Hornby R965 controller

Hornby R965 controller failure and repair. Faults - no drive, model speed not adjustable, model stops after a minute or two or longer.

Drill out the ends of the thin brass rivets with a 6mm bit and lever the base off

Remove the nuts from the AC terminals, pull off the speed controller knob, remove nut/washer and then remove the PCB from the case

Plug in and test the voltage across the output of the AC PSU wall plug (16-17.5vac - set to AC range - if no output, check for breaks in the cable if none you will have to replace the SMPS - Hornby C912 16VAC/14VA or similar, most likely trains stopping after a while is due a fault in the the SMPS as they is no cutout circuit in the controller.  - again replace but do follow the checks below before making any expenditure). Test at the DC output after the rectifier bridge of the board (around 15vdc - set to DC range). The final output is around 20vdc (make sure the direction switch is activated - centre is off). You will not be able to measure the switching of the SCR but you can test on a train using a track or directly to the wheels. First make sure there are no simple breaks in the wire cable to the model track connector. If you get a break at the connector, push out the crimp pin, open up the cable crimp and solder in place, re-crimp the cable and reinsert into plastic connector block. If after these checks no output or no speed adjustment is possible, follow the procedure below

Test all resistors for open circuit and check the potentiometer wiper (centre contact) is working (increasing/decreasing resistance), you will not be able to test the capacitors without a specific capacitance tester, but look for short circuit on the large electrolytic 220uF 25v, the .1uF disk and the 1000pF axial cap. (un-solder one lead to test)

Test diodes
There are 4 x 1N4001 diodes in the rectifier bridge if you get the readings above, they are working correctly. There are 2 more diodes on the board (see below for location), measure their forward voltage using the diode setting on your meter – about 0.5volt.

If a diode setting is not available, measure the resistance, reverse the leads and test again – one way should be very high and the other low

If any fail the test, unsolder one end and test again – if same result, replace (white or silver line in same orientation – see photo)

Test the SCR (click for datasheet)
The measured resistance across K (cathode) and A (anode) should be high (>1 MΩ). Reverse the meter leads, the resistance should be again high. Connect the negative lead to K and the positive lead to A. Short the gate (G) to A. A reading of approximately 15 to 50Ω is normal. When the gate to-anode short is removed, the same reading should still show on the meter until the leads are removed from K or A. Reconnecting the meter leads to K and A should show no reading until G is again shorted to A. Make sure that the resistance measurement is being taken across the device and not something else in the circuit, by unsoldering and removing from the board. Note also that the only valid “failure" resistance indications are “open" and "short.” If so replace make sure the leads are in same orientation K, A, G as marked on the board

Test the PUJT (click for datasheet)
Measure the resistance between b1 and b2, then reverse the leads, both readings should show the same high resistance regardless of the meter lead polarity. Connect the negative lead to the emitter (e), measure the resistance from e to b1, and then e to b2. Both readings should indicate high resistances approximately equal to each other. Connect the positive lead to e and measure e to b1 and e to b2. Both readings should indicate low resistances approximately equal to each other. Replace as required making sure the leads are in same orientation b1, b2, e as marked on the board