Installing a DCC decoder in a Heljan Class 128 Diesel Loco

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Class 128

Before installing chip into any loco it is advisable to check the current being drawn by the loco. You should test the current drawn when the loco is stalled to find the peak current and select a decoder that will cope with that maximum current requirement.

A number of purchasers of this model have reported issues with parts falling off whilst other have received models intact. This particular model was in the former category with three loose parts floating around inside the packaging when the box was opened. Fortunately a decision was made not to glue these parts back in place until after the model had been re-assembled following the installation - fortunate because three other components parted company from the assembly during the installation without any encouragement!

There are a number of very fine and fragile detailing parts on the underframe and at each end so care is required when handling the model.

The loco comes DCC ready with a 21-pin socket so a TCS EU621 decoder was used for this installation. You will need a small flat-headed screwdriver, preferably one smaller than that shown in the photograph here.


Class 128

The loco comes with a leaflet which briefly describes how to remove the body and install a decoder. In theory straightforward, but in practice a little more challenging.

The body is secured to the chassis with four screws. They are easily located and fairly straightforward to remove, though being very long one or two were difficult to coax out once undone.

The body has a VERY tight fit to the solid chassis so some patience is required to separate the two. Be especially careful where the bottoms of the exhaust pipes sit just below a very slight lip in the chassis. Running finger nails along each side and pulling gently outwards should loosen the body and it is then a case of pulling with some force to completely separate it from the chassis.

Remove dummy plug 21-pin connections


Once inside the location of the dummy plug is obvious. Gently lever out the dummy plug from the socket being careful not to bend any of the pins sticking up from the circuit board.

There are no pin markings so it is necessary to identify where the missing pin (out of the 22) is located.


Location of decoder Location of lamp holder

Make sure the 21-pin decoder is the right way up and place it on to the 2 rows of pins. Push down firmly making sure you do not bend the pins in the process!

Program the decoder as required BEFORE replacing the cab and check that everything works as expected. Using DC control cab lights are on permanently in both directions but with the decoder installed cab lights under DCC control are activated with F1 and F2. Directional marker lights are activated as usual with F0.

Do make sure that the connectors on the ends of the main circuit board are firmly pressed in - I have found some which are loose and one totally disconnected. Also make sure that the fine wires have not been trapped by the fixing screws, another fault I have found on one model which rendered the cab lighting inoperative.

When everything has been tested replace the body. This is the most difficult part of the installation, it being difficult to coax the body back into position. Make sure the cab lighting units are correctly seated before commencing re-assembly. In addition you need to be careful with the exhaust pipes again and they might need gently levering over uneven chassis parts. Also be careful of the two metal lamp holders which are located just above the buffers. These can be easily bent out of position and can hamper getting the body back into its correct position.


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Copyright Bromsgrove Models 2006