Example 1: Heljan O Gauge Class 73

Kit: MS990L; YLR302815; RE Slim Boom Box 50x15x15

Voted one of the top models of 2024, Heljan's O gauge Class 73 is covered in detail, and looks especially good in Pullman livery. It comes with an 'XL' socket for easy DCC sound conversion.

There is even a mounting for an ESU smoke generator, though we found this was not particularly practical because it is not close enough to the outlet in the roof!

The XL socket is great, except that ESU's XL standard is slightly different from the Zimo standard (the servo connections are different). The model doesn't have any servos though, so we can make it compatible by simply removing the header pins for servos. Lift off the plastic socket from the appropriate side and snip the unwanted pins flush. Then our Zimo MS990L should fit in more easily.

We decide to go with a pair of speakers, which fit snuggly inside the body... a YLR302815 2W traditional box, and a Rail Exclusive Slim Boom Box (50x15x15mm). This combination seems to work very well and gives lots of volume as well as a great frequency range.

A modified version of our Class 73 project gives control of the various lighting features, as well as syncrhonisation of the arc-sparking white LEDs in the bogies, which will briefly flash whenever the random arcing sound is played (only in motion of course).

A few more pictures below of the modifications to the MS990L decoder and positions of speakers with a different model...

Example 2: Another Heljan Class 73 with better speakers

Kit: MS990L; YLR453520; YLR302815

We were never completely convinced with the speaker choices previously - they are easy to fit, but the RE speaker in particular struggles to cope with the power from the big MS990, and this becomes even more evident with our latest 2025 Class 73 Premium sound project. So, time to revisit and fit something better...

Measuring up suggests that the big boomy YLR453520 3W traditional speaker can be squeezed in with the mounts are in the model for an ESU smoke generator. We'll need to grind those metal mounts off though, so some careful masking and protecting of the wiring in that area is required.

With the mounts gone, the big speaker fits in nicely, with just enough room either side for routing of the wires.

On the other side of the decoder socket, our smaller YLR302815 2W traditional speaker can be seated (on BlackTack) without further modification.

The MS990L has 2 separate audio outputs, so each speaker is connected to its' own output. Rather than mess about with any plugs, we identify the SPK1 and SPK2 pairs of pads and solder directly to those instead.

MS990L plugged in (usual pin chopping required for XL socket fitment, and this combination proves to work very well indeed.

Please note that these guides are provided as useful resources for you, as-is. YouChoos cannot be held responsible for errors in the information, or for any damage caused to your models or equipment if you choose to follow any of the steps detailed here.