Capacitors on Model Locomotives

[Theakerr]

I have a Hornby B16 that has developed a strange habit.  Every so often it will be going normally and will suddenly slow down and the voltmeter goes weird.  I apply more power and after a bit off it goes back to normal behaviour but  at a slightly faster speed.  If I do not apply an increase in power it continues at the same slower speed until stopped and re-started.  I am using DC and it does this on both a PWM controller and my ancient HM Powermaster.  I have checked all circuits and find nothing wrong but when I connected the motor directly to the Powermaster I did not see  any slowdown over an extended run time.  So to my question; I found four separate capacitors (flat type), some (maybe all) appear to be wired across both + and - wires.  It was my understanding that a capacitor NEVER failed but based on my test I am wondering!  So before I do a Tony W and go snip snip just what do they do and can they fail?

[sharris]

They are for RF interference suppression.

 

Usually they have a chemically inert dielectric (typically a plastic or ceramic) so are less likely to degrade than electrolytics which can dry out with age. 

 

Unless there's any obvious physical damage (are they cracked or chipped?) The cause could be a partial short or the mechanism binding in the chassis. Can your meter measure current? Does the current jump when the slowdown occurs? Does the motor run hotter when it slows down?

[Steven B]

Any capacitors connected across the terminals of the motor will still be there when you power the motor directly.

 

If, as you say, the model works perfectly when connecting the control wires directly to the motor then the capacitors are not at fault.

 

The problem's more likely to be mechanical or linked to the path between rails and motor.

[34theletterbetweenB&D]

I have a Hornby B16 ...

 I'll buy it off you, wobbly capacitors and all. Is this an advance specimen of one of Hornby's upcoming delights?

 

Capacitors certainly do fail, and I have had it happen in several Chinese manufactured RTR models. While they are failing current draw can be very variable

[sharris]

Incidentally 4 capacitors? 

 

In all the Hornby engines I've opened up (admittedly, only 1970s models) there has only ever been 1 ceramic capacitor for supression. Four seems excessive.

[Theakerr]

I agree 4 capacitors does seem a bit excessive.  1 in the tender and three in the loco, two on top and one near the wire to motor connections.  Re the wiring and binding I have checked and double checked but cannot find anything.  I think it is going to be snip and cut starting with the tender one