The Di3.623 has an interesting story. It was originally built for the Finnish railways but that order got cancelled. Then it became a demonstrator locomotive for NOHAB and it made a tour to Hungary and it was at the basis of the M61 order. Then it was sold to the NSB and became the 623.

During the 1980s the Di3.623 still had the old colors (gammel design), as this is one of my favorite eras I wanted a model of this particular model. There has been no production model of the 623 in these colors though.
A good starting point is the Heljan model as the 623 is a long Di3 (the 602, 622, 623 and 641-643 were longer than the other models). I was able to get a shell of the Heljan Di3.622 in the gammel design color. This model was made by Heljan on behalf of NMJ in the early 2000's.

This page will describe making a model of the Di3.623 from this 622 shell.

There are a few key differences between the 622 and the 623, especially in the mid-1980s. The key difference is that the stripe is not continuous. The side door doesn't have this stripe anymore. This meant that this had to be removed. As I documented in other renumbering projects I used Model Master Airbrush thinner to remove the old number and stripe,

After the number is being removed, it is time for cleaning the shell and then applying the new numbers. The decals are coming from Romerike Modellservice. One of the issues of removing the original number is that the top coat is being removed. So after the decals are being applied a new top coast using Tamyia TS-80. This gives a very nice flat and even finish.

The Heljan models are known for having poor wheel conductivity and therefore a power pack is a must in these locomotives. For this install I've used the Train-O-Matic SPP. The power pack is installed in the diesel tank under the frame. A small whole is drilled in the frame for the wires. To do this the motor has to be taken out.
The original PCB is not going to be used, a Nixtrainz decoder buddy is used instead. This board is much smaller than the original PCB. This board has a small connector for all the light connections. Finally an iPhone 4S speaker is installed.

6 LEDs are used for lights. The top-light uses a lens from Scale Sound Systems. Cab lights are made by glueing a small LED to the back of the interior cab insert. Then all the wires are hooked up to the decoder buddy connector. This allows for removing the shell in the future. All the various outputs are clearly marked on the connector board which makes for an easy hookup.

The end-result. A NSB Di3.623 in gammel design, as it ran from 1985 to 1989.