Controller R965 dead - what is 'link' on the circuit board?

[Will-352865]

Hi. New to this, just picked up some old sets for my son's outing into the world of model railways. The locos need a bit of soldering to repair some dodgy wiring repairs but also the controller is now dead. Was kind of working on and off for a day but now no output.

The power pack reads 18V across the plug so no issues there.

The through AC reads 2.8V and the track DC supply reads 0V in all switch and resistor positions.

I've popped it open and followed the board layout to test across positions.

Bridging the power side of the 'link' to the opposing AC output I get 18V as I'd expect.

Bridging from the otherside of the 'link' I get 2.8V so I'm guessing the 'link' is at fault.

What is the 'link'? Is this the fault as I would assume this should be allowing the unaltered 18V AC power in through to the AC output pins.

Thanks for any thoughts. I'm fairly good at replacing things and fixing the obvious but I'm not savvy when it comes to circuit componentry.

[96RAF]

The orange ‘link’ is a PPTC, essentially a resettable fuse. It should have a code on it to search on.

This links to a typical such device- very unlikely to be the exact one you want - https://www.mouser.co.uk/datasheet/2/643/ds_CP_0zrc_series-1960444.pdf

[P-Henny]

Note that this version of the R965 uses Thyristor control of the output to the track. In essence and in simplistic terms, this makes the controller a 'current' controller rather than a 'voltage' controller. This means that to read the track voltage with a voltage multimeter, the output needs a current load attached to it, else without a load you will get a fairly high voltage reading that does not seem to exhibit any voltage value variation using the speed control knob rotation.

The PPTC is explained here:

https://en.m.wikipedia.org/wiki/Resettable_fuse

P.S. A Thyristor is very similar to a SCR (Silicon Controlled Rectifier). The output to the track is therefore a series of halfwave rectified pulses that are controlled by the waveform applied to the Thyristor gate. This is not the same as PWM (Pulse Width Modulation) but does give a very similar effect with regards motor control as PWM i.e high torque at low motor speed.

[Will-352865]

Thanks 96RAF.

So from a quick search into PTCs I see they get hot if they trip; this is the case. On further investigation the problem is with the variable resistor as if I twist and hold it at 0 the PTC cools down and I get 18V AC out on the pins. Soon as I let go of the VR nob it drops back to 2.8V and the PTC heats up again.

[96RAF]

Sounds like that VR has a short problem. Maybe the track has worn through. At least you are making progress.

Likely that VR is the same as used in other Hornby controllers. I have a data sheet for the HM2000 one which has an off switch. Is there a code on the board next to it.

Edit - by the way this forum section is for the HM6K app controlled analogue system. Once we have resolved your issue I will shift the thread over to the General Discussion section. I have asked for the section header to be amended for clarification.

[P-Henny]

A technical review of the R965 including oscilloscope screenshots of the controlled output can be seen on 'Scott Pages' (link below). The following is an extract of the linked review.

"..........it is a feedback type that uses a thyristor (or SCR) as an electronic element switching a rectified but unfiltered source of dc, at line frequency........."

Model Railway Controllers

See also this 'How to repair the R965' page.

Hornby R965 controller