Ontrack Controller Fault

[Simon G]

At our Club, we have an ONtrack Dual Track Controller, model MPC3. Until recently, it has performed faultlessly for years. Now, one track has developed a fault. On locos that take a heavy current, such as Hornby Dublo, Triang etc, the locos run for a few seconds, then slow to a halt. Modern Hornby & Bachmann locos can run happily round the layout without this problem.

 

We dont know exactly what might be causing this! A thermal overload would normally be a more rapid stop - unless anyone knows differently!

 

Any suggestions would be appreciated.

 

Thanks.

[Frank Sharp]

Simon,

 

Just dropped on this, hope it isn’t too late to be of help

 

You are on the right track. I assume that like all modern controllers this unit doesn’t have a bimetal strip cutout that is either on or off but a device called a polyswitch. These are cheaper than the old cutouts, but very fast acting, won’t reset or try to reset until the fault is cleared and very sensitive. Even if your controller is nominally good for one amp, and you check an older loco and find that it draws 0.75 when running, it might draw fractionally over one amp as the motor starts, that’s enough to set a polyswitch to action. The switch, and there’s no ‘moving’ parts in it, sees the overload and raises its resistance, now it is drawing not only the current wanted by the running motor but that needed by its own higher resistance, so it warms more, resistance increases and eventually goes high resistance.

 

As you say, a few seconds delay, and not a sharp shut down. I’ve had the same with Athearn motors. Though a polyswitch reset with a low amp motor takes a few seconds, in fact they take hours to properly reset and can be very sensitive during that time. I’m coming to a belief that they also become more sensitive with age.

 

Cure. Disconnect mains, open up controller. You are looking for a yellow disc, rarely a square with two wires, egg yolk coloured . It should be somewhere either on the 16V AC feed from transformer to the control, or more usually before or after the 4 rectifier diodes. Black cylinders with silver bands. The controller instructions should say what the controller is designed to deliver. You can replace the polyswitch with an old style bimetal strip type. If the controller is intended to give 1 amp, get one that trips at 1.2. Squires or Rapidonline will have them, All components may also have them. You'll have to watch for overloads and do something about them straight away whereas the polyswitches wouldn't reset until faults were cleared.

 

If you or someone at the club knows what they are doing it is possible to wire polyswich and bimetal strip on parallel with a switch to select between the sensitive (polyswitch) or robust (thermal)overloads.

 

Frank

[Simon G]

Frank,

Thanks for the reply.  We have not got round to investigating the fault further yet, so your post is very timely!  From what you say, a faulty Polyswitch could easily be causing our problems.  Hopefully, we will be able to check it out at the next club night.  I will post any progress back onto this thread.

Cheers, Simon

[Simon G]

Frank - you are a star!  It was indeed a faulty Polyswitch.  We opened up the case and located the 2 Polyswitches and checked the resistance across each before trying to run anything.  The good one was effectively 0 ohms, and the faulty one was about 2 ohms.  After running a high current loco until it stopped, I rechecked the faulty one and it was up to 12 ohms and was only settling slowly, and seemed to stop at 4 ohms long after it had 'cooled'.  I managed to source some ( a pack of 5 on ebay was cheaper than 1 on rapidonline after P&P.  We removed the faulty one and put a new one in place, and... nothing!  Further investigation showed that while soldering the new one in place, we had bridged a connection on the PCB.  Once this had been sorted, everything runs as sweet as it should.  My Triang Class 31 ran round for many circuits at high speed without a hitch.  And we now have 4 spares!

Many thanks for your help.

Simon

[p-lidster]

Dear Simon,

 

I have the same problem with my On-track controller as you had. I have tried to source the bi-metal strips on Ebay & I have looked on Squires web site but it does not matter what variation on bi-metal trip / overload/ surge switch I put in I get no results. Did you get them from a specific supplier, if so could you please give me details so that I can purchase some.

 

Regards,

 

Peter L.

[cliff park]

Rapidonline have them, various current ratings, make sure you get the right ones. They start at 33p each, but postage might be silly.

[p-lidster]

Simon,

 

Just dropped on this, hope it isn’t too late to be of help

 

You are on the right track. I assume that like all modern controllers this unit doesn’t have a bimetal strip cutout that is either on or off but a device called a polyswitch. These are cheaper than the old cutouts, but very fast acting, won’t reset or try to reset until the fault is cleared and very sensitive. Even if your controller is nominally good for one amp, and you check an older loco and find that it draws 0.75 when running, it might draw fractionally over one amp as the motor starts, that’s enough to set a polyswitch to action. The switch, and there’s no ‘moving’ parts in it, sees the overload and raises its resistance, now it is drawing not only the current wanted by the running motor but that needed by its own higher resistance, so it warms more, resistance increases and eventually goes high resistance.

 

As you say, a few seconds delay, and not a sharp shut down. I’ve had the same with Athearn motors. Though a polyswitch reset with a low amp motor takes a few seconds, in fact they take hours to properly reset and can be very sensitive during that time. I’m coming to a belief that they also become more sensitive with age.

 

Cure. Disconnect mains, open up controller. You are looking for a yellow disc, rarely a square with two wires, egg yolk coloured . It should be somewhere either on the 16V AC feed from transformer to the control, or more usually before or after the 4 rectifier diodes. Black cylinders with silver bands. The controller instructions should say what the controller is designed to deliver. You can replace the polyswitch with an old style bimetal strip type. If the controller is intended to give 1 amp, get one that trips at 1.2. Squires or Rapidonline will have them, All components may also have them. You'll have to watch for overloads and do something about them straight away whereas the polyswitches wouldn't reset until faults were cleared.

 

If you or someone at the club knows what they are doing it is possible to wire polyswich and bimetal strip on parallel with a switch to select between the sensitive (polyswitch) or robust (thermal)overloads.

 

Frank

Hi Frank,

 

I have a dual control On-track controller with the same problem as Simon G., however I am a novice at electronics, so your advice would be helpful. I have contacted Rapidonline about 1.2A  Bimetal strip cutouts, & they recommend either a 1.1A or a 1.35A. Will one of these do, & will I need more than one for each side of the controller.

 

Regards,

 

Peter.

[Frank Sharp]

Peter,

Long forgotten this thread and turned it up looking for something else. Bimetal strip cutouts aren't as sensitive and once reset aren't bothered by remembering that they've recently been tripped. I'd go with the 1.1 amp. If you go too high you risk burning out the electronics. Usually that's the four diodes that make up the rectifier. Usually they are just 1 amp. I've repaired a few at shows in the past, usually with 3 amp diodes which almost makes them bomb proof.