I received my Heljan class 20 with centre headcode the other week. Class 20s were some of, if not he very last locos to run in BR Green before being painted blue. 20141 was the very last one and was finally painted blue in May 1980, so it is just possible for me to have a green one with full yellow ends running in my modelling time period of 1980 to 85. I have a blue one already that I bough on Ebay a few years back that someone had done a very nice job of weathering and installing sound, I also have a green one from the first Heljan batch of locos. The plan had been to paint that with full yellow ends and run those as a pair. The new loco has spurred me on to finally do something. So here is how I installed sound in both green locos. The first thing I did was run one the locos on DC to make sure everything worked OK. I tested the current draw with a meter and found that at high speed the locos were each drawing 1.5 amps and close to an amp at low speed. The plan was use a Zimo MX645R decoder from Coastal DCC. These have current rating of 1.2 amps so not really enough for retaining both motors, so I decided to remove one of the motors. Starting with what will be 20141, 4 screws hold the body on 2 at the front and 2 in the middle, these came out easily and there is no difficulty accessing them. There is a centrally mounted PCB with for the very first time colour coded wiring in the normal DCC colours. This PCB is held in by 4 screws which are easily removed. Her's the Zimo decoder I now generally prefer Zimo decoders to ESU Loksound ones, as they are cheaper, they are easier to add a stay alive to and often have more functions. This one was £95 . This is an MX645R meaning it has the NEM 652 8 pin plug on it. It also has wires for the speaker in purple, a brown wire for F2 output and at the other end a blue wire and a grey one. The blue is for the positive of a stay alive capacitor and the grey the ground (negative) . I'm going to use a TCS KA2 but you could make your own and the decoder has the current control diode and resistor already built in. The KA2 has the current control diode and resistor but that is ok and it works just fine.. So to add a stay alive there is no need to solder wires to the decoder which is a really good thing. ESU should take note of that. I have now removed the front motor, all you need to do is loosen the screw next to the motor and just lift it out and then unscrew the motor wires from the PCB. Remember to tighten the screw back up. I then had a good look as to what space is available for speakers. I have these 2 differnt speakers available that I got from DC Kits (they sell the Leggoman biffo sounds but only on ESU decoders), they also sell the enormous speaker that went in the class 128 parcels DMU. That size speaker isn't going to fit here. after a bit of trial and error the longer thinner speaker is the easier to fit in the large slot in the middle of the chassis.