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:
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 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)
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.