Decoder Fitting for OO Scale Models

13.1 Autumn 2012:9_4Summer2009 21/08/2012 14:55 Page 8
Decoder Fitting for OO Scale Models
The Bachmann Class 04 shunter is based on a chassis which was originally designed by Mainline Railways for its ground-breaking
model of the BR Class 03 shunter. It has a split frame chassis with current collection through split axles and the chassis frame
to the motor which is directly connected to each half of the split chassis. It makes it an interesting, if challenging, model to
convert to DCC.
Part 2: DCC for Split Frame
Chassis Locomotives
By Nigel Burkin
S
imple decoder installation was not, in all
likelihood a priority for the designers of
the split frame chassis concept. The benefits
which were considered to be more of a
priority included simplified electrical paths
from track to motor, easier manufacturing
methods together with the elimination of
what were considered to be troublesome
wiper pick-ups. It is easy to understand why
the split frame chassis design became
popular when taking one to pieces. They
consist of two diecast sections which are a
mirror image of each other where each
makes up one half of the chassis. Insulation
sleeves and bushes together with the use of
split axles for the driven wheel sets ensures
that each side of the chassis is electrically
isolated from the other.
In effect, the design ensures there is an
electrical path to the motor without the
need for circuit boards and wires. Split
chassis design simplified things for the
manufacturer with no need for complicated
electrical pick-up devices, complex circuitry
and motor connections. However, the
advent of DCC soon put an end to this
design in OO gauge.
Electrical current from the track is collected
via the wheels and direct contact between
the axle and the bearing surface (steam
locomotives and fixed frame diesel
shunters) which is usually part of the split
chassis frame shell. The motor and gears are
fitted between the diecast chassis sections
with a direct electrical contact between the
chassis and motor contacts. Both sides of
the split frame chassis are live, with the
insulation sleeves and bushes fitted between
the halves of the chassis preventing a
short circuit.
There were a large number of 4mm scale
ready-to-run models of various types
produced with split frame chassis of one
sort or another, from small shunters to
larger tender engines, many of which are
very robust and reliable designs and likely to
be still in service on layouts today. Primarily,
they are found in Bachmann models
originating from the Mainline Railways range
and includes our subject models: the Class
04 shunter and J72 tank engine.
The first significant issue for modellers to
consider is the lack of space for a decoder
in split frame engines. The opportunity
to pack as much ballast into the model
as possible was taken by the manufacturer
to meet the demand for good
haulage capability.
The second point that DCC-users
encounter is fundamental to the function of
a decoder. They receive and interpret
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packets of data sent from the command
station and ultimately the controller the
modeller is using to operate the layout.
Those packets of data are transmitted as
part of the track supply current. Decoders,
with their unique addresses, await their 'call
sign' and when that is received, they will
respond to any instruction that is
transmitted to it. Decoders also utilise the
constant supply of power from DCC
systems and rectify this so an electric motor
can perform normally. Consequently,
the decoder has to sit between the track
current supply and the motor with no
direct connection between the track and
the motor at all. Isolating the motor
from the diecast chassis is critical to a
successful installation. There must be no
electrical contact between the motor and
the chassis frame, bypassing the decoder or
it will be destroyed!
The following projects demonstrate how
to fit decoders to split frame chassis. The
technique is basically the same no matter
the model involved. When the conversion is
complete, the model should be tested on a
service track before being exposed to full
track power. All NMRA compliant decoders
are programmed with default address 0003
when produced at the factory. After the
model has been tested for a satisfactory
decoder installation, a new address can
be allocated.
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For each of these projects, the
following tools and materials were
used:
• Electrical insulation tape
• Bachmann 6-pin adapter (36-562)
• ‘Kapton' tape
• Jeweller's screwdrivers
• A selection of 0.6mm electrical cable
• Soldering iron and electrical solder
• Needle files
• Tweezers
• Scissors and wire strippers
• Hacksaw
• Bench vice
• Bachmann 6-pin decoder
• Power drill and 4mm diameter drill
suitable for metal
• Wet and dry abrasive paper.
A small tank engine, the J72, also presents
the same space constraints as the Class 04
shunter. In fact, the primary concern in
virtually all split frame chassis engines is the
lack of space, even in larger locomotives.
Points to Consider:
When undertaking this type of conversion,
the choice of decoder can be determined by
the available space as well as one of decoder
performance. Most modern decoders work
well with the majority of ready to run
models, so performance is rarely a serious
consideration. The Bachmann 6-pin decoder
(36-558) fitted to the J72 model with the
help of a 6-pin adapter (36-562) has a
sufficiently high current rating to cope with
the motors fitted to smaller split frame
models. A 6-pin dummy plug was fitted first
for testing of the converted model on
analogue power before the decoder was
plugged in.
Isolating the motor from the chassis frames
by cutting away the metal that makes
contact with the motor terminals is
considered to be best practice. However,
the design of some locomotives makes it
difficult to do this without weakening the
chassis or removing a screw mounting hole.
In those cases, it is necessary to rely on
insulation tape for isolation. Use high quality
tape specially designed for high performance
situations such as this and 'Kapton' tape will
do the job admirably.
With so much metal making up the internals
of these models, insulation tape is essential
to protect the decoder from direct contact
with the chassis. Insulation tape, be it old
fashioned electrical tape, or where space is
tight, specialist tape such as 'Kapton' tape,
should be applied to metal surfaces where a
decoder will be fitted or could come into
contact with it.
Before commencing the actual conversion,
the model has to be dismantled into its
component parts, something which can be
quite daunting. However, there's no cutting
up chassis blocks until everything is
removed and safely put aside. Do not be
put off by having to take things apart.
The construction of split frame chassis
models is relatively straightforward; only
those locomotives with complex valve gear
may present a challenge.
When deciding where to locate a decoder,
choose a spot where the least amount of
metal has to be removed. This is important
in several respects: to make the job easier
and to retain as much ballast weight as
possible so impact on locomotive
performance is not affected. Remember,
there has to be some space around the
decoder to allow air flow over it. However,
the large metal chassis block will ensure that
any heat from the decoder is dissipated.
An easy way of checking that a motor is
completely isolated from the chassis frames
in a converted model is to apply test leads
to either side of the reassembled chassis
and see if the motor rotates. A connection
to each of the new motor leads alternately
with the chassis block on the opposite side
will soon reveal if the connection between
motor and chassis is broken, as required for
this type of conversion.
Project 1: Bachmann OO Gauge
Class 04 Shunter:
In common with all split frame chassis
models, the body of the Class 04 conceals a
lump of metal designed to give the model
maximum haulage capacity. The chassis
completely fills the interior of the model
except the cab space, with no room for a
typical decoder - this being of no
importance when this chassis type was first
designed. It's not desirable to put the
decoder in the cab because both it and the
harness wires will be visible under normal
viewing conditions.
To fit a decoder to this model, some cutting
of metal from the top of the chassis frame is
necessary. However, once 3mm or so of
metal has been removed, a Bachmann
HO/OO gauge decoder such as the 1 Amp,
two function type (36-552) can be easily
accommodated. The motor must be isolated
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from the chassis frames with 'Kapton' tape
and the orange and grey harness leads
soldered directly to the motor terminals
1
Two screws located at the extreme ends
of the chassis hold the chassis and
body together.
2
Two more screws to note: the pair which
secures the bottom plate to the chassis,
retaining the wheel sets. You cannot take
the chassis to pieces until the wheel sets
have been removed.
3
The logical place for the decoder is on top
of the chassis itself to avoid cluttering the
cab with harness wires. About 3mm of
metal must be filed away to create a
recess for the decoder.
4
This view shows the corresponding area
of the bonnet that will be occupied by the
decoder. It is possible to carve some
plastic from the inside of the bonnet just
to ease things by a millimetre or so
without risk of damaging external detail.
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Finding the right decoder for a model is as
much to do with the available space as it is to
do with decoder performance. Choose the
largest decoder possible if space can be made
inside the model to accommodate it.
It is time to completely dismantle the model to
modify the chassis block and break the
electrical connections between the chassis and
the motor. This is an important part of this
type of DCC conversion because the motor
terminals are in direct with each half of the
chassis block. The way a split frame chassis
works is now more apparent with the
insulation gap between the frames (A);
insulation bushes (B), insulated securing screws
(C); electrical contact between split axles and
the chassis frame (D) and insulating bushes on
each wheel set (E).
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Release the three screws holding the
chassis together to reach the motor and
gears. Note the connections between
motor and chassis block (A); insulating
bushes (B) and insulation sleeve fitted to
the motor (C). Do not loose the
insulating bushes!
The split frame chassis block was
completely stripped of all components. It
was then clamped in a bench vice to allow
a section to be cut away from the top with
a junior hack saw. Approximately 3mm is
removed and filed smooth to
accommodate the decoder. Clean away all
filings before refitting the components.
The motor and it insulating sleeve just
pops out of the chassis. Insulation tape
was applied to exposed metal surfaces
that would make contact with the motor
terminals on the inside of the chassis
because cutting the chassis at this point
would weaken it. Insulation tape is also
applied to the motor terminals too, just to
be sure that there is no chance of
electrical contact being made. Wires are
soldered to the terminals to supply
current directly from decoder to motor.
Similar in design, the J72 model can be
tackled in more or less the same way except
for a few detail changes. The narrow boiler
and smoke box points to the use of the
Bachmann 6-pin decoder (36-558) and an
adapter. The adapter makes conversion back
to analogue control using a spare dummy
plug simple to do, but this is not essential.
The motor terminals were not isolated from
the frames using 'Kapton' tape in this
instance, but by completely drilling metal
away. Furthermore, part of one chassis
frame half was removed to make room for
the decoder without the decoder or
harness wires showing in the gap between
the locomotive frames and the boiler. A lot
of drill filings result from this conversion, so
care is taken to clean them all away before
reassembly of the chassis.
1
7
9
Project 2 - Bachmann OO Gauge J72
Tank Locomotive:
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There's no more room for a decoder in
the Bachmann J72 of this vintage than
in the Class 04 or many other
models with split frame chassis. However,
the conversion takes around three
hours to complete, which is not bad
considering the amount of chassis
modification required.
2
The installation is basically complete. The
red and black harness wires (A) were fed
through to the base of the chassis to be
held in place by the bottom plate thus
forming a positive connection with the
split frame chassis block, one cable making
contact with either side. Exposed metal of
the chassis block has been covered in
insulation tape to protect the decoder
from accidental contact (B). The decoder
has been connected to the two cables
leading to the motor terminals (C) thus
completing the project. The model was
reassembled and the body replaced.
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This picture offers some idea of how little
space actually exists in one of these
models. However, the fit was not so close
as to leave about 1.5mm all round the
inside of the water tanks. This was a real
bonus for running harness wires from the
decoder to the motor terminals without
pinching them between the chassis
and body.
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3: Two screws on the underside retain the
wheel keeper plate (A) whilst one screw
at the front secures chassis and body
together (B) with the help of clips at the
rear (C).
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The best pace for the decoder is at the
front of the model where a chunk of
metal can be cut from one half of the
chassis without difficulty.
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6
5
Fully dismantle the model and remove all
parts to a safe place before cutting and
drilling metal from the chassis blocks. The
J72 chassis picks up current in the same
manner as other split frame chassis
designs with direct contact between the
frames (A) and wheels (B) with no wiper
pick-ups; insulating bushes on the wheel
sets (C) and insulating bushes between
the chassis blocks (D). The securing
screws (E) offer useful connection points
for the decoder's red and black leads.
The motor is fixed within the frames.
The section removed to take a decoder
(A) is at the front of the locomotive. The
motor mount (B) is to the rear with
sprung motor contacts on each side of
the chassis (C). When modifying the
chassis, the fixing screw locations (D)
were avoided.
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8
The motor to chassis contacts are simply
removed by drilling the metal away with a
4mm diameter drill. It might look crude,
but it is effective!
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Assembly is complete, with the model
ready for testing on analogue power,
hence the use of the dummy plug fitted to
the 6-pin adapter. In effect, the model is
now DCC-ready!
A Bachmann 6-pin decoder (36-558) (A)
is installed to an adapter (E) with the gap
between the frames and boiler left clear
of wires or insulation tape. The sleeve
fitted to this decoder can be trimmed to
ensure it fits the available space. Kapton
tape is used in large quantities to protect
everything (B and F) including the motor
frame (D) from accidental contact with
the metal chassis frames. Grey (C) and
orange (H) wires lead to the motor
terminal wires whilst the red (G) and
black ones are attached to each side of
the chassis using the securing screws as
convenient connections.
Converting Other Models:
This picture shows how the motor
contacts are now clear of the chassis
frames. The metal tags were eventually
removed too because there was chance of
contact with the inside of the hole.
The chassis frame modifications are
completed and leads soldered to the motor
terminals which means it is time to fit the 6pin adapter and decoder. This is the time to
test the motor to check that power only
reaches it via the leads and not from one or
other of the chassis frames.
*The current catalogue B1 models have an 8PIN decoder socket.
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The conversion principles described in this
article apply to almost any split frame
chassis model which works on 12v DC
power and almost anything from the Palitoy
and Bachmann family. Once the thought of
cutting up diecast chassis frame shells has
been reconciled, there is no reason why any
ready could not tackle such a conversion.
The chassis frames are tough, so it is unlikely
that they will become damaged in a bench
vice unless it is over tightened. Remember
to clean filings away, isolate the motor, fit a
sharp new blade to the hack saw and split
frame chassis conversions are no longer a
serious challenge!
Apply the techniques described in this
article to any of the Bachmann split frame
chassis models including large valve gear
fitted locomotives such as the Thompson
B1*. In fact, the tender might offer an option
for accommodating a decoder without
having to modify the chassis too much
(other than isolating the motor from the
chassis frames) as long as connecting wires
between the engine and tender are not
going to present a problem.