Hornby R8216/R8247 Accessory Decoders


This report details the procedural teardown of a Hornby Accessory Decoders, highlights any parts that are known to have gone faulty, tells of their physical circuit board code, identifies their original part number and where applicable provides an equivalent part by supplier code.


The procedure totally dismantles the unit, but you only need to dismantle as much as is necessary to access those parts you wish to work on.


This information is provided in good faith, but as I have no idea of your skill levels it is assumed you know what you are doing and that you work on your accessory decoders at your own risk.


Be aware that opening the case and/or making changes or attempting repairs to these units will invalidate any warranty on your accessory decoder and may also preclude any future chance of Hornby being able to repair or even update the unit. This also applies to any unit bought second hand, whether working or not, as the previous owner may have opened the case or attempted repairs.




















The original accessory decoder R8216 was limited to a pulsed output on each of its four ports and required special procedures to pre-charge the unit before programming was possible, whereas the superceding R8247 has either a pulsed or steady output and is much easier to program.


The unit is essentially a CDU (capacitor discharge unit) designed for DCC (Digital Command Control) use. Each port is rated at 200mA steady output, but enough on pulse to reliably fire a single solenoid point motor, but service experience shows that a pair of solenoids can be fired together such as one would see in a linked cross-over of tracks.


The unit has a DCC signal input on the Track A-B terminals and four ouputs each as shown (+ C -).



























The unit comes apart in the standard Hornby fashion, qty x 4 special screws located under the rubber footpads.


















There are very few components, just regular heavy wattage resistors and large capacitors, along with terminal blocks and a jumper area used for update programming of the firmware (not day to day user address programming) on the top side of the PCB.


















The underside of the PCB is fairly bare with a dual mosfet (Q1-Q4) and a pair of hefty diodes for each output port and a PIC chip (U1) for the main operating firmware. Replacing the mosfets is a fiddly task but most parts, which are clearly part numbered are readily available from standard suppliers.

















This report is Work in Progress but the information I have to hand is presented for your information in the meantime. Cross reference of board codes to part numbers remains outstanding.


Known Faults


Short circuits on track can cause a decoder to lose its addressing requiring a reset and re-address. It is better to run your accessory decoders from their own DCC bus to avoid track induced problems.


There is one known serious failure mode due to a bug in the Select controller but only seen at firmware version 1.1 which can cause an R8247 to fail. The problem is not seen with the Select at other firmware revision states or with other controllers.


The Select bugged scenario is this:


If port 1 is activated the R8247 will start to pulse on/off rapidly. If a point motor is connected to the port it will discharge the on-board CDU and keep it flat such that the unit will not operate a point on any other port.


If the unit is powered down port 1 will reset and the unit will work fine until port 1 is accessed again. If any other ports are selected first then the unit will work until port 1 is next used whereupon the unit fails again.


If the system is left powered in the failure state it is possible for the output transistors to fail completely. The pulsing is not easy to detect and the chance of final failure makes diagnosis also difficult.


The main problem is the R8247 can be repaired or replaced only to fail again as the Select is the root cause.


The best advice is never to connect an R8247 to a Select at revision 1.1. Get the Select upgraded instead. You may be able to get this done free of charge by Hornby (contact Customer Services) and you may also be able to claim for a new R8247 if the problems are proven to be connected.

 


Acknowledgments:

Many thanks to Ken Wards in the Hornby Research and Development Department for his help in providing part numbers for the circuit board codes quoted in this article.


RH/Acc-Dec/Oct2017/v1.0



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© R Honnor