308...

[Noopy]

@Noopy,
Thanks for these nice insight pictures.
Do we actually know, who the manufacturer of Q308 is and is there a datasheet around ?

I was happy to get such an special IC! 

Unfortunately I also wasn´t able to identify the manufacturer. Couldn´t assign the "M" logo... 

NEWS:
The "M" stands for Micro Power Systems! 
Unfortunatelly you don´t find very much about the company and nothing about this FET. 

[balage]

Hi fellow 617 users,

I will join the club soon! I have bought a non-working as-is 617 on Ebay that does not turn on. The device seems cosmetically good in and outside, so I am eagerly waiting to get the item and start the repair. I am sure I will need your help and experience.

I hope all the reeds are good.

Cheers!

[_Wim_]

I will join the club soon!

Welcome! I wish you lots of fun, both with repairing the 617 and playing with it afterwards (or even doing some actual work with it)

[balage]

Here I am! I was sure it is not a "fuse replace complexity" of problem but it seems I have to go deep into the digital circuitry (first...).

I can see that the input stage was repaired; the input protection transistor pair and the input JFETs are changed. Some LDOs on the analog board are also changed. But the main problem is that the device does not turn on.

What it does is turning on some LEDs and digits on the front for 2-3 seconds. Power rails are okay, but the motorola microcontroller does not run the crystal. Now I am waiting for a new crystal.

But anyway, has anybody seen such symptoms?

Thanks,

[Johnny10]

Very Cool

[_Wim_]

What it does is turning on some LEDs and digits on the front for 2-3 seconds. Power rails are okay, but the motorola microcontroller does not run the crystal. Now I am waiting for a new crystal.

But anyway, has anybody seen such symptoms?

Thanks,

I also had a broken crystal, but the one used by the ADC, not the one used by the micro. I find it strange the leds & digits go again out after 2-3 seconds. That does seem like the micro is doing something, or a power rail that shuts down (overload?).

[balage]

I also had a broken crystal, but the one used by the ADC, not the one used by the micro. I find it strange the leds & digits go again out after 2-3 seconds. That does seem like the micro is doing something, or a power rail that shuts down (overload?).

It seems the power rails (at least around the micro) are stable and noiseless.

But, I've discovered that the micro's reset pin is tied to the ground while it should go up to ~4 volts after a second. I cannot find where the reset signal is coming from. On the schematic there is a label for the reset signal so maybe it is controller from somewhere else.

Fast access to the schematic, page 163: https://www.univie.ac.at/photovoltaik/umwelt/ws2015/K617_original.pdf

UPDATE 1: The reset signal goes to the GPIB controller IC U113.

UPDATE 2: There is a typo in the manual. It says the reset pin is pin 3 but in reality it is pin 1. See the section below. And the real reset pin goes to high as it should.

[_Wim_]

Do you see any other pulse trains on for example pin 4 and pin 6? Does the unit has any reaction to button presses?

[balage]

I have changed the xtal but nothing has changed. Maybe the MC146805 uC is dead?! But it is only avaliabe on ebay and that's what I have when you have to wait for a component for 1 month. 

Anyway. There is totally no response from the microcontroller, buttons have no effect. What I also see is that the display board does not produce the button release pulses. Its strange because the display board has the power supplies and there is nothing there that could likely go wrong.

[TurboTom]

If supply voltage is present at the 6805 and the oscillator isn't running, I'ld check the 10M resistor parallel to the crystal and the two load capacitors. If all that checks okay and you still won't get oscillation, it's likely the processor is at fault. As a final test, you may try supplying an external clock signal to pin 39 of the microcontroller. If it then shows some signs of life, only the oscillator section may have shifted out of tolerance (gain / offset).

[_Wim_]

What I also see is that the display board does not produce the button release pulses. Its strange because the display board has the power supplies and there is nothing there that could likely go wrong.

The buttons are scanned by the micro controller via pins 9,10,11. If the micro is not running, no pulses will be seen.

[balage]

Hey,

Sorry for not reacting. I was busy (and lazy) and had only time to order the microcontroller. Has just arrived! It works now! Scans the buttons and runs the oscillator.

However it seems this is the only thing that is working good. The voltage source was bad but it seems only one of the optos is wrong.

The instrument is reading the followings:
Input state                    Opened                 Shorted
Volts:                increasing to overload         90mV
Ohms:                      inc. to overload          900Ohm
Current:                         0,9mA                   overload

For me it seems the input circuitry gets 90mV or something from somewhere. That is why it measures 9xx in different configs. 

Do you have any idea where should I start? The input protection BJT pair (near the end of the red minigrabber) is replaced a rude way.
[images later]

[_Wim_]

What do you measure on the preamp output at the back? The preamp output has the signal before is goes to the ADC, so this way you can already see if the problem is on the analog board or the digital board.

When you are in manual ranging, does it behave the same on all ranges? Or are only some ranges faulty?

[balage]

The preamp outputs 0,09 always in all ranges. At least it is stable    For current is outputs -0,09. So changes the polarity.

Both the preamp and the display show the same value in all ranges, e.g. I mean 90mV in all ranges.

NO! Sorry, I was wrong. For volts it shows 90mV in all ranges. But in resistance it shows this in the ranges, with rounded values:
0,09G -> 0,090G -> 0,0900G
9,00M -> 0,900M -> 0,0900M
9,00k -> 0,900k -> 0,9000k

0,90mA -> 0,9000mA
0,90uA -> 0,900uA -> 0,9000uA
0,90nA -> 0,900nA -> 0,9000nA
For picoamps the sequence is the same. Actually for current it is straightforward, it is always 0,9xxx.

It seems an offset voltage is injected somewhere in the measurement loop.

[_Wim_]

Ok, so the problem is definitely on the analog board. 

What happens is you actually measure a resistor, voltage or current? Does it go up with the correct amount, or do you see no change?

[balage]

The voltage source is now working. U124 needed to be changed.

So the displayed value is not changing in any of the functions. The LT1012 (U309) has a moving bootstrapped power supply as far as I can understand it. It is good. The reeds are also clicking when I change function.

I have measured that 89mV is going back to the motherboard on J1014 between pin 1 and 10 and 4. At pin 10 there is signal if K301 is operated. Should I brave enough to move W303 jumper to + or -5V? What is this for?
Also should I put a voltage here to the middle of W303?

UPDATE:
I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.

[Kleinstein]

The W303 jumper is for coarse trim / compensation of offset of the FET pair.  Remember where it started and watch out for ESD as the FET can be ESD sensitive. 

10 mV across the input of U309 is normally too much - especially for an LT1012. So the U309 should be close to one side of it's supply (bootstrapped +-5 V) With a fixed output, there should not be much movement for the bootstrapped supply - more like close to +-5 V. The 90 mV shift is not much.

[_Wim_]

I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.

Did I understood you correctly that, when applying a voltage between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminals of the 617 no change is seen on the display. This would indicate something is probably wrong with Q308...

[_Wim_]

What happens when you do "zero check". Does it change the reading on the display? I presume it makes also no difference?

If you measure on pin3 of Q308 with a voltage applied to the input terminals, do you see any change with the input voltage on or off?

[balage]

I measured voltage between U309's pin 2 and 3. It was 10mVolts. When I have applied a voltage to the input, the voltage was the same. But while doing this the displayed value changed.
Also I have measured the output of this opamp and the output was constantly -77mV while I changed the input voltage.

Did I understood you correctly that, when applying a voltage between pin 2&3 of U309 the display changes, but when applying a voltage to the input terminals of the 617 no change is seen on the display. This would indicate something is probably wrong with Q308...

Yes, I was not so accurate. 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 any voltage on the triax input has not effect. But zeroing works fine.

[balage]

The W303 jumper is for coarse trim / compensation of offset of the FET pair.  Remember where it started and watch out for ESD as the FET can be ESD sensitive. 

10 mV across the input of U309 is normally too much - especially for an LT1012. So the U309 should be close to one side of it's supply (bootstrapped +-5 V) With a fixed output, there should not be much movement for the bootstrapped supply - more like close to +-5 V. The 90 mV shift is not much.

Then the 90mV is coming from the center rail of the bootstrapped supply. I mean from between R306& R307. Confusing.

[_Wim_]

Yes, I was not so accurate. 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 any voltage on the triax input has not effect. But zeroing works fine.

"Zero check" works fine? That would mean Q308 is at least doing something, and I would also expect you see the effect of a voltage at the input terminal (triax). "Zero check" grounds the input using K307 just before Q308.

Could it be you confused "zero check" with "zero correct" (which is only a micro-controller delta function)?

[_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.

[balage]

  Sorry for this. So zero check has no effect but I hear the reed-relay clicking.

Q308 was replaced so something was around here before I get the 617. 9121TN is written on the case. Google shows nothing for that. The paired BJT is replaced a rude way...

So if at the inputs of LT1012 nothing changes when a voltage is applied to the triax then Q308 is dead, right? +/-5V from the bootstrapped supply is good.