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BBC Goerz Metrawatt MA 4E Service Manual and calibration

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KLA564#:
Hi Ernest,

Like you, I bought the exact same model of meter, also from marktplaats. Since you mentioned you have the calibration done: How did you open it? I unscrewed the metal cover from the front and took out the dial, but I can only see one trimmer on the top PCB, and that one does not seem to be the one to calibrate, or is it? Is there a way, unknown to me, to open up the back of it?

I discovered the same non-linearity of the meter. It measures fine in the lower part of the scale, but points too low higher up. What I did on a totally different meter is mis adjusting the zero point, thus accepting lower accuracy in the lower 1/3 of the scale, which you usually don't use anyway, but gaining accuracy in the right 2/3 of the scale. Maybe this can be a solution for that problem, let me know what you think.

Regards, Karl

ErnestB:
Hi Karl. To answer some of your questions:
1) To open the meter you need to unlock the 4 corners from the top side. You first remove the thin aluminum cover (4 small screws) and take out the meter movement part. There are 4 access slots in 4 corners. I have made a "tool" I could slide in the slot to push the hooks aside. When pushing hooks aside you need to pull gently to separate that part of the meter and make sure it does not lock back when you are unlocking the other 3 sides. The tool would be some plastic or metal piece of about 10x3mm flat to be able to slide in. I made one of a wider plastic clip of a pen, but maybe you could even use some regular metal key that is flat and thus not sharp at the beginning. But when you look into those slots from a side you will understand what you need to do. The hooks are coming from under towards the top.
2) Regarding the calibration (AC and DC). When you just remove the thin aluminum cover in the lower left corner you can see 2 holes. Through 1 you have the access to 1 trimmer to adjust for AC and the other hole is for adjusting the DC calibration. Beside 1 hole you can read something like AC I believe. The other is obviously the DC setting.
3) I was planning to work further on the meter to try to solve 2 issues.
       3a) One that I thought that I have solved is the seemingly lower impedance when nothing is connected in the range x10kohm. At the time I worked on it the open circuit value was quite high like 10MOhm, but now like 350kOhm. It has maybe / probably to do something with the feedback capacitor. And I don't understand the change back to the lower value. Maybe some oscillation of the opamp...
       3b) The same thing you noticed is the non-linearity, that I have not expected. I was thinking that the 2 issues maybe had the same error source. Interesting to see that you notice the same issue.
I have disassembled the meter several times, every time thinking I fixed the issue of the low open circuit impedance, but some time later the problem would appear back.
So in general I am really disappointed in this meter. I would be happy to understand the working to the detail and try to understand what is causing the issues. It would be a educational journey. Full schematic is available. Maybe there is something wrong in the (regulated) power supply (2 sides, plus and min).
I will post several pictures of the meter that I have made when it was completely open.
Let me know if you figure something out before me. Greetings Ernest

KLA564#:
Thanks Ernest, Those were helpful answers. I found the calibration trimmers, how could I have missed those.
What I will do one of these days is unsolder the movement and check it out of the box for linearity. I will put it in series with my DMM and put some micro amps through it. This way, it will at least be obvious wether this is an electronic issue or a movement issue. In the latter case, there is nothing we can do about it. If it's the electronics, it may be solvable.

I had another issue with the movement, it had some friction, that kept it from returning to zero all the way and when measuring, I had to tap it to reach the right value. I solved it by loosening the bottom bearing of the movement, by turning the adjustment screw like 10 degrees counter clockwise. That makes me distrust the movement in the first place. So, like you, also disappointed in this one.

BTW, can we somehow exchange e-mail addresses, it's way easier to communicate in Dutch. If we have any common findings later, we can still post them here.

ErnestB:
No problem. Mine is: ernest072@yahoo.com

KLA564#:
I just checked the movement for linearity, it seems fairly linear, certainly not as bad as the instrument as a whole.
It is a 30 micro amp movement. I put a 1 M ohm resistance and my Bremen BM 869S in series with the movement, then connected this to my power supply.
This is what I found:

Current μA   Readout
0,00                   0,0
4,92                   5,0
9,96                   10,0
15,00           15,0
20,00           20,0
25,05           25,0
30,10           30,0

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