Hornby R8238 DCC Reverse Loop Module


This report details the procedural teardown of a Hornby Reverse Loop Module (hereafter called an RLM) used to automatically switch track polarity when a train enters a reverse loop. The unit is powered from the main track and outputs the correct polarity to the isolated reverse loop section of track as required.


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 RLM at your own risk.


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


A common mistake is for users to provide a reverse loop section that is shorter than their longest train, which prevents the RLM working correctly.


The way this unit works is to detect a short circuit as a train passes from the main track into the isolated reverse loop section. The unit immediately switches polarity of the isolated loop to match the main track. As the train leaves the isolated section the RLM again senses and switches to the correct polarity.


The unit can be set for sensitivity by way of the Select/Elite switch - in effect this switch is selecting whether to be aware of a 1-Amp (Select) or 4-Amp (Elite) power supply unit and react accordingly.



Top Side



















Isolated Track Output Side




















Main Track Connection




















Lower Side




















Dismantling:


Undo the 4 x security screws found under the 4 x rubber foot pads. These are the usual triangular recess screws.



















Lift the top case off. The circuit board lifts out but remains attached to the top case by the switch and led cables which can be disconnected at their plugs.



























Reverse side of PCB.



















Note one of the component tags on my PCB was visible in the case mounting screw hole and could possibly cause a problem.






















Components:


All component codes are clearly visible on the PCB. These are mostly standard components used by Hornby and some have previously been identified in the other Teardown Reports.  Refer to the table for identification.






















Item Code          Description                      Part Number

F1                     Resetting fuse              30V 3A 30R300


K1 & K2             SPDT Relay                   G6RN-1 12VDC


Q1 & Q8            NPN Transistor SOT23    MMBT5551LT1G


D1                    Voltage Regulator Diode 27V 500mW SOD80 BZV55-C27


D2                    Voltage Regulator Diode 2.0V 500mW SOD80 BZV55-C2.0


D8-D13             Fast Rectifier                 RS1A SMA


D5                    Red LED 3mm


U1                    Voltage Regulator 5V      LM78L05ABUTR


U2                    Dual Low Power Op Amp SOIC-8 Narrow LM358M


U4                    PIC12F510 8-pin 8-bit     Microchip 12F510-I/SN


R4 & R5             Wire Wound Resistor      5-Ohm 3W 5%


SMD Resistors    Size 0603                     10k 5k1 4k7 2k2 470R 47k 1k


SW1                  SPDT Switch




Problems:


Very little is reported as going wrong with this unit.


Most faults are likely to have been user induced by incorrect connections to the layout, simple as they are (2 wires in and 2 wires out). Reference to the user’s manual will show how to connect the unit and where isolating rail joints need to be.


Acknowledgments:

Many thanks to Ken Wards in the Hornby Research and Development Department for his help in compiling this article.


RH/RLM/May2018/v1.0



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© R Honnor

Board Code

Description

Part Number

Alt PN

Purpose

C3

Capacitor

As marked


15V to track

C6

Capacitor

As marked


Track output

C10

Capacitor

100μF 25V


5V regulated

F1

3-Amp 30V Resettable Fuse

As marked


DC input

F2

0.9-Amp 60V Resettable Fuse

As marked


Dc input

L1

Inductor

7μθ


Track output

L2

Inductor

7μθ


Track output

L4

Common Mode Choke

CM1812R 600R-10


Track output

L5

Common Mode Choke

CM1812R 600R-10


DC input

L6

Common Mode Choke

CM1812R 600R-10


DC input

L7

Common Mode Choke

CM1812R 600R-10


Boost output

U1

PIC

18F4520


Main functions and display

U6

12V Regulator

LM7812CT


DC input

U7

5V Regulator

LM7805CT


5V regulated

Q1

Dual mosfet

IRLU024NPBF


Track output

Q2

Dual mosfet

IRLU024NPBF


Track output

Q3

Dual mosfet

IRF9024N


Track output

Q4

Dual mosfet

IRF9024N


Track output

Q8

Dual mosfet

IRF7509


Boost output (Xpressnet)

Q9

Dual mosfet

IRF7509


Boost output (Xpressnet)

XX

Display

TRI-T CL010-7009-01 Rev C


Display screen