Issue with a Megger like insulation tester made by MWS.

[Breakinger]

Hi,

I'm trying to fix a insulation and grounding tester made in 1960 by a German company called MWS. The device is powered by a hand crank and I believe the principle of operation is similar to "Megger" meters.

It contains mostly switches, few resistors, 3 capacitors with values of 4uF, 0.5uF and 25nF, and a number of what looks like coils or precision resistors made from resistance wire. The generator is connected to the board with 5 wires, each producing a different voltage when the handle is turned. The generator shaft also drives two switches that look like they are some sort of a commutator - possibly to provide DC current for the measurement.

The problem with the device is that, when the handle is turned the needle jerks very quickly between the measurement value and zero position. When I saw this first I assumed it would be an easy fix. I assumed a capacitor that is used to store a charge that powers the movement is leaky and this means it gets discharged too quickly. However replacing 2 out of 3 caps made no difference(the third cap is a ceramic 25nF so I thought it unlikely it would go bad).

Next I plan to take an oscilloscope and try to find where the current that is supposed to drive the movement leaks.

I thought I'll also ask on this forum as this meter seems to be very similar to Megger meters and perhaps someone encountered this jerky needle issue before.

I attach a photo of the meter. If people are interested I can also take some photos of the inside.

This is a link to high resolution version of the same photo: ****************/fkCZY0

[Breakinger]

Also, in the meter there is one part I can't identify. It is at the back of the photo labelled Gr (I've just placed it there to take the picture - I desoldered it to measure it).

It looks like a paper capacitor, but all other caps are labelled C1, C2 etc. This is labelled Gr. The writing on the part is E200/75 0.001 and +RFT14.58- Additionally one end has a red dot on the inside as if to show polarity.

I measured its resistance at 35MOhmds, but I don't trust this measurement as it was done with a cheap multi meter. I haven't got an LCR meter to verify it is indeed a capacitor.

After I removed it from the circuit the meter behaves same as before which makes me think it might be the part that causes the malfunction.

[Refugee]

Perhaps the GR stands for germanium rectifier?

[karesz*]

Hi,
maybe Herr Lutz Weissig hase some information for you?
rgds, Karl

[trobbins]

That meter appears to be used for ground/earth resistivity measurements, as typically needed for electrical installations to assess earth potential rise, and touch potentials, around a facility for nearby lightning strikes or facility fault currents. Nowadays there are standardised test setups for how far apart probes are inserted in to the ground, and reporting of earth resistivity of the soil.

[Chaparal]

This Megger ist from GDR (German Demokratik Republik)
MWS MessgeraeteWerk Schlotheim, Thueringen.

Type Erdungs - Isolations - Meßbrücke A 307 E - I
Ground - Insulation - Measuring Bridge
It was used for Resistance measurments: Ground, Insulation and Line.
With 2 probes it was possible to measure ground resistance in 2 ranges.

E200/75 is a Selen Rectifier 200 Veff 75mA
RFT 1458 means week 14 1958

Peter

[Breakinger]

Thank you very much. Well, this probably explains why it is working as it does with the jerky needle movement (The movement gets AC instead of DC). The voltage from the hand cranked generator is not getting rectified due to selenium rectifier being bad...

I wonder how do I replace that selenium rectifier. Probably a diode and a resistor in series.... The main question now is how to size the resistor without knowing the current the device is expected to draw, the expected voltage etc.

[Breakinger]

It turns out the selenium rectifier is fine

It lets half of the waveform through and then it goes to a capacitor. The result is -100V with about 20V AC on top of it. I'm still struggling to find how is that AC finding its way to the meter movement as I think it is what causes the needle to jerk.

In addition to the board I attached a photo of before there is a bank of 6 of what looks like wire resistors(they are labelled with letter W and a number each ).

If it turns out they are OK I guess the only remaining option is that there is a short somewhere, or one of the switches has high resistance.

It is a pity radio museum page about the device has a picture of the first page of the manual and nothing from the inside....

[Chaparal]

Letter W is for Widerstand = Resistor

[Breakinger]

Thanks. I've managed to "fix" the meter. It turns out 3 of the many contact switches required to be slightly bent. As they were contacting when they shouldn't. Now the needle moves smoothly without jerking.

However, I wrote "fix" in quotes as I haven't been successful in adjusting the meter to provide any useful measurements.

I started on the Leitungsmessung setting which according to google translate means cable(or line) measurement. So I expect that reading the value should be done by looking at the scale labelled 0 to 10 Ohms plus the multiplier from 0.2 to 1000 can be set by a switch.

Two potentiometers with knobs next to the movement appear to be there for coarse and fine adjustment, but when I adjust it to show 1ohm (on multiplier set to 1) while measuring a 1ohm resistor, the meter shows almost the same measurement when I measure a 2ohm resistance...

Additionally the needle of the meter can't be zeroed properly. The screw present for the purpose moves the needle from 0.2 to 0.7, but it is impossible to actually set it to 0. Can anyone suggest how to fix that? Do I need to open that movement and move something inside manually?

Many thanks for all your previous replies. I wouldn't have gotten that far with the device without them.