Hi all,

Last week, several times my MS refused to start up. I found the problem was that the 18VA transformer (the ‘ugly black box’) was not providing any power.

I decided to have a look inside, and to my dismay, it was not a broken cable, but a broken component. There is a little resistor (see photo) that is in series with the AC winding of the transformer. If I remember resistor color coding well, this is a 3.3 Ohms resistor (correct me if I’m wrong!). The thing is broken: when measured, it has infinitive resistance.



I’m not too much into AC circuitry, but did anyone experience this problem before? Any idea why it could have broken? A faulty component? Did not seem to have been too hot for a too long time (burned resistors look different – burned).

/rudi

For the colour coding of resisters: gold is 'under' black, and silver is 'under' gold.

Orange-Orange-Black is 33 Ohms (this one is well knowm).
Orange-Orange-Gold is 3.3 Ohms
Orange-Orange-Silver is 0.33 Ohms

But to be honest I do not know about violet as the tolerance ring.... (or am I mistaking and is this not a resistor?)

/r

Interesting site ,very useful.

Hallo Rudi,

Never seen that problem before. As a matter of fact, never had an ugly box go bad....

Charlie (looking for a piece of wood...)

Is it a resistor? Or is it a coil?

I cannot tell from the picture, but the green thingies inside M* locos with 5 pole digital motor are coils, not resistors (one connected to each brush plate).

Resistor or coil: with infinite resistance it sure is broken...

Thanks all for replying. Tom, I have indeed wondered whether it is a coil instead. Also because I have not yet found what the additional violet ring is about. A saw a few sources on resistor color codes, designating this ring as temperature coefficient, but violet is not defined color there. (e.g. http://en.wikipedia.org/...i/Electronic_color_code)

Hans, indeed it is in the primary circuit, in series with the winding of the mains transformer. No lose parts there, though I did notice that the soldering of the rather heavy transformer on the PCB did lead to some transformer pins not being well making contact. I guess it was not a wise thing of M* to solder such a heavy component to the PCB, without additional strain relief. Then again, they are probably procuring it from a cost-effective source.... (which does not relieve them of their responsibilities, though).

Cannot see anything that might have led to a spike. But you might nevertheless be right. Seems something destroyed the outer pretective coating. For the time being, I shorted the component, and look for a replacement.

The 'cheap fuse' hypothesis is still the most probable, in my view.

/r

Depends on the material. I once killed a coal resistor with a M* transformer - and it was a long death (glowing red and becoming very hot). I wouldn't use that type of resistor as a fuse.

But it could be a non-coal resistor.

You got me wrong: I plugged the resistor into the red and brown connectors of the trafo and killed it intentionally (a resistor from a damaged device (don't remember what it was)).