need help on a prema 6040s attenuator board, looking for circuit diagram

[sony3000]

Hello everyone, this is my first post.

I need help on a prema 6040s 7.5 multimeter with an unknown past.
Its a beautiful device full of discrete elements, no asics in a rock solid case.
After some common problems like blown tantal caps (board 6040-vw4:c25) and resistors (board 60-S-Gr-2:R6,R7) i ran into trouble
in form of a zener diode (board 5020-V: CR22) that had one leg dislocated from the signal path hiding its misery under the cooling umbrella of the U4 opamp.

The problem I'm facing now seems to be in the attenuator section.
The meter is currently working in and only in the 1000V setting. All other ranges display low and noisy values, independent of integration period  .
So my guess is there's some issue with the attentuator. Actually, I found two problems already:

1. The resolution of the schematics is so bad its nigh impossible to read.
Does someone have a better copy of the circuit for "Abschwaecher 6040-S-AB-2"?
The relays are switching on range changes, but I have not yet been able to figure out what the individual switches are switching

2. The board itself is confusing with perpendicular sub-boards and a resistor network (I'm guessing) in a shielded black box (yes, black). Does anyone know if its worthwhile opening the box and if yes, how do you open it without breaking?

I have the circuit diagrams that are available on the net in the german and english version, but the quality of the diagram is poor in both. i have attached the copy i have.
I have also asked Prema, but they have ceased their famous support service middle of 2020, so no luck here.

suggestions much appreciated

[willi]

Your schematic is the input attenuator AB version 2. Your picture shows AB 3. AB is the short form for "Abschwaecher" (german for damper, attenuator).
I attached a better copy, but it is also version 2. I have a reconstruction of AB7 in eagle format but it is totally different form yours.

Hope it helps a little

[sony3000]

Hello all,

I've made a simplified diagram of the dc operation of the attenuator plus a truth table for the relays/switches.
Resistances and relay/transistor settings are as on my board.
The resistor values do check out with the bom of the board.

I'm having problems understanding how the attenuator operates.
The output of the attenuator feeds into the pre-amplifier opamp u3.
u3 is encapsulated in a black box, probably together with the feedback resistors.
The circuit diagram of the preamplifier does not mention any, but they must be somewhere ?!
Anyway, the R28 / R40 branch does not seem right. I can't figure out how the 1000v and the 10v ranges operate.
The only difference between these is the switch from the r40 (53k32) to the r28 branch (10M05).
Any way i look at it, the ratio between these just doesn't account for a factor 100.
Where am i wrong?

u3 might be a differential amplifier, but even so the ratio r40/r28 seems odd.
I've attached an article on differential amplifiers. Equation 9 to 15 are the most interesting.

inputs and ideas much appreciated

[Kleinstein]

Some of the prema input amplifiers are rather confusing, as the input can be a kind of virtual ground and the low side (negative) terminal can actually be the amplifiers ouput. This seems to be case here in the lower ranges (0.1 / 1 / 10 V).

The branche with R28 also has a resistor to the "low" side of the divider going to the number 3 pin, likely on the connector J10 an this going to the amplifier ouput. So when in divider mode the amplifier would work as a king of inverting amplifier with 10 M to the input and  likely some 100 K in the feedback.

U3 is likely an old style hybrid OP. definitely not an LM733, more like something similor to the old ADI 234 (does not have a differential output).


edit:   The resistor(s) R40 is likely just for protection. So the actual value does not matter.

[RR6502]

Hi Folks,

I recently bought an 6040 S at ebay and it basically works but is a little out of spec. 
Currently I cannot help you right away, because I'm on the way to completely disassemble the whole DMM, clean it, reverse engineer all PCBs, draw new and much better readable schematics, check for errors between schematics and PCBs (there are many so far: like reference designators used twice, different pin numbers between connectors and schematics, missing parts on the PCB, ... - ah it is sometimes frustrating). 
As soon as I am done I will notify you and perhaps I post them here for everybody. But I am sure it will take a while.

What I can tell right away is that U3 is neither an old style hybrid OpAmp nor an LM733 or anything like it - it is just an OpAmp made of discrete components (Dual-JFET inputs, ...), you can open that little black box very easy, just a few screws and that's it.

As soon as I figure out more concerning your described issue, in particular how the "Abschwaecher" is working, I will come back to you.

But to give you an idea on how long it may take before I am finished with my work on the 6040 S, I have that DMM for about 3 weeks now and have just redrawn the "O"hm-, "V"orverstaerker-, and "G"leich"r"ichter-PCB and ... not the schematics - yet.

By the way, your picture of the "AB"schwaecher-PCB showed me a missing cable in my 6040 S. I was wondering why only my bananas on the back are working but not the bananas on the front?! 
After opening the case it came clear why they refused to work: no cable at all was attached to the

• and [-] terminal - strange thing. And more astonishing was the fact that the solder joints are unused like nothing was ever attached to them since they were build in ... ?!? Hmmm, maybe they were replaced once with new ones but that is out of my knowledge. Anyhow the complete cabling between the "AB"schwaecher and the front terminals is (still) missing. 

At least one question arise from my point of view: One slot is missing a module - the "R"atio module (Option 05) - and I am keen on getting this, besides all other nice modules of course that can be inserted as:
- DAC output (Option 08),
- BCD-Output/Remote Control Input (Option 07)
- and there supposed to be an RS232-Interface, but because of U8 (a 2758 PROM) where I cannot get hold of the BIN-file, this will be a dream only I must admit.

Sooo, let us keep contact.

[sony3000]

Hi 6502,

congratulations and welcome. Glad that yours is working.
I'm happy to share and collaborate. I sent you a PM so we can have small signal-to-noise discussions without cluttering the forum, if you like.

Speaking of cluttering, my first attempt at the attenuator operation schematic is misleading and i'm going to remove/replace it.

I also noticed the problems with the design diagrams you mention. Yes, it is a real piece of german engineering. More like german desperation to get something that meets the specs 
This is particularly apparent in the attenuator board. The schematic does not match the component placement diagram, but the component placement diagram is matching my physical board. According to their timestamps, both have been made with only one day between them. How have they come up with this ?!

I'm not sure, but it might be that they had several different iterations of every board in production.
I guess willi was hinting at this mentioning the different versions for the attenuator board.
Here's a list of what's in my device physically:

• Grundplatine,backplane,6040 S GR 3
• Microprozessorplatine,cpu board,6040 MCS 3B
• Vorverstaerker, pre-amplifier (DC), 5020 v7
• Anzeigeplatine, display board, 6040 TAW 4
• Trafoplatine, power transformer, 5040-T-4
• Digitalplatine, amplification control,6040 D3
• Integrator, integrator board, 6040 I-4
• Ohmteil, Ohm board, 5040 O-7
• Abschwaecher, attenuator, 6040S ab 3
• Ohm relais, Ohm relays, RER/6040S OHM
• Wechselspannungsvorverstärker, pre-amplifier (AC), 5040 VW 4A
• Gleichrichterplatine, rectifier board, 5040 G 3a

I omitted the display and key interface board for (my) convenience

Here's the boards front- and backsides: see below

Here's also a text file of the bill of materials from the manual available on prema's website : see below

Your findings on the U3 sub-circuit are astonishing. So far, i was inclined to go with kleinstein in assuming it is a AD 234 or AD 261. Their pinout seems to be compatible with whats on the board. That together with U4 as a low-pass filter against the chopper noise seemed to make sense.
Datasheet is here: check below
After reading your mail, i went back and measured the width of the black box. It is at 36mm too small for the 38mmx38mm 234 package.
It is hard to believe they went an built something custom.

You mention that the disassembly of the black box should be easy. To me, it looks more a one-way option. I don't know how to safely disengage the blue pin connectors.
Plus, there's probably a metal foil sheet and other nasties before you even see the components. How did you do it?

So far, i was unable to find a DC feedback path into the inverting input of U3. Is the feedback directly implemented in the sub-assembly?

It's not uncommon that only one set of jacks is connected internally. This was probably done to reduce thermal and noise in general.
In my case, it's just the front that is connected. The wiring from attenuator to front panel is straightforward, i'd be happy to describe in more detail, if you like.

The ratio module is indeed interesting. I have to see one yet. The DAC, BCD and RS232 are less appealing, after all there's a GBIP interface ?
I have a 2716 somewhere, but don't know the 2758 or where to get the required image.

...My attachments are apparently too large, am looking for a way to upload them
...here you go: https://sabercathost.com/folder/355525/prema6040s

[RR6502]

Okay sony 3000,

here are some first results from my reverse engineering:
The thing I forced myself to complete was to get an idea of what's inside the U3 of the "V"orverstaerker.
On the PCB it's labeled "CHOPPER 5", and here is the PCB and schematic:



This little black box has the dimensions of 58x39x17mm and has two M3 screws on top and one plastic screw on the bottom to be fixed with the "V"orverstaerker PCB. Oh, by the way, this "U3" is not soldered to the "V"orverstaerker PCB - it's a (blue) plug-in socket - the same as with the INTRONICS R310 RMS module on the "G"leichrichter PCB. Well, at least in my 6040 S. 

I'm working with Sprint-Layout 6.0 and sPlan 7.0, which are great for this job and their price is very fair.

Well, I'm not really done with analyzing this circuit, but it's not something I have seen before ...
The text in red color are the pins coming out of U3. I have searched the www for datasheets and can confirm that everybody can find some if he/she wants to. Getting those parts for replacement might be a complete different task.

After comparison of the build-in modules/PCBs I have almost the same as sony3000, except for two things:
1. My CPU board is an MCS-5 and
2. The PCB for Ohm/Volt switching is a complete different one and its labeled "6040-S-F-1". Its the little PCB mounted vertically just in front of the "AB"schwaecher. I'll take and post a picture soon for better comparison. The components are almost identical except for the SDS relays - however, mine are more vintage types, which are mechanically moving a slider to make the switching. ;-)

Now, happy analyzing of the circuit of U3.
I'll keep you updated.

[sony3000]

Hi and Thanks,
check your personal message folder

... the reference to the external resistor should probably be R21 instead of R2 ?

Your diagram is great, i've never seen anything similar.

[RR6502]

Hi sony3000,

nope, the reference is acually R2 - you remember(?), I told you about some Ref.-Designators that are used twice in the schematic, and that's unfortunately one of them!
If you look at the non-inverting input of U4, go straight down, and right before C11 to its left, there is the only correct R21.  Same sh... at the "Lageplan" for that resistor, one is right below R1 at the left of the PCB just below the big C2 and the other one is right below the now revealed U3. So I said to myself, since all Ref.-Designators are (more or less) sequentially named from the top-left to the bottom-right, why shouldn't I do it the same way ...
But because of the circumstance that I bought at ebay in addition a German Manual for that fancy 6040 S, which came with a BOM, the Ref.-Designator I have just chosen was the correct one. 

The input to and through R2 comes (via R4 of the "GR"undplatine) from a separat winding of the mains transformer (pins 12 and 13). This 50(/60)Hz sine signal goes directly to Q1 which controls in turn the two N-channel MOSFETs.
What exactly this U3 should be doing and how the signals look like I must discover one day with LTspice in more detail. But as I mentioned I have still a lot of work to do to get all PCBs redrawn.

Hi everybody else out there,

Maybe other users could be so kind to contribute to that work somehow, borrow other versions of the PCBs, in particular the once that are not installed in mine, like the RS-232, the DAC, or most interestingly the RATIO module.
In addition to all redrawing stuff I have planned to do some rework in the manual. Get rid of that typewriter style font and may be adding a chapter about trouble-shooting with diagrams, etc.
Furthermore I would like to disassemble the source code of the EPROMs and document the (assembler) program listings. In case someone wants to edit the sources and write his own routines. 

Any contributions and help is welcome. The same goes for new ideas on what to document like to add, edit, delete, ..., to/from the 'new' manual.