|Adding Sound to the Bachmann Class 70 Heavy Freight Diesel Locomotive|
This page shows how a Legomanbiffo LokSound V4 decoder and speaker will be fitted in place of the original TCS decoder to add a sound system to Class 70 number 70006.
Class 70, 70006, viewed through the dirty window of a Northern Rail Class 156 at Carnforth in September 2010
|Scope of the project:
Speaker Location: My preferred speaker solution would be an ESU 50334 20x40mm or a good base reflex unit, mounted facing the track between the bogies, but I'm not sure if it can be done. Second choice is to use the space provided below the roof fans, with the speaker facing upwards. However, in this location, I would hope to seal the speaker around its rim to the underside of the roof around the fan vents. No local enclosure would be used, instead the locomotive body will provide isolation between the front and back of the speaker.
Big problem with option 2 above! I've just checked the available width in the upper body shell directly below the fans. It's around 18mm and would not accommodate a 20mm wide speaker! So I will have to very seriously see what might be possible below the motor between the bogies. I have bitter experience of using smaller speakers than a 20x40mm in past projects, with less than satisfactory audio performance..... until they were replaced by a 20x40! .......See how it was done below!
LokSound function output interfacing: The original TCS (non-sound) decoder had 6 full performance function outputs for lighting control. The LokSound V4 has 4 equivalent function outputs plus two low current "logic level" outputs that can be converted to full performance outputs by using ESU's adapter PCB, which contains two simple amplifiers for this purpose. It is proposed to use a pair of small surface mount N-channel MOSFET devices to perform a similar function to ESU's adapter board amplifiers, but in a much smaller space.
A 22mm long aperture can be cut in the now redundant switch panel on the loco underside between the bogies. To increase the available depth below the motor, the chokes and capacitors (not required for DCC operation) can also be removed. This makes enough room for a full size ESU 50334 speaker enclosure to be comfortably accommodated between the lower locomotive frames.
The original underside structure, with the bogies removed.
Top of the original switch mounting PCB, before removal.
The motor underside after the original suppressor chokes and capacitors have been removed.
(The 100k resistor simply maintains the motor contacts separation, in place of the original capacitor.)
Underside view of the new aperture.
Topside view of the new aperture
Topside view of the speaker enclosure in place
Underside view showing the ESU 50344 20x40mm speaker glued in place within the enclosure.
(The inside surface of the frame girder is cut away to clear the speaker cone.)
When the speaker glue had set, the ends of the cylinders that were originally mounted below the switch panel, were re-attached to the end of the speaker enclosure to improve the appearance between the frames, when seen from a 3/4 side view.
The speaker is not at all obvious when the loco is on the track.
|Circuit Diagram proposed for
the sound equipped locomotive:
Small N-Channel MOSFETs (2N7002) are used to convert the low current logic level Aux3 and Aux4 function outputs to have a normal switching capability.
The SOT23 device was glued upside-down to the PCB and thin wires soldered to the inverted gate & source legs.
This is the Aux3 interfacing MOSFET connected to the base resistor of the original PNP transistor enabling the rear lights.
Showing the Aux4 device adjacent to the 21 pin decoder socket.
Excellent, the MOSFETs work fine without the need for any additional hold-off gate resistors!
|Proposed function button
assignments & CV changes required:
|Active Button Sound Slot
Assignments (& Lighting Controls):
Most of the sound slots (at least all of those I've checked) are set at maximum volume, so to make any of them sound louder compared to the diesel engine sounds, I've had to reduce the volume of the engine sounds in sound slot 1.
Motor Control Settings
Smooth running is achieved across the entire speed range, but I'm sure many other combinations will achieve the same result!
Some Unique Class 70 Sound Effects:
Two of the sound effects on Biff's decoder are new to me......
1) Skipfire idle engine sounds..... The class 70 employs a novel fuel saving technique. When the locomotive is stationary and the engine is idling, a computerised system arranges for fuel to be injected in rotation to only a few cylinders at a time. This greatly reduces fuel consumption during idle, but keeps the engine turning and warm, ready to revert to normal operation quickly, when power needs to be applied. The sound is of a distinctive intermittent nature, with a lower frequency content than when the engine idles normally. When enabled, via button 17 in Biff's decoder, the engine switches to this mode immediately if in stationary idle state or automatically a few seconds after entering idle mode when the locomotive comes to a halt. When the locomotive is next moved, first the engine reverts to normal mode and after the revs build up, the loco gradually moves away.
2) High air mass cooling fans supporting the dynamic braking....... Another unusual Class 70 feature for a U.K. mainline freight diesel is the dynamic or rheostatic braking system. When the driver applies the brakes at medium or high speed, the axle mounted electric motors are disconnected from the normal driving electrical supply and re-connected to a large bank of fan cooled resistors. In this mode, the motors act as electrical generators, turned by the driving wheels, while feeding electrical current into the resistor bank. This action has a significant braking action on the driving wheels as while the wheels are turning, the motor current induces torque in the motor rotor assemblies that works against the direction of motion. However, to prevent the resistive load from overheating, powerful fans blast cooling air through the resistor bank as long as the motors continue to rotate during the braking period. This creates a distinctive "screaming air sound". Button 8 enables this feature in Biff's decoder. The fan sounds are only produced when the speed control is shut down to slow or stop the locomotive from medium to high speeds. Slow-speed braking results in conventional braking sounds only.
Both effects work very well in the Bachmann model and provide a lot of added interest to the driving experience. I've tried to illustrate the effects in the test track video below!
The finished Class 70 on the test track.
|Supplier website links:
Photos of the real class 70 were taken at Carnforth in September 2010. The photos of the model were taken hand held on the kitchen worktop.