I recently bought a Marklin BR120.1 3553 engine, it was listed as not tested. Initial running repairs were completed and a quick test seemed to indicate that this was a runner, however when I first ran it on the track, the train shuddered as it moved and then stopped moving, I then tried to pulse the transformer to change the direction, and it appeared to change but it still refused to move and the lights no longer worked.

Upon first inspection there was no damage to any of the connections and the brushes were making contact with the motor which was able to spin without resistance when the wheels were turned.

I then disassembled the motor to clean the contacts, I noticed that the field magnet surrounding the motor was fairly strong to the point where it would pull the motor off centre, I also inspected the gears to make sure they could freely move and there was no damage to the teeth.

After cleaning the contacts and reinstalled the motor I then applied a voltage across the brush terminals to see if the motor would spin with a directly applied voltage to which it did, however it was not smooth at all and it was almost like a ratchet jittery motion and when I change the voltage, I did not notice a change in speed.

The controller board has not been tested as it is clear that there is some issue with it due to the lights no longer working which are fed by the board and that a directly applied voltage will turn the lights on, I have tried applying a voltage over the terminals where power is supplied to the board with no response.

I would like some help regarding the magnet and its strength to determine if that is what is causing the jittery motion as at this point in time it appears to be the only culprit left in causing the strange operation of the motor it is the first time I have dealt with an arrangement like this as all of the other trains I have use coiled magnets but this one is just a magnet block. I will attach images of the magnet and motor for clarification as well as an image of the controller board.

Thank you. My son and I will follow up on your advice.

I have tried looking for the wound field magnet for a drum collector motor and not really had any luck.
It's probably a bit harder being UK based unless you want to pay a lot in postage.

Since your motor works ok on DC you could just try to get hold of a cheap and basic decoder which will run in analogue mode such as the Marklin 6090 ones or the ones out of the 60760 kit.
You might find that the decoder in your photos still works. Someone on here who knows more about digital than me might be able to tell you what it is and how it should be wired.
Most if not all of the Marklin decoders will work in analogue mode - they usually automatically detect it.

What I have done on a couple of locos where a previous owner had put in a permanent magnet is to fit a Roco 89940 reverser. This is a small circuit board that rectifies the AC to DC and feeds it to the motor. It has a little relay in it that reverses the polarity in response to the over voltage pulse from the Marklin controller. Roco used to fit them to their analogue locos that were sold to be used on the Marklin system.

Paul

All "five star" locos were analogue and had field coils. The 35xx five-pole armature was different from the five-pole armatures of later digital locomotives.
I cannot tell from the photo if this is still the original 35xx armature or a modern digital type.

Looks as if the loco had been converted with a permanent field magnet and a decoder, maybe ESU.
When buying a new decoder, it could be a good idea to make the full conversion with 60941 motor parts. The 35xx armature requires different decoder settings and it can take time to find them.

This is one of my BR120 locomotives I converted many years ago from delta to digital with a Marklin high propulsion kit being the 5-pole digital armature, brushplate with screws & brushes & permanent magnet. At the time only the Marklin #6090 decoder was the one available to me & still has never let me down. If I had to do it all over again now I would have fitted it with an ESU lokpilot decoder:

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We have managed to retro fit with a coil magnet and switch (similar to the 3151 / 3353 models). Tested and running on our analogue 3-rail track.

Thank you for your comments and advice.