EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: jeroen79 on January 08, 2017, 08:08:51 pm
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I found an old 'Wee' Megger that has a slight flaw.
With nothing or a high resistance (10M?) connected the needle does not fully deflect but stops near 4M?.
1M? reads correct.
A short will also fully deflect the needle to zero.
I found nothing stopping or slowing the needle mechanically.
If I pass current through the coils I can also fully deflect it.
Open circuit output meets the required 500V.
The circuit:
(http://i.imgur.com/ia6TyeQ.png)
A is the (wirewound) resistor for the deflecting coil.
D is the resistor for the control coil.
B and C are a capacitor and resistor to stabilize the inductor's output.
Here (http://www.richardsradios.co.uk/megger.html) it says that A and D should be 100k? each.
B and C should be 0.1mFD (=100nF?) and 47k?.
What I measured:
LD 260? (deflecting coil)
LC 115? (control coil)
A 49.6k?
B 0.44uF
C 5.27k?
D 210k?
This does not match the specified 100k?.
Is it possible for these wirewound resistors to somehow change?
Weird thing is that one is twice as lage and the other twice as small.
My theory:
The higher resistance on the control coil and the lower resistance for the deflecting coil will make the latter much stronger so it will favour the zero side over the infinity side.
Am I right?
Would the solution be to replace the old resistors with new 100k? ones?
As they will be loaded at 500V they should be at least 2.5W rated, right?
The inside with parts marked. (not mine)
(http://i.imgur.com/I8SjlHs.png)
The bridge over the meter movement is the common for LC and LD and is also connected to the generator and the capacitor.
The two coils have separate connections at the bottom and these go to the resistors.
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Mine does that as well.
I don't think it's normal though, but besides that, it is still accurate on the remaining of the scale.
Are you sure you have the schematics of your model ?
The one you show seems odd.
The early versions were driven with a DC generator and shouldn't have a capacitor I guess.
Later ones (Since the 50' if I remember) switched to an alternator (which I happen to have) and the picture you show is from this later alternator version.
On this picture (again from memory) A & D are the two selenium rectifiers and B is the dual capacitor, all mounted as a multiplier.
I can dig in my documentation to see if I find more info but I restored mine a few years ago so I'm not sure.
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The resistor in the picture that I can see (C) looks like a carbon comp resistor, not wirewound. Those are well known for varying with age.