Solartron 7071 / 7081 last digit is jumping all over the place

[acbern]

Thanks for the hint re. chopper isolation. Charge injection could be an issue, and I have not thought about that yet.
I had tested the input resistance, it varied, some ball park was 10 meg., it varied, hence the input drift.

The problem I have with stability in principle is acutally similar to the one e61_phil has, at a higher level. I have read somewhere on voltnuts someone had an issue with a protection diode in the input, causing noise/leaks, and mine could be just more leaking, just at a much higher level.
As far as parts are concerend, I think the IFN401 is a good bet, and re. input low leak protection diodes, the PN4117A jfet seems a good solution, so I may try this. Any opinions?

e61-phil: I am in the Munich area. Not sure swapping (at this stage) is the best for both of us, these are rare parts, mine may be broken, while yours at least are only a little noisy. Thanks, I may come back.
Still trying to find parts. The ones I mentioned should be ok, other suggestions wellcome.

[Kleinstein]

The protection diodes at the input only see a very low voltage ( < 100 µV) - so a normal low leakage diode like BAV199 should be well good enough. The circuit might not like too much light.

I would be very careful without those diodes, as I don't see an other ESD protection.

For the input current one usually measures the voltage reading with something like an 1M resistor - so the result would be a measured voltage or current.
The alternative test is a low leakage capacitor in the 1-10 nF range, and than watch the capacitor charge discharge - this way one can get the input current at different voltage levels too.

Before changing the rather expensive input FETs, I would check other parts first. One would be the drain voltage of the input FETs - in case the transistor array failed. For a first test one could get away with two single JFETs. At first glance the data for the IFN401 don't look that special (except rather low capacitance which should not be critical here). DC performance is set by the chopper part anyway.

As far as I understood the meter, there is an option to test the ADC without the input amplifier.

[e61_phil]

For the input current one usually measures the voltage reading with something like an 1M resistor - so the result would be a measured voltage or current.
The alternative test is a low leakage capacitor in the 1-10 nF range, and than watch the capacitor charge discharge - this way one can get the input current at different voltage levels too.

My method to measure input current at different voltage levels was simply to connect a high resistance (10G for example) in series to a calibrator. With this setup one can calculate the drop over the resistor and accordingly the current.

[acbern]

For the input current one usually measures the voltage reading with something like an 1M resistor - so the result would be a measured voltage or current.
The alternative test is a low leakage capacitor in the 1-10 nF range, and than watch the capacitor charge discharge - this way one can get the input current at different voltage levels too.

My method to measure input current at different voltage levels was simply to connect a high resistance (10G for example) in series to a calibrator. With this setup one can calculate the drop over the resistor and accordingly the current.

I did that, using an external resistor (1M), and also did that using just the internal 2x18k protection resistors. Same results re. leakage current/input resistance (again, it anyway fluctuated). It also matched the input resistance measured with a simple meter. Since the input resistance was so low (10M range), I didn't even have to go to a capacitor to test it.

[wiss]

What voltage range are you in? 10 M is way low for the lower ranges.

[Kleinstein]

A 10 M resistance would point to something like the high voltage divider always active - so it could be a hanging relay. For the higher voltage ranges (> 20 V) the 10 M input resistance is normal and so is some extra noise. to really test the input amplifier noise  one should use the 100 mV range and for the input bias the 10 V range might be more suitable (to test at higher voltage).

[acbern]

The range I was focussed on was 10V. Therefore the roughly 10M is really a leak issue (it is not the 10M input divider that is on by mistake, and the leakage was even lower than that). I found this when measuring the input voltage and the voltage inside after the 2x 18k. There was a considerable voltage drop accross these two resistors, and drifting. Actually, the input resistance started at about 1M, but is now up at about 10M.
I have eliminated D101, C103, have decoupled the chopper (R406, 407, 408 removed). The voltage changes a bit of course then, but the high leakage and slow drift is still there, so its not the chopper.
I am thinking this only leaves D401, D408, TR401 (and TR412). D401 and 408 are in plastic TO92 case, so I would think photo current is not an issue, and leakage (forward and reverse) are probably neither, since the voltage accross them is low. Still, as its easier, these are the next candidates. And then TR401.
Any thoughts?

[Kleinstein]

One cannot measure leakage current in MOhms - it should be nA or pA !

When removing the chopper part, some drift is normal. If the voltage did not change much when disabling the chopper part, it could be that the chopper part did not work all the time. Also still having a similar drift points towards a problem with the chopper part. Either because the chopper part is broken, or the offset of the normal amplifier got too large.
With to much voltage (e.g. 10 mV range) at the chopper part this might look like leakage. As the copper can have a relatively low (e.g. 2 M) impedance between it's inputs. This is normally not a problem because the voltage is essentially zero if the chopper is working correctly.

So how much offset was there with and without the chopper part active ?

[acbern]

One cannot measure leakage current in MOhms - it should be nA or pA !

That is obvious, but I did not say this.

When removing the chopper part, some drift is normal. If the voltage did not change much when disabling the chopper part, it could be that the chopper part did not work all the time. Also still having a similar drift points towards a problem with the chopper part. Either because the chopper part is broken, or the offset of the normal amplifier got too large.
With to much voltage (e.g. 10 mV range) at the chopper part this might look like leakage. As the copper can have a relatively low (e.g. 2 M) impedance between it's inputs. This is normally not a problem because the voltage is essentially zero if the chopper is working correctly.

So how much offset was there with and without the chopper part active ?

The chopper was totally isolated. All three resistors were removed, so chopper disconnected. The change in voltage displayed was minimal, in the 20mV range, expectable range. In that state, with essentially TR401, IC401 and 402 remaining as a differential amplifier, there should have been no leakage current at all. I do not see a rationale why in that state the disconneted chopper should cause an input leakage current (with sligtly varying value, causing a drifty reading, caused by a voltage drop accross R5 and 6, in the 5mV range), when disconnected. The input only sees FET gates arround the Input Amp and the two antiparallel diodes. Still there is a high leakage current flowing. That is definitely wrong (may not be the only issue). RL7/4 also in the path, and not eliminated yet as cause, but not that likely for leakage, but still tbc.
Also, with the chopper connected, the small changes in the mV range at the TP403 chopper demodulator output (which was at about 2V) were not correlated to changes in the diff amp input voltage/meter reading. The chopper may still have an issue, beyond whats going on with the FETs/diodes, but the 2V output at TP403 does not look wrong at a first glance.

[Kleinstein]

2 V at TP403 sounds reasonable, especially if there is some movement too. So chances are the chopper could work and the problem is in deed leakage. Of cause the copper can not cause leakage when disconnected - it was not clear that there was still leakage with the copper disconnected. There was just not much information on how much leakage / offset problem was there before and after disconnecting the chopper.

When the output without the chopper is only changing by about 20 mV this indicates the normal differential amplifier is also still working relatively good. It could still show some drift - even with JFET pairs drift in the 10-50 µV/K range is normal.

5 mV at the 36 K input protection resistors would be a leakage current in the 100 nA range, so definitely too much, and more than one should expect from a dirty board.

The easy part to test would be probably the protection diodes. For a first test even 1N4148 should be much better than 100 nA. Ane could also use base-colector junctions of small transistors (like Bc547) as low leakage diodes.
This would mainly leave TR401 and TR412 as the possible leakage paths.
IC401 could contribute indirectly: if broken the voltage at TR401 drain could be too high - worth a test as there are quite some reports of those CA3xxx transistor arrays to go bad (for no good reason).

One could check the function of TR412, by looking at its offset.

[perdrix]

Did anyone ever reach any conclusions on how to fix or at least mitigate this problem (apart from turning on the Digital Filter)?

Thanks
Dave

[Kleinstein]

Did anyone ever reach any conclusions on how to fix or at least mitigate this problem (apart from turning on the Digital Filter)?

Thanks
Dave

There are several problems describes in this thread: One is ADC noise, a second a not so well working AZ function and at last a broken input amplifier.

The relatively high noise is a known property of this meter. Form the correlation of the samples it looks like a higher frequency noise effecting the first / last comparator reading and is thus not adding up. So it gets better over time. It does not seem to be the LM311 and the noise seems to be higher than expected from the circuit. So it's more like a problem with supply noise or similar.

The AZ problem is more like a principle problem - that might be partially corrected with a software update. But it also has a root cause in the type of ADC with a large integrating cap that changes voltage quite a bit. So there is some effect of dielectric absorption - so there might not be a really good solution except a different type of ADC. This could kind a limit to this type of ADC.

[pigrew]

I'm in the process of restoring an abused 7081 at the moment. Based on the above discussion, autozero seems to be a major issue with these meters. I've not yet turned on my unit (it had some bulging capacitors, among other things. I'll create a repair post at some point.).

The AZ problem is more like a principle problem - that might be partially corrected with a software update. But it also has a root cause in the type of ADC with a large integrating cap that changes voltage quite a bit. So there is some effect of dielectric absorption - so there might not be a really good solution except a different type of ADC. This could kind a limit to this type of ADC.

I noticed that the download package from xdevs/ko4bb has tweaked firmware which extends the autozero pre-delay time (both a version 1.0 and 1.1). Is it generally suggested to install the tweaked firmware?

[borghese]

Hello
My 7071 jump 3 digits in 7x9.
It is known that the 10.48576 MHz oscillator, in the floating logic, for synchronisation with the main supply is not very stable; if you have a stable signal generator, you can disable TR805 and inject the signal on TR806 base.
I think Mikle T. has already done this
I have not read all the posts; my apologies if anyone has already suggested this.

[tggzzz]

Hello
My 7071 jump 3 digits in 7x9.
It is known that the 10.48576 MHz oscillator, in the floating logic, for synchronisation with the main supply is not very stable; if you have a stable signal generator, you can disable TR805 and inject the signal on TR806 base.
I think MikeT has already done this
I have not read all the posts; my apologies if anyone has already suggested this.

It should lock to whatever the mains frequency happens to be at the moment. When I got my 7081, it didn't.

Ensure you have the right division ratio for the 50/60Hz, and, if necessary, tweak the trimpot 6-30pF C807 near the varactor.

After tweaking there was a significant improvement.

[branadic]

What is the reason that the oscillator is instable? If it's due to temperature one can use a  crystal heater to stabilize it. All you need is some voltage between 8-12V for it.

-branadic-

[e61_phil]

It is due to the instability of the mains frequency. There is no crystal. It is a PLL which is locked to the mains frequency.

[tggzzz]

What is the reason that the oscillator is instable? If it's due to temperature one can use a  crystal heater to stabilize it. All you need is some voltage between 8-12V for it.

It is locked to the mains frequency to reduce some noise/interference effects. That's why you sometimes see people specifying high resolution measurement time in terms of NPLC, number power line cycles.