Thanks Trev,
This where it is at:
supplier sent new board & parts for cost of postage.
I replaced the board and ran it for 5 mins, no problems, with tank full of water.
When I fitted the lid with the fan.
The card failed again, no output.

The heat sink where extremely hot, lid on or off.

I then replaced the failed components & disconnected one of the transducers.
I have been running it for durations of 30 mins now, with no problems.

I also added pieces of additional heat sink to each switching device, using screws and transfer paste.

My conclusion,
This board will only power one transducer.
Don't put it in a small box, even if you have a fan.
Heat sinks get too hot causing switching devices, transistors, what ever they are & diodes D4 & D5, fuse to fail.


[Linked Image]
[Linked Image]

Happy is he who penetrates the mystery of things.