BBC Goerz Metrawatt MA 4E Service Manual and calibration - Page 1
Forum section: electronics / testgear

author: ErnestB
author_rank: Contributor
timestamp: September 30, 2023, 09:31:25 pm
content: Would anybody here have some information on how to calibrate and/or have schematics of analogue multimeter BBC Goerz Metrawatt MA 4E (besides the German user manual)? T.i.a.

author: ErnestB
author_rank: Contributor
timestamp: September 30, 2023, 10:19:57 pm
content: Edit: Calibration done, but service manual incl. schematics would be a nice to have.

author: ErnestB
author_rank: Contributor
timestamp: September 30, 2023, 11:38:12 pm
content: Edit 2: The meter works great EXCEPT on the resistor measurements. On the range kOhm x 1 and kOhm x 10 the meter does not go to "infinity" with open leads, but rather shows about 100k. I have a theory that the battery acid come into the meter and have influenced the PCB and created this parallel "resistance". I have bought this meter on our national second hand "ebay" (called Marktplaats). The seller had some photo's where I could see some white tarnish (the battery compartment is clear plastic). So the seller cleaned it up at made some new photo's and said "I sell the meter with clean battery compartment, as you can see". Well I think the seller is kind of honest, as he was convinced that the meter is just good, and didn't realize that that battery leakage has done anything. As the battery compartment is not directly within the electronics and the defect was not obvious. Anyway, I knew almost for 100% sure that the battery leaked once, but was hoping that it doesn't influence the meter. I do not have any evidence that my story adds up, but I think it makes sense. As said I could calibrate the meter for DC and AC voltages. I could sneak a peak inside and could see that one of the battery contacts was partly "eaten" with a small hole in it. It could be difficult task to find those 100k somewhere in the PCB I suppose, and to eliminate that maybe even impossible...

author: ErnestB
author_rank: Contributor
timestamp: September 30, 2023, 11:48:27 pm
content: This problem is quite typical one. Battery left in the equipment that destroy the equipment. I had this CASIO fx-82SX for years and years. Problem of the calculator is that it uses only 400uW of power, and that battery at the time for replacement already leaked it's toxic blood, that it destroys the calculator. So when I have noticed that the digits have less contrast I opened it up and could see that the battery had leaked. I cleaned it up and put a new battery, but damage was done. The acid already crawl under the display contacts and yeah, not good

author: ErnestB
author_rank: Contributor
timestamp: October 03, 2023, 01:27:00 pm
content: Update: Found the schematics at radiomuseum.org, AFTER I have repaired the multimeterThe whole time I was on a trail that the acid from a leaked battery caused the extra lower unwanted input resistance of about 100k.But at one point I found a small capacitor of 220p to be causing the trouble (the function I was not sure of until I found the schematics).Replaced it with 47p as that value was working the best, and that value is suggested on the IC datasheet... Anyway, happy I could repair it to it's full functionality.

author: HighVoltage
author_rank: Super Contributor
timestamp: October 03, 2023, 02:02:01 pm
content: Nice findCan you share the schematics with us?

author: ErnestB
author_rank: Contributor
timestamp: October 03, 2023, 02:57:30 pm
content: So I have needed to replace the compensating capacitor (component number 116), that seats between pin 1 and 8 of the IC CA3130S. It is a "Miller effect compensating capacitor". I have measured aprox. the same value capacitance, as stated on the schematics, but maybe the capacitor developed some faulty ESR, I don't know. It is that old round transparent foil capacitor type. I have replaced it with a regular through hole ceramic one. It seems it solved the problem. But now I can at least study the whole circuitry to try to understand how it works.Edit: I realize that ESR is a bad term, as it would be more a leakage through the capacitor that causes the trouble, I suppose..

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 03, 2023, 03:30:30 pm
content: Quote from: ErnestB on October 03, 2023, 02:42:52 pmHere you go..

author: ErnestB
author_rank: Contributor
timestamp: October 03, 2023, 09:34:33 pm
content: The schematics.

author: ErnestB
author_rank: Contributor
timestamp: October 08, 2023, 03:18:43 pm
content: Well, I can tell you I'm not there yet. I don't understand yet what is exactly going on. I was on the trail of the leaked battery, then was thinking that a bad feedback capacitor caused the problem, then when I realized that maybe that was not really the case I was again on the trail of the leaked battery. Especially when I have seen that the place where the clip for negative polarity is (the place where the battery have leaked) is very close by the main Op-amp. Then I was almost sure that the acid have caused the "shortage". I have cleaned, as good as I could.. Then again tried some different capacitors... At the end I had a quite good result of more then 10M with open leads. BUT, one day later I still have 1.2M !!!???So I don't really know what is going on. Without the feedback capacitor the op-amp goes to oscillation. And I was thinking with meter open and testing it would be different parasitic capacity once closed and so on. So I thought I got it fixed, closed the meter, but no. One other thing, the meter seems to be not really linear across the scale. If I calibrate it to be accurate on the first (left) part of the scale, like on Voltage measurements, then for the higher values and full scale the values are lower and lower. So in any way still things to do...

author: KLA564#
author_rank: Newbie
timestamp: October 27, 2023, 01:09:02 pm
content: 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

author: ErnestB
author_rank: Contributor
timestamp: October 27, 2023, 04:27:40 pm
content: 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

author: KLA564#
author_rank: Newbie
timestamp: October 27, 2023, 06:58:00 pm
content: 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.

author: ErnestB
author_rank: Contributor
timestamp: October 27, 2023, 08:59:18 pm
content: No problem. Mine is: ernest072@yahoo.com

author: KLA564#
author_rank: Newbie
timestamp: October 28, 2023, 09:01:15 am
content: 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 Readout0,00 0,04,92 5,09,96 10,015,00 15,020,00 20,025,05 25,030,10 30,0

author: ErnestB
author_rank: Contributor
timestamp: October 28, 2023, 09:15:30 am
content: The other meter has also some degree of accuracy, so this is all just good. Thanks for the check.