Loksound sound decoder failing

[ColinB]

Ok, I know it is not Hornby but I know a lot of members have these. A friend asked me to look at his loco because everytime he put it on the tracks the controller tripped out with a short circuit. The loco has a Loksound decoder in it with the sound option. After taking the loco apart and using my decoder tester I found that the decoder had gone short circuit, in fact I narrowed it down to two of the bridge rectifier diodes going short circuit. Now obviously it is dead, but the thing that surprised me, on any other decoder that goes short circuit there is a burning smell and the diodes fry. Definitely with TTS decoders that is what happens. With this decoder there is no obvious damage. The friend said it just stop working and put a short across the track. It was in a Bachmann Jubilee tender, so no concerns with cooling. So my question is, is this a common failure mode with these? I have since replaced it with a Zimo 21 pin decoder and it is working happily, so no wiring issues on the loco. So why did it die?