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

[e61_phil]

Hi,

today I got my Solartron 7071. After a quick check of the meter I removed the LK17 jumper to get an 7081.
I thought it is a great upgrade of my HP 3456A and a relatively cheap entry in the world of 8,5 digits.

I connected the meter to different sources (Knick JS3010, Fluke 343A and a LTZ1000) and regardless of the source the last digit is jumping all over the place. That means in 5,5 digit mode it is only a usefull 4,5 digit meter, in 6,5 a 5,5 and so on... At the same time my 3456A was connected and the reading was stable to 6,5 digits even with only 10NPLC.

The meter itself seems to be stable. In 8,5 digit mode measurements against my sources are all well below 0.5ppm over an hour. It seems to me like a noise ADC.

Is this a normal behavior of this unit? So, I think without removing the jumper to get a 7081 there is no benefit to my HP 3456A.

Philipp

[Andreas]

Hello,

according to test spec at 10V input the rms noise should be below 3uV RMS for 10 successive readings.
(or 20 uVpp).
http://www.perdrix.co.uk/Solartron7081/Solartron_7081_Test_Spec.pdf

with best regards

Andreas

[ManateeMafia]

Congratulations on your new gear. Check the usuals... power supplies. Also, some of those had the Varta battery. I have seen a couple of leakage issues that went all over. Some went down the plastic standoffs and created a path for the power supply rails. Also, it can get under parts that look fine but are eating away at the traces. No cals are lost with the battery, but it can make it boot/hang with errors.

[e61_phil]

I did some further measurements with my HP 3456A in parallel.

The Solartron was set to 6,5 Digit Mode and the HP 3456A was also set to 6,5 digits and 10 NPLC. So, both meters deliver a new value every ~400ms.

To be sure there is no problem with cable EMF or something like that, I shorted the cable directly on the calibrator (Fluke 343A was used). The measured peak-peak value was about 600nV on the Solartron and a bit higher on the HP 3456 (much cheaper cables were used) (short.PNG). Everything was well below 10µV so I shouldn't care about it in the 10V range with 6,5 digits.

The next measurement was 10V from the Fluke 343A. The HP 3456A showed a very stable measurement with only a few times one last significant digit up (1ppm peak-peak). Unfortunately, the Solartron showed 14ppm peak-peak and 1,3 ppm rms (10V.PNG).

I can't believe that this should be ok for this meter. Is that really the case??

[Andy Watson]

Are you running it as a 7071 or a 7081? And does it make a difference to the last digit?

[e61_phil]

I'm running it as a 7081. I don't know if it makes a difference. I will have a try tomorrow.

[e61_phil]

In 7071 mode, there is no difference in the measurement.

[acbern]

I have a 7071 (not modified to 7081), and this is certainly not ok, provided the source is stable (as seems to be the case here).

[e61_phil]

I have a 7071 (not modified to 7081), and this is certainly not ok, provided the source is stable (as seems to be the case here).

Do you have any measurement over a few minutes with a stable source in 6,5 digit mode?


If one activate the digital filter with a walking window it looks fine. So it is really noise and not an instability, I think

[e61_phil]

With the gliding average window (aka digital filter) switched on everything looks much better. The attached measurement was done in 7,5 digit mode and the 3456A set to 100NPLC. The measurement shows the warmup of the Fluke 343A (which makes some problems now, but that is another story..). I added some offset to one of the meters to align the curves.

[Mickle T.]

With the gliding average window (aka digital filter) switched on everything looks much better. The attached measurement was done in 7,5 digit mode and the 3456A set to 100NPLC. The measurement shows the warmup of the Fluke 343A (which makes some problems now, but that is another story..). I added some offset to one of the meters to align the curves.

I think this 7081 have a problem with a PLL to mains synchronization. Look at the "9.8 Analogue checks" of the Solartron 7081 Maintenance Manual to adjust the PLL frequency.

[e61_phil]

@DigilentMinds.com it is a doom-loop

I think this 7081 have a problem with a PLL to mains synchronization. Look at the "9.8 Analogue checks" of the Solartron 7081 Maintenance Manual to adjust the PLL frequency.

To not destroy anything: Is it right to disconnect the power cord from the 7081 and only supply 2.5V from an external power supply Between TP904 (-15V) and the low side of C806?

I'm a little bit confused about: "monitor the voltage at the negative end of C806 with respect to TP904. The voltage should settle to 2.5V +/- 0.3. Adjust C807 to correct if necessary."

If I supply 2.5V why should I measure it? And how can C807 adjust my external power supply?

And do you know if 5.21MHz to 5.27MHz is also the right frequency for 50Hz?

The next step is: "Disconnect the bench supply and switch on the 7081 using 60Hz mains frequency. Check that, after settling, the voltage on C807 is 2.5V +/-0.5V." Know I should measure von C807?

Thx
Philipp

[e61_phil]

My first measurement with line cord connected on Pin5 of the IC835 shows 5.24 MHz is it ok for 50Hz or should it be 4.36 MHz?

[Mickle T.]

1. 7081 must be connected to the mains, switched on and preheated during >15 minutes.
2. PLL in the steady state must have a 5.2428 MHz output frequency (at any 50 or 60 Hz mains frequency) and 2.5 V on the input (C806). If not, slowly adjust C807 to correct it. Absolutely no any external supply needed!
 

[e61_phil]

1. 7081 must be connected to the mains, switched on and preheated during >15 minutes.
2. PLL in the steady state must have a 5.2428 MHz output frequency (at any 50 or 60 Hz mains frequency) and 2.5 V on the input (C806). If not, slowly adjust C807 to correct it. Absolutely no any external supply needed!

Thx

I've attached the measurement between TP904 and C806. It seems to be a bit low, but within the +/- .5V tolerance.

If I adjust C807, I think it will change both frequency and voltage. Is it right, to have a look on the frequency to adjust C807 and not on this 2.5V?

[Mickle T.]

I think it's all Ok. No adjustment needed.
The next of the possible problems is a zener 

[e61_phil]

I think it's all Ok. No adjustment needed.
The next of the possible problems is a zener 

To late I adjusted it a bit more to 2.5V but you're right: no change in the reading noise


I am reassured that it does not seem to be normal with the reading noise. I've read in your documentation pack: 6µVpp in 7,5 digit mode and 4µVpp in 8,5 digit mode. Even 6µVpp in 7,5 digit mode can cause the last two digits to jump. If the noise increases in 6,5 digit mode I thought it can be normal for the meter (which I think is very bad).

What is your meaning on the measurement noise I showed here?

[Mickle T.]

It's a difficult question. The most silent of my 3x Solartron 7081 has a zero noise on the 6.5-digits 10 V mode with 10 V reference connected. The most noisy has a 2-3 ppm jumps. (without any moddings, of course)

[Kleinstein]

The significant higher noise for the 10 V measurement compared to the short circuit measurement is suspicious. It points to a problem with the reference voltage or possibly leakage at the input protection if the 10 V are to close the the limits.

It might help to do a test at 5 V as well (to exclude beginnging leakage) - noise from the reference would be half, leakage from getting close to onset of limiting should be much lower.

Another test would be measuring resistance - here reference noise should cancel out, but still using the ADC at high values. So if readings are more stable this would point to a bad reference.

One could test the line frequency suppression by measuring the DC volts from a pure AC voltage (e.g. about 5 V AC (line synchronous) in 10 V DC range) - a bad PLL lock would result in a low difference frequency. Adjustments at the PLL should not chance much on the lock, so it would mainly change the lock range and maybe the speed to get a lock, but hardly the final readings once the PLL is locked. With the PLL control voltage not at it's limits the PLL is very likely locked.

[e61_phil]

I measured the input current earlier today and it was about 8pA on a 10V input. (100G in series to the input will cause 800mV of drop)

Is it possible to have a very noise but stable reference? A measurement on the LTZ1000 in 8,5 digit mode and digital filter on was stable to below .4ppm over a few hours.

I will do some measurements with 5V. For ohms measurement I have to install a second pair of binding posts first.

Thanks for your great help!

[e61_phil]

I had a short try with 5V and it gives also 14ppm peak-peak.

Now, I set both meters in manual range 10V and short the cable at the calibrator. In this setup the Solartron is measuring 110µV peak-peak


I think it is a problem with the ADC. I tested the manual ranges 1V, 10V, 100V and 1000V and the noise is in every range around 10 counts.

Edit: The maintenance manual stated, that this could be the chopper amp (IC201 ICL 7650).

[Mickle T.]

Shorting the DMM's cable isn't enough to check ADC noise. 7081 have a special test mode to do this: type "mode,test0" in the RS232 terminal.

[e61_phil]

Shorting the DMM's cable isn't enough to check ADC noise. 7081 have a special test mode to do this: type "mode,test0" in the RS232 terminal.

I've connected the Solartron via GPIB, but I think that doesn't matter.

In TEST0 mode there is also these around 10 count noise (see attachment).

[e61_phil]

I replaced IC201 (ICL 7650 chopper amp) but that doesn't help

[Mickle T.]

The next thing is a power supply (+15, -15 and 5 V).