Documents or repair experience for adret 103A DC current and voltage standard

[Krampmeier]

I was lucky enough to score an old adret 103A DC calibrator which had been taken out of service many years ago, as it was not possible to bring it back into spec.
The unit is basically working, sourcing voltages and currents up to 100V / 100 mA. The accuracy, especially the linearity, is not great though. Of cause, in combination with an accurate DMM, the device is still useable for many applications, but of cause I'd like to try to recover as much of the original performance as possible.

Unfortunately, I got only the device, without any manuals or documents. This means, I don't even know how to use the sweep mode (not that I would need it), and more importantly, I don't have alignment instructions or schematics.

With nothing to lose, I played with the alignment pots, trying to find out what each of them does, but never got to a point where I was totally happy with the result. Something seems to compromise the linearity and symmetry of the output voltage and current. I used a Keithley 2010, which should be in spec, as a reference.

I replaced all electrolytic capacitors by new one (very ugly bodge job, as I had to replace axial caps with radial ones), and removed the leaking NiCd batteries which were hiding under the main PCB. This did not solve the accuracy problems though 

Does anyone here have more information about this box, such as manual, service manual, schematics, specification sheet? Or experience solving a similar problem with this type of device? I'd be grateful if someone could help me out with these!

I attached some photos of the inside and outside of the device...

[BFX]

Here you are 
https://doc.xdevs.com/doc/ADRET/
https://www.eevblog.com/forum/repair/repairrestoration-adret-103a-current-and-voltage-standard/

Don't forget to check ripple on power lines, there are plenty of tantalum troublemakers

[Krampmeier]

Wow, this is great! Thank you very much for pointing this thread out, and for sharing those valuable documents! That manual is really great and should help a lot!
The last time I actively searched for documents on this unit was like 2 years ago, when I got the thing, and I had not imagined something new would ever show up...

Did your unit also have problems with chattering contacts on the keypad? Mine suffers from that pretty badly, and I wonder if there is an easy fix for that. Just cleaning, and maybe contact spray? I hope I can get the switches apart far enough at all...

[edavid]

Did your unit also have problems with chattering contacts on the keypad?

My unit has a very bad keypad also.  How the heck do you get access to get contact spray into the switches?

BTW, here are the spec sheets in English and French:

http://www.davmar.org/Spec/Adret103A-spec.pdf
http://www.davmar.org/Spec/Adret103A-spec-fr.pdf

[Krampmeier]

Thank you for the spec sheets, edavid!
I did not try to open the switches yet, I will look into that later when the other problems are solved...

BFX, did you actually find a way to clean the keypad contacts?

[BFX]

I have no problem with my keyboard.
May be push some contact cleaner around buttons would be enough.

I have still no idea about this board with battery. How it works or how it behaves correctly.
In my unit I have disconnected batteries for now.

[RLMCN]

Hello,
I have 2 ADRET 103, on one of them I spent quite a time to make it work again.
It can be used without battery, in such a case the increment function setting is lost when power is turned off.
Quoted from the manual:
"Any prolonged interruption in operation causes suppression of increment, loss of the contents of memories and cancellation of START/STOP limit values in SWEEP mode. On the other hand, the instrument circuits are not disturbed by mains cuts of less than 5 seconds as the effects of such cuts are compensated by a nickel-cadmium battery (ref : 4 x SANYO - 450AA). When the instrument is first put into operation, the battery is not fully charged until after several hours of operation."
This instrument is not difficult to maintain, all parts can be found easily even today. Changing OPAs (OP07) on the Preamplifier Board for something more up-to-date is an easy way to improve performances.
I have User/Service manuals for it (French and English) also I'll be happy to share my experience about it.
Just let me know.
 

[Krampmeier]

RLMCN, that sounds great! I'll not use the sweep stuff and I don't mind if any settings get lost when I power down the unit, so I won't bother replacing the batteries. It is good to know that this does not incluence the device function.

I am pretty happy with mine now, after I did the alignment as described in the manual. Improving the start-up time (temperature stability) would be nice though.

May I ask which OPA you used as replacements for the OP07 (I see only SN-003 on the preamp board)? OP77? I suppose low offset voltage and bias current drift over temperature are important parameters here... OP77 and OPA277 look like good candidates to me.

Wouldn't replacing SN6 (µA 741) on the reference board with a OPA277 also make sense?

Did you also replace the OPAs on the reference board (LM308A)? I did not find a part with such a stable offset voltage, except the OPA2189 and other chopper stabilized devices...

Did you somehow "refresh" the keyboard contacts? Any hints on how to do that without breaking something?

And do I understand it correctly that they supply a hex inverter with the reverence voltage and use it as the final PWM switch? Feels a bit weird to me!

[RLMCN]

Hello Krampmeier,
You are right, the OPA2189 is a perfect candidate for voltage reference with the LM399.
It was not available when I did the rework (more or less 8 years ago).
On the VOLTAGE REFERENCE board, I decided to keep the LM308. First it's good for this job and was not sure if some special adjustments or compensations was made at the factory on the pair LT399/LM308 that could be jeopardized by changing one item only.
I replaced SN6 (741!) buffering the ref. voltage and SN2/SN3/SN5 (LM308) by LT1012AM, offering lower voltage offset and voltage offset drift, lower current offset and current current drift.
On the PREAMPLIFIER board, I also replaced SN1 (741) and SN3 (OP07) by LT1012AM.
I didn't face the problem you describe regarding keyboards. On the FRONT PANEL I had to replace all LEDs nearly emitting no light at all, and some 7-segment displays. I guess the instrument was 24/7 powered up on an ATE or something…
My longest fight was a (very) intermittent failure caused by the ILS relays. Those relays are encapsulated in a plastic (or epoxy resin) that aged poorly, having nearly invisible cracks on the surface. These cracks in fact were deep enough to cut the wire of the coil. I found identical replacement parts from Digikey.   
On the FITTED CHASSIS board, the PCB was brown-colored around Q3/Q4. These T0220 bipolar transistors are switching the 120V power supply, grounding either the positive or negative net according to the requested polarity of the output voltage (a way to reduce heat dissipation in the output power stage). I removed Q3/Q4 from the board, and thanks to a T-shaped aluminum profile, I mounted these TO220 on the rear heatsink.
Of course I replaced all aluminum/tantalum caps.
And yes, your understanding is just correct. In the 103, the DA conversion is achieved by a PWM followed by Low-Pass filter. The instrument has 6 digits. Two PWM/Low-Pass channels are used: one for the 3 MS digits (Switch C1 and filter FL1), one for the 3 LS digits (switch C2). Both Low-Pass outputs are summed through two resistances having a ratio of 1000 by SN3. The PWM switch for the MS channel is made by 6 parallel CMOS inverter SN9 (CD4069) simply powered by the reference voltage.
This IS weird! But even worse to come…
The designer (Roger Charbonnier) assumed that the LS channel could be one thousand times less accurate than the MS one. If you look at the VOLTAGE REFERENCE schematic, you can see that the MS channel (Switch C1 and filter FL1) involves clock re-synchronization, hex-inverter powered by the reference voltage and Sallen & Key Low-Pass filter, but that the LS one is just 1/2 CD4001 followed by an RC filter, and this CD4001 is even not powered by the reference voltage, but by the output of a 741 wired with Gain=1 as a buffer… 
I still dont really understand how the linearity adjustment P11 is supposed to work.
The concept of the instrument is clever and elegant (at least to my eyes) but the work of the analog designers is a bit "dry" (the RF generators from ADRET truly show a higher level of analog skill).
I believe that this concept could be pushed to better overall performances without too much effort.
Anyway, the instrument is cool and easy to use, also I find the front panel design time-resistant, compared to other things from the same period.
By the way, the front panel switches are made by MARQUARDT, it's the 6450 SERIES and some are still produced today.
They have a buy-on-line store, it is not that expensive:
https://www.marquardt-shop.com/products/switches/tact-and-key-switches/6450/6450.0001.html
Of course you will get un-legended stuff but a fresh mechanism, just in case…

[Krampmeier]

RLMCN, thank you very much for these comprehensive answers and the background information! This was a super interesting read, and I wish I understood the circuit of this beautiful device as far as you do...
I am amazed that Adret got away with using a 1:1000 resistance matching at the adder, which must track precisely over temperature, even with a pot (P10) in the signal path. It is really fun to learn from all the tricks and arts used in this calibrator (and other vintage test equipment as well), and of cause it would be tempting to use the same concepts with today's parts and see how far it can be pushed!

I also like the appearance and usability of this unit, and I hope that the one I have will continue working for many years to come. It is surely worth the effort do do some upgrades and maintenance, so the the pointer towards Marquardt is highly appreciated. These switches resemble the small ones only though, not the square ones - or are the button caps the only difference?

I am already looking forward to December, when I will have some time to take the thing apart and swap (at least) a couple of opamps. I am still not sure if I'll use the LT1012 which you selected, or the OPA277. The first one has less bias current, the latter less drift over temperature (and lower price). As most of the source resistances are not too high, I am tempted to use the OPA277...

Is it really necessary to replace the 4.7 µF tantalums? I did not find non-SMD MLCCs with this capacity, so I'd have to put in tantalums again...

The area around Q3 and Q4 still looks good on my unit's board, but I'll get some small clip-on heatsinks for them.

Thanks again for sharing your experience and knowledge!

[BFX]

RLMCN , thank you for this information's. Have you some thermal images of your unit? I can share some of mine.
Is your unit also working with batteries?
Also could you please share documentation what you have? Or is it the same as we have? Better quality will be also helpful.
I still didn't goes throe the calibration because I was waiting for triaxial cable for my Keithley 602 which can be used as null meter.
Have you guys also some hints/experience around calibration?
@Krampmeier replace all tantalums is highly recommended from my point of view. I used also tantalum caps again.

[Krampmeier]

Hi, BFX,

I did the alignment with a Keithley 2010. It should be good enough, especially when the zero volts offset is rel'ed out using a short circuit before.
The procedure worked as described in the service manual, except that the 100V range seems to have an accuracy problem at my unit.

[Krampmeier]

I just noticed that my unit has the LT1008 instead of LM308 in the filters on the reference board (SN2, SN3, SN5). I saw the same on BFX's photos, so Adret must have done a revision of these boards. The µA741 and OP07 are still there though.

The OPA277 should still be an upgrade to the LT1008, but I may have to remove the 100 pF compensation caps, as the OPA has the offset correction input at pin 8 (which was compensation at the LM308). The same problem should have shown up with the LT1012.

Removing the front panel PCB was a nasty job. The screws are behind the glued-on decoration foil (don't know the technical term for that). Mine is not as beautiful as it used to be any more now... I don't like this particular aspect of the mechanical design - everything else is nicely accessible...

I found the Marquardt 6425.0101 switches (for the square shape buttons) at mouser, and the labelled button caps can be taken from the old ones. Also found X7R ceramic caps with 4.7 nF for the "Fitted Chassis" board, which will replace the tantalum firecrackers.

[RLMCN]

ADRET 103A

[RLMCN]

ADRET 103A

[RLMCN]

ADRET 103A

[RLMCN]

ADRET 103A

[RLMCN]

ADRET 103A

[RLMCN]

Hi every body,
The Adret 103A User Manual and Etalonnage (I only have a French version, sorry) are uploaded above.
(Please rename files ADRET_103A.z01.zip to ADRET_103A.z01, ADRET_103A.z02.zip to ADRET_103A.z02, etc, in a single directory to able to unzip ADRET_103A.zip)
Working far away from home for the time being I cannot provide pictures of my unit.
It's working without battery, i.e. some settings are lost when power is removed (this was acceptable for me). I did the calibration using an Agilent 34401A, not by choice but nothing else was available.
Calibration was not difficult, I remember however that the instrument needs nearly one hour warm up.
Regarding OPA choice (honestly I forgot what I did about the compensation when replacing for the LT1012) instruments produced after 1984 have LT1008 on board, as you can see in the uploaded  schematic. The compensation capacitor is still there, and this in accordance with the datasheet of the LT1008. Other OPAs may need different configurations.
Krampmeier, I may have a solution for your front panel. In fact I had similar problem when repairing my unit, so I produced a small batch of panels adhesives from a local supplier. Could be I have some left, I'll be able to check this when returning home around mid of February. It's an ersatz of course, but can give a decent look to the instrument.

[Krampmeier]

Hi, RLMCN,

thank you very much for uploading your documents! I don't speak french, but the better scans of the schematics will fill in some blanks. I had trouble inflating the split zip file (both under Linux and Windows, despite renaming and using 7-zip), but I'll try again another time.

It would be absolutely great if you could sell me one of your replacement panel adhesives, in case there is one left! Waiting until February would be no problem. Thank you very much in advance for your kind offer!

[edavid]

Here are download locations for the manuals provided by BFX and RLMCN:

BFX's manual, glued together and with the pages rotated the right way around - but still needs more cleanup:
http://www.davmar.org/pdf/Adret103A-FR-EN-V1.pdf

RLMCN's manual - mostly a better copy than BFX's, but the foldouts are chopped up:
http://www.davmar.org/pdf/Adret103A-FR-EN-V2.pdf

RLMCN's "Calibration et Maintenance" - French only:
http://www.davmar.org/pdf/Adret103A-CalMaint-FR.pdf

RLMCN's "Adret 103A REFERENCE DE TENSION avec LT1008":
http://www.davmar.org/pdf/Adret103A-VoltRef-LT1008.pdf

[RLMCN]

Hi edavid,
Thanks for hosting the files.
I did the compression on MacOs, traditional compatibility issues...
Sorry about that.