Replace all optoisolators (PLL is not consistently locking)
Thanks for the suggestion. I tweaked it a few days ago. I changed the control voltage from 1.9V to 2.44V, and the PLL was still intermittently unlocked. I ordered the optocouplers, and they were delivered earlier today.Replace all optoisolators (PLL is not consistently locking)
Try tweaking C807. Floating logic sheet 5, rear right corner lower board.
on the eeproms i would put back a new protective tape ... some of them aren't protected at all
Signal | Wire | connector 2B.305 pin |
Lo | Black | 3 |
Hi | Red | 4 |
Guard | Drain+shield, isolated from connector housing | 1 |
Lo-sense,High-sense | shielded twisted pair, drain to lo-sense | 2,5 |
(floating) | shielded twisted pair + unshielded pair |
Also, I noticed that in TEST10 mode, a relay switches for every measurement. I'm guessing that's normal.).Yes, it's normal.
What does the "PU3" mean on the schematic?Those "PU" thingies designate connections to IC825.
My option "1" uses a 10 Hz reference to drive the PLL, and my option "2" uses a 50/60 Hz reference, and also overrides the
QuoteMy option "1" uses a 10 Hz reference to drive the PLL, and my option "2" uses a 50/60 Hz reference, and also overrides the
Have you got any improvements using external 50Hz?
Out of topic, do you have a drawing of the calibration key or maybe one to sell?
QuoteMy option "1" uses a 10 Hz reference to drive the PLL, and my option "2" uses a 50/60 Hz reference, and also overrides the
Have you got any improvements using external 50Hz?
I wouldn't expect improvement with an external 50Hz, but I would expect improvement with something tracking the mains frequency (i.e. nearly but not exactly 50Hz).
Out of topic, do you have a drawing of the calibration key or maybe one to sell?
Oh... one other thing... My meter came with a front-panel of a binding-post breakout box. I may try to re-use it. I wish it had come with the box itself... Cry. Needing a fuse is weird; maybe it was a current-shunt box?
Unfortunately, the measured voltages seemed way off, about 3000 ppm low.If the output of IC304 (LM741) is -10V this shift low the zener voltage of several mV; in a corner of the CS there is a label with the value of the DAC; you can check the programmed value by sending the commands with the calibration key in the CAL position:
Out of topic, do you have a drawing of the calibration key or maybe one to sell?
I found that adjusting the main clock has a huge impact on noise thought.
You mean tuning the oscillator's capacitor to make the control voltage around 2.5V? Or injecting a lower-noise clock?
I think using a CMOS switch chip instead of the discrete MOSFET circuit is an option. However the ADG1404 might need a level shifter as the control signal is -15 V to -10 V. Especially for a first test a higher on resistance switch might be OK.
A current through the FETs to ground sounds plausible - more like a design weakness than a broken part. A faster control signal could help, but it would not totally eliminate the problem.
Is it possible to resolve the wave-from of the spikes to seen if they go up or down first.
I don't fully understand, is there are INL problem with this DMM? All 7081 I seen, has a nearly perfect INL in range of 0.1-0.2 ppm /+-10V.
PLL to follow line frequency. The circuit used at 7081 has very narrow voltage gap where it works OK. So it is difficult to set it correctly.Can you explain your method to adjust the PLL or is it the same as described in the manual?
But it is worth of doing it, if You need low uncertainty at low voltage levels (1V, 0.1V).
Re input cable:I think you uses a dirty/failed cable >:D I had a several 7081's and full sets of optional leads for it, including low thermal ones. All leads are well screened, have a PTFE insulation and gives the same measurement results in 1MOhm range. No 0.5...1 ppm due to leakage.
I would rather avoid using close Hi with Lo:
1. isulation issues: if Your cable has poor iinsulation, You will get leakage between them. It may be meaningfull at 10^4 kOhm measurements.
Iven at 1MOhm level, original Solartron cable lowers reading 0.5 ..1 ppm due to leakage. Pure cable may lower readings 5ppm or more..
2. Solartron 7081 is somewhat sensitive to noise ( noise changes reading by temporary saturating amplifiers). Therefore good screen around test leads is important whem You will measure Vdc on Low levels, as well as when measuring AC voltages.The 7081 noise sensitivity has nothing to do with the input amplifier saturation. On the old revision of DMM (PCB5 rev. E and earlier) the floating RATIO inputs has a poor FET control levels (TR505, 506, 508, 509 switches), so a fast transient/noise, picked by RATIO inputs, could get to the ADC input. The second problem with old 7081 was a poor GUARD routing, which was corrected in a new DMM revisions.
Without exact tracking of line frequency and integrating Ux exactly N periods of line voltage, it is very difficult to dump net noise to uV level (influence of AC on DC reading)..Phase locking to the mains raises more issues than it resolves. Are you uses an ancient auxiliary diesel-generator for DMM powering? A 51.2s integration time and a narrow limits of frequency deviation, specified for the national grid, gives an excellent NMRR in theory and in practice.
Phase locking to the mains raises more issues than it resolves. Are you uses an ancient auxiliary diesel-generator for DMM powering? A 51.2s integration time and a narrow limits of frequency deviation, specified for the national grid, gives an excellent NMRR in theory and in practice.
I want a way of measuring the effect of the mains/PLL, before trying to tweak things.
What's the best way to measure the goodness of the clock/PLL? Look at the voltage noise of a shorted input (or test0)? Or wind the input cable around a power line a few times to actually inject the mains into the DMM and check that the input is still zero? Because the frequency seems to change on the order of every minute, I think I need to focus on the results of the 8.5 digit mode.
For some reason I can only find Web pages to show the current European grid frequency, e.g. http://www.mainsfrequency.com/ (http://www.mainsfrequency.com/)
For measurement of 1V DC with 0.1V AC line noise, it will result in -4 to +4 ppm influence. For me it is too much.0.1V AC noise in DC measurement is a sign of very bad setup. It's unacceptable in most metrological environments, unless you are using like a femto / picoammeter.
...
0.1V of line AC noise on laboratory table cable connections is quite often, especially at higher (>10kOm) Vx voltage source resistance.
+0.1V of DC signal + 0.1V ac rms (0.14V peak) would give temporary 0.24V * 100 =24V causing saturation of 7081 amplifier (allowable voltage is +-14V.This does not mean at all, that "Solartron 7081 is somewhat sensitive to noise". It's just wrong range selection and nothing else.
AFAIK the steps during AZ cycles with a non zero input voltage are a kind of bug. Mickle did some investigations on this, and may even have a work around / fix. AFAIK this is some settling (possibly DA related) problem when switching to Zero.Kleinstein,
I am going to replace batteries inside my 7071 and 7081 meters. Could someone verify my thoughts that there are 2 batteries:
1) Lithium cell 3.6V (on the bottom pcb on earthy side)
2) Ni-Cd battery 3.6V (on the top pcb on earthy side)
Am I right that Lithium cell is holding calibration data and if removed, they are lost and unit must be calibrated again?
Ni-Cd battery is for RAM backup and can be removed/replaced easily? I read that someone replaced Ni-Cd with supercap. May I ask for details? I am thinking of installing batteries inside some batt. holders (not directly soldered to pcb) to prevent leaking on boards in future.
Any tips and tricks how to change batteries will be much appreciated ! :-+
Thanks
Replacement VFDs on eBay - what! Please would you provide a link...
First inspection revealed, beside missing feets, completely dead battery and leaking NiMh accu as well as dirt from smoking.
Could neutralize and rescue the traces, pads of the sockets, but lost two pads and the relating via on one of the positive accu connections. However, I could rescue the pads too and glued them back in place with UHU endfest, which is currently curing.
Replacement battery and accu are already ordered and will hopefully show up soon. Meanwhile I will go on to clean the boards from smoking dirt and neutralize the smell of it.
-branadic-
Sorry to highjack this thread, but as it is related to a repair I think it is the right place.
A 7081 showed up today on my desk. Unfortunately it is missing all the feets, so with some pictures provided by Marco Reps and the dirt marks on the unit I designed some replacement feets for it. But before I print them I wanted to ask if someone can check the design files. Attached are the files for both sides, thus a mirrored version is included. Thanks.
-branadic-
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
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 got a set of 2764 EPROMs with the latest firmware version DD and the bugfix for ic803, that was developed by MickleT.
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
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.
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.
I did a quick print in PLA to test the new front panel. Was done really quickly, so this is not the final quality (and color ...).
I did a quick print in PLA to test the new front panel. Was done really quickly, so this is not the final quality (and color ...).
I presume you have a daughter, and that you are trying to help her realise that your toys and experience might be valuable to her :)
I must admit, for something as precious as a 7081, once I was satisfied with the design I'd probably want to get this professionally made. Nylon isn't a bad material, being slightly flexible, but a black SLA would be good too.
Are the PLA/ABS materials hygroscopic, or would gaps between layers trap gunk over the years? If so they wouldn't be good w.r.t. high impedance or high voltage.
It's a simple correction (balancing) of R305 TC with copper wire.