Hornby B12/3


Another loco aimed mainly at our extended "Little Histon" layout, to enable post war period running through the station.

Hornby photo of 61580

Likely configuration:

If the motion is smooth enough, I would like to use a Zimo decoder..... in the absence of a dedicated B12 sound project, I plan to use a Paul Chetter generic 2 cylinder LNER steam sound project to which he has added real B12 whistle sounds. I'll update this when Paul gets around to producing a complete B12 sound project from his recordings on the North Norfolk Railway.

Although Hornby provide mounting arrangements for a 28mm round speaker, its not easy to make the tender body into an airtight speaker enclosure, so instead, I'll fitted the speaker into a custom enclosure, just a couple of mm above the tender ballast weight. The space in the tender is quite limited, so after a little plastic surgery, the Zimo SC68 super cap stay-alive device is mounted hard against the left hand side of the tender, semi recessed into the speaker enclosure, where it is just touching the underside of the coal load. The entry slot into the speaker enclosure is carefully sealed to restore an airtight speaker fix. A zener voltage protection circuit is used to ensure that the voltage across the capacitor never exceeds the maximum 15 Volts rating. The decoder will be mounted on an ESU 21 pin adapter board, within the coal hopper, hidden by the coal load. A "stopping train" front lamp (top centre) plus red & orange LEDs in the firebox will be added to the locomotive. I think there is just enough space between the motor plus flywheels and the underside of the metal boiler top, to run the front lamp wiring through to the rear of the firebox. A small hole will be cut in the cab front bulkhead in the location of the firebox door. The red and amber LEDs will be mounted directly behind the hole. Series resistors for the LEDs should fit behind the firebox door and the 3 new wires will hopefully be run in parallel with Hornby's 4 wires, back to a connector on the tender underside.

I'll also add a footplate crew.

The only remaining Class B12 locomotive in apple green LNER livery, on the North Norfolk railway.


DC Testing:

The locomotive is a very smooth runner in DC configuration, straight out of the box. Slow speed running is amongst the best I have ever seen for a Hornby steam loco under DC control. Using my bench power supply which provides pure DC, the motion is virtually silent.

However, when controlled by the DC pulse width controller used on our village layout, while speed control was equally smooth, a pronounced growl was apparent from the motor at slow speeds.


Probable Circuit :

I plan to use a similar arrangement to that in my B17:

The original 8 pin DCC socket is discarded. The motor and track connection wires are connected instead to an ESU 21 pin adapter PCB, mounted in the coal hopper. Motor and track wires are routed through a hole cut into the bottom of the hopper. The speaker wires are also soldered to the 21 pin adapter board.

A sub-miniature 4 way connector socket will be glued to the underside of the tender to provide lighting connections to the locomotive. The socket will be connected to Common positive, headlight and FO1 pads on the 21 pin adapter board (via the hole in the bottom of the coal hopper).


LED series resistors mounted behind the cab bulkhead. The left hand resistor simply

provides an anchor point at one end for the common positive connections.


Zimo MX644D mounted on ESU 21 pin adapter board in the coal hopper.

The blue tack blob is used to hold the decoder in place.


The stay alive super cap protection circuit is super-glued to a plasticard substrate,

above the decoder and immediately below the coal load moulding.


Decoder CV Adjustments:

To remove the standard default conventional mapping: Set CV33 to zero and CV34 to zero.

Then, using Zimo "Swiss Mapping":  CV430=29  (button 0), CV432=14  (FO headlight forward), CV434=14  (FO headlight backwards)

Some motor control adjustments:   CV29 to 7 (Hornby motor wiring colour code back to front), CV3 & CV4 both to 100 (acceleration and deceleration rates), CV9 to 87, CV57 to 80.

Fine tuning the chuff rate to 4 per driving wheel revolution: CV267 to 57.

Also dropped the sound volume of the Westinghouse brake pump on button and random operation.


Key Number

Function Description

F0 Loco Front Oil Lamp & red background firebox glow
F1 Sound ON
F2 Brake Key
F3 Toot Whistle
F4 Long Whistle
F5 Heavy Train (off) / Light Engine (on) Selection
F6 Coal Shovelling (This sound is also a random sound)
F7 Live Steam Injector (This sound is also a random sound)
F8 Blower
F9 Wheel Flange
F10 Safety valves lifted (This sound is also a random sound)
F11 Hand Brake
F12 Tender Water Filling -  variable length
F13 Wagon Snatch
F14 Buffering
F15 Cylinder Drains Opened
F16 Westinghouse Pump  (This sound is also a random sound)
F17 Toot Toot Whistle
F18 Guard’s Whistle
F19 Fade All Sounds
F20 Shunt Key
F21 -
F22 -
F23 -
F24 -
F25 -
F26 -
F27 Overall Volume Down
F28 Overall Volume Up


Annoying early life failure of DCC Concepts LNER style oil lamp:

Having tested the locomotive, and made some adjustments to the CVs as shown above, I was about to head back to the work bench to fit the Hornby "add on extras" when I noticed that the front oil lamp was no longer functioning.

A bit of simple testing with the multimeter revealed that the lamp had gone short circuit. I carefully released the low tack glue from the wiring, so that the lamp could be extracted from its mount and sure enough, the short was inside the DCC Concepts lamp moulding....... they are not cheap, so I was not impressed!

I'm now painting another example matt black to eliminate the light bleed exhibited by these lamps. (The black will be over-painted in matt white when dry). The new lamp is working fine at present.... I hope it does not fail like its predecessor!


Failed Oil lamp proves an unexpected good fortune:

When the loco was dismantled to replace the failed oil lamp, it was apparent that not all the new wiring had dropped into the channel in the chassis assembly. Some was trapped and partly compressed between the locomotive upper and low body assemblies.

In order to route the wiring correctly into the channel, I filed a notch into the metalwork of the upper body immediately below the location of the LED series resistors behind the cab bulkhead.

The wires to the new tender connector, are soldered to the appropriate resistor leads. They were then glued in place & bunched before routing via the notch, so that this time they all dropped directly into the Hornby wiring channel in the lower body, during re-assembly.


Adding the Hornby Detailed parts & the loco crew:

The loco and tender obstacle deflector brackets and brake hose parts were glued in position using gel super-glue.

A driver and fireman were then selected from a Bachmann loco crew pack and super-glued in place within the cab.


Motor noise in DCC operation:

A distinct buzzing noise is produced from the locomotive motor during slow speed running.

With the Zimo MX644D decoder fitted, the volume of the buzz can be reduced by minimising the frequency and duration of the BEMF sampling via CV9. In previous applications, I have usually set this CV to around 87 but on the Hornby B12/3, a value closer to zero improves the motor noise level. Using CV58 to reduce the level of BEMF control at slow speed also reduces the motor noise.  However both these CVs also impact upon smooth very slow speed running. So to avoid a small amount of judder in this situation, I've ended up with CV9 = 3 and returned CV58 to 255.  To mask the residual noise during the early stage of a departure, I have increased the length of the automatic cylinder blow off sound sequence, using CV272 and speeded up the acceleration and deceleration delays by resetting CV3 & 4 to 75. 


  The completed B12/3:












For the You Tube Video, please click here


Supplier website links:
Hornby    B12 Sourced direct from Hornby thanks to a good discount provided by completing their new product survey
Digitrains    Source for Zimo stay alive super cap., ESU 21 pin adapter board, 28mm speaker and DCCC LNER headlights


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