Author Topic: Repair of Solartron 7081 SN#718  (Read 11964 times)

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #75 on: April 29, 2020, 05:38:25 pm »
Battery arrived and is already soldered in place. Rechargeable battery is still on its way and will hopefully arrive tomorrow.
Cable clips were removed from old glue and reinstalled with double sided tape. Unit step by step returns into something of value.

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #76 on: May 01, 2020, 05:38:42 pm »
Rechargeable battery didn't arrive, but I found two 0.47F / 5.4V supercaps, that I installed instead. Handle rubber had a nice cooking in water with vinegar, the old cigarette smell is now gone  :-+
Meter is now warming up, then we can give it a first measuring and see where we are.
I started designing a new front panel for binding posts. Unfortunately LT binding post don't fit, so plan b is to use Pomona 3770 like Marco Reps instead. Hopefully I can finish that design this weekend. Other than Marco I want to keep the calibration lock at the front.
A lil bit of a longer lasting project is the design of a new reference board like MickleT. did, but also different: LTZ1000CH, TDP resistor network for temperature setting, ...
And I will start analysing the schematics to see, if we can get noise down with modern opamps.

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Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #77 on: May 01, 2020, 07:55:30 pm »
There was an older thread on the 7081 meter. AFAIR the meter is relatively noisy and there was not much success reducing the noise very much. From a short look at the schematics, there may be a few point where a more modern OP could help and reduce the noise, but there are several points that can contribute. A difficult part is the PLL for the clock. Not really sure it is worth replacing the reference. A selected 1N829 is not that bad.
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #78 on: May 02, 2020, 07:12:27 pm »
Well, I can't agree that the meter is relatively noisy, attached first results with the meter, measuring my LTZ1000#1 reference. Not to bad for 9x7, only about +/-2 counts, but need to check 9x8 as well. And I need an EPROM with MickleT.'s bugfix for the drift correction.

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Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #79 on: May 02, 2020, 08:25:58 pm »
The noise level is about where it should be to call it 7 nines. However this is already at a relatively low speed. It is impressive that the noise is that low even with a 10 V voltage, and not only with a short. So the 1N829 reference is really low noise. At least for 7 digits no real need for a lower noise reference.

Maybe not right to call it high noise, more accurate slow to get low noise. Chances are one will see the limitations in the 8 nines mode, as there are a few drift / low frequency noise components that are not compensated by a AZ mode, other than the very slow drift compensation mode with it's own problems. After all the meter is more like a 7 digit meter with an extra slow mode to go a little beyond 7.

The stability without the drift compensation is impressive.  Before the drift compensation the readings are more comparable to the non AZ mode in conventional DMMs. It has the advantage of continuous sampling the input and thus minimal noise-bandwidth for the input signal. Quite often the external voltage to measure is more noisy than the ADCs or reference. In the lower ranges the input amplifier also can be more noisy than the ADC or reference. So a low noise BW be a plus. Some meters have an extra analog filter for this purpose to reduce the BW, but this adds to the settling.
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #80 on: May 03, 2020, 06:10:01 am »
9x7 doesn't look to bad either, with drift off and no filtering active.

Edit: My unit has serial number 475, contains 2764 EPROM and firmware is marked DC

Code: [Select]
Vdc calibration constants
-17.4045562E-06
 164.748121E-03
-3.09944152E-06
 1.64744222E+00
-1.07288360E-06
 16.4753368E+00
-2.98023223E-06
 164.631444E+00
-1.13248825E-06
 1.64640850E+03
Vac calibration constants
 327.509400E-03
 3.27633542E+00
 32.6723201E+00
 326.904304E+00
 3.27080905E+03
kOhm calibration constants
-17.4045562E-06
 166.011106E-03
-3.09944152E-06
 1.66011920E+00
-1.07288360E-06
 16.6019517E+00
-3.09944152E-06
 166.040122E+00
-1.07288360E-06
 1.66032950E+03
Top Ohms range calibration constants
-1.07288360E-06
 9.90003223E+03
 602.343082E-03
Ratio terminals calibration constant
 16.4756781E+00
Zener current token value
103

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« Last Edit: May 03, 2020, 06:47:38 am by branadic »
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Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #81 on: May 03, 2020, 07:04:21 am »
The noise looks in deed relative good. The very low frequency noise parts is kind of normal without the drift correction.
Even than there is the question how much is from the ADC and what is due to the noise of the 2 references involved. One known contribution is 2.5 times the noise of one of the OP07 in the reference amplification. Beside the reference this could be a significant part for the slow part.
The drift correction part would remove some of the very low frequency noise, but also add some noise from the steps.

As far as I have understood the ADC, the 9x8 mode should be the same as averaging over readings int the 9x7 mode. The integration in continuous anyway and the timing of the comparator essentially ready the time for the zero crossing. So adjacent readings just add up with longer measurements.  The only difference could be with the drift correction, that would likely be slower and lower noise in the 9x8 mode.
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #82 on: May 04, 2020, 04:20:55 pm »
Thanks to someone close to me and the amazing work of MickleT. I will have the drift bug fixed during this week. So let me say thank you here.
And for everyone who doesn't know, here is the package that includes the ROM files with the bug fix for IC803:

http://www.ko4bb.com/manuals/2.205.132.12/Solartron_7081_Documentation_pack_-_Mickle.T.zip

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #83 on: May 04, 2020, 07:26:35 pm »
Attached is an INL measurement of the 7081 in 9x7 mode and auto zero off, with 20 values averaged.

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Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #84 on: May 04, 2020, 07:41:33 pm »
The INL looks really good. It this still without drift correction or with the old drift correction ?

20 values averaged is quite slow, though using 9x7 mode only. AFAIR 20 readings in 7 digit mode would be about the same time as 1-2 readings in 8 digit mode. So probably without drift correction as this would be to noisy in 7 digit mode.

Depending on what type of noise dominates one could get a little less noise if one does not use the simple arithmetic mean, but with a little different weights. Maybe something 1/2 the weight to the first and last reading.  This would effectively average over the first and last 2 readings for the comparator timing. This is a little like frequency calculation from multi time stamp frequency measurements. The difference may not be very large, but also not very much effort.
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #85 on: May 04, 2020, 09:08:57 pm »
Drift compensation was turned off. One reading in 9x7 equals 3.2s, so the average of 20 readings equals almost one reading in 9x8 (51.2s).
Will repeat that measurement with drift compensation on once the "new" EPROMs have arrived and been installed.

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #86 on: May 06, 2020, 11:50:02 am »
Some minor progress on another part of the meter. I'm not a fan of those low thermal Fischer connectors. Inspired by Marco Reps I designed a new front panel for Pomona binding posts, but wanted to keep the calibration key switch.
If someone is interested I can share the CAD files as STEP. Will have them printed soon and yes, they do fit all 70x1 meters.

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #87 on: May 09, 2020, 09:12:10 am »
Thanks to a helping electronics community member nearby I got a set of 2764 EPROMs with the latest firmware version DD and the bugfix for ic803, that was developed by MickleT. I measured multiple hours with drift correction turned on and off (9x7). The jumps after the meter has warmed up are much smaller now.
Mickle states: "It is almost impossible to get rid of these spikes and random level shifts due to a DA effect and ADC noise. The main goal of patch-fix was reducing a zero offset, introduced by drift correction and depending on the magnitude and sign of the input voltage of the voltmeter. This is done simply by adding a fixed delay after a MUX switching while drift correction."
So this is almost the best I can achieve on this meter.
Black low thermal binding posts with black, red and blue inserts are on their way and will arrive soon, so I can finallize the CAD files for them and the CAD drawing for the front panel, while the Pomona version is intended for the lower grade 7061/7071.

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« Last Edit: May 10, 2020, 08:04:32 am by branadic »
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Online Kosmic

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Re: Repair of Solartron 7081 SN#718
« Reply #88 on: May 09, 2020, 02:15:14 pm »
Some minor progress on another part of the meter. I'm not a fan of those low thermal Fischer connectors. Inspired by Marco Reps I designed a new front panel for Pomona binding posts, but wanted to keep the calibration key switch.
If someone is interested I can share the CAD files as STEP. Will have them printed soon and yes, they do fit all 70x1 meters.

-branadic-

I would be interested in the files.

I was planning to do this operation on my Solartron 7061 and 7081. Fischer connectors are ridiculously expensive and I found some good quality binding posts relatively cheap.

On my side I was planning to simply print the part in PLA. Are you planning something else ? aluminium like Marco ?
 

Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #89 on: May 09, 2020, 03:13:57 pm »
The jumps from the drift correction are quite visible. It meter running in 9x7 or 9x8 mode ? Averaging 16 reading 9x7 would be the same for the normal readings, but the drift correction is likely still different, with less noise in the 9x8 mode.

The regular outliers to the negative side look a little strange. So the first reading after the drift correction may have sometimes problems.

For the drift-correction jumps it may be interesting to watch a step in the real signal: some 30 minutes at a higher voltage like 10 V (the exact value and stability does not matter) and than jump to a short, and watch settling for the next 20 -30 minutes. During the zero reading phase there can be settling from DA and thus jumps in the drift correction and also some "drift".
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #90 on: May 09, 2020, 05:28:50 pm »
Attached is the STL-file.
I will use a DLP printer, with a ceramic filled resin to print the panel. But it's up to you how to realize it. It can be cnc machined from a block of aluminium, but you can also try FDM.
Unfortunately the STEP file is to large to upload it here. So STEP file only up on request, to send it per mail for your own modifications on it.

Meter was running in 9x7, but currently measures in 9x8 for comparison. If the meter really suffers from DA, it would be interesting to replace the integrator caps by PTFE ones, though we've already seen, that DA is not the main issue on R6581, so meanwhile I'm a bit sceptical about DA statements. On the other hand some hermetically sealed PTFE caps would fit the hermetically sealed resistors inside S7081 optically.

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Offline picburner

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Re: Repair of Solartron 7081 SN#718
« Reply #91 on: May 09, 2020, 07:24:13 pm »
I got a set of 2764 EPROMs with the latest firmware version DD and the bugfix for ic803, that was developed by MickleT.

Which version of MickleT's ic803 patch did you install, 1000mS or 3200mS?
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #92 on: May 09, 2020, 08:09:29 pm »
I have patch 1.1 installed, thus 3200ms.

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« Last Edit: May 09, 2020, 09:50:10 pm by branadic »
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Offline TiN

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Re: Repair of Solartron 7081 SN#718
« Reply #93 on: May 09, 2020, 08:14:27 pm »
Quote
Unfortunately the STEP file is to large to upload it here.
You could always share it on standalone hosting and post a link.

Great work on S7081. Little meter that can, unlike Advantest  :-+
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Offline picburner

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Re: Repair of Solartron 7081 SN#718
« Reply #94 on: May 10, 2020, 06:49:51 am »
My 7081 is very old and still used the tms2564 so I thought of this solution that fits on sk404, sk405 and IC416.
The palce16v8 replaces the 74ls138 also.
« Last Edit: May 10, 2020, 06:52:31 am by picburner »
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #95 on: May 10, 2020, 08:13:34 am »
Quote
Great work on S7081. Little meter that can, unlike Advantest 

Thanks. :)

Quote
My 7081 is very old and still used the tms2564 so I thought of this solution that fits on sk404, sk405 and IC416.
The palce16v8 replaces the 74ls138 also.

Why don't you keep the original parts? Is there any reason to not buy NOS parts and get them programmed?

9x8 measurement finished. Can't finally conclude if drift correction is doing any good to the meter or if I prefer it turned off. Unfortunately drift correction isn't explained in detail, neither in the user manual nor in the operation/service manual, so I haven't got the point yet, what drift it should actually correct for. Can somebody elaborate on that?

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Offline Kleinstein

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Re: Repair of Solartron 7081 SN#718
« Reply #96 on: May 10, 2020, 10:53:31 am »
AFAIK the drift correction does a zero measurement, a little like in the AZ mode / manual Zero in a meter like the Keithley 2000/2002. The ADC measures a zero reading to correct for drift AFAIK this does not include the input amplifier, as switching is done behind the amplifier. So this is for correction of  ADC zero drift.

The integrator in the ADC used an AZ OP, so essentially drift there. However there can be drift in the reference amplification part. It the balance in the +10 V to -10 V reference changes, this will effectively be zero point dirft for the ADC. There are mainly 2 parts: the double resistor (R305 a/b) and the OP IC 306 (OP07).  An 1 ppm dirft in the R305a/b matching would result in an offset of some 16 ┬ÁV. So it does not need that much drift there.
 

Online Kosmic

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Re: Repair of Solartron 7081 SN#718
« Reply #97 on: May 10, 2020, 01:15:13 pm »
Attached is the STL-file.
I will use a DLP printer, with a ceramic filled resin to print the panel. But it's up to you how to realize it. It can be cnc machined from a block of aluminium, but you can also try FDM.
Unfortunately the STEP file is to large to upload it here. So STEP file only up on request, to send it per mail for your own modifications on it.

Thank you, much appreciated!
 

Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #98 on: May 11, 2020, 04:53:53 pm »
Attached a slightly modified version for Pomona 3770 and a version for LT binding posts.
Today I received black LT binding posts. Both versions of the front panel will be printed soon, painted black, while the labeling will be filled with white color.

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Offline branadic

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Re: Repair of Solartron 7081 SN#718
« Reply #99 on: May 16, 2020, 05:38:31 pm »
Some minor progress, but not yet finished.

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