If you have the service manual then it's a question of following the diagram. Most likely a shortened diode or Mosfet.  Keep in mind that the psu is controlled by the uP. But first you'll need the 5V. So search the cause of the shortcircuit.

Hi beobuddy

Thanks for your reply, I am pleased to say that I have at last tracked down a dry jiont in the P34 TR10 area so now I have my stable 5V supply. And. A STBY light ! I am going through the diagram bit buy bit. My cause of the Mains relay fuse blowing (100mA)  is somwhere in PCB P9  as soon as I connect it to motor controller PCB 35 then it's lights out. I will keep you posted as you were kind enough to respond....Fingers crossed  BTW what's uP?

regards Maurics

You are one very cleaver chappie!  that's exactly what it was, also there was a dry joint on the 5v rail to supply the fuse on the relay PCB it's now up and running  so now I have two of them!!!! BS 9000 That is  thanks very much for the help guys so very much apprechiated... Maurice

Well done! Congratulations 😁

Nicely done!

Did you check any diodes around the Mosfet? Apart from rectifiying they protect the Mosfets. But then again, if they fail it will end up as a shortcircuit.

Maurice wrote:

 so now I have two of them!!!!

You can never have enough of these icons.

Congrats on saving one more.🤒

Hi All

Hoping for some hints here :)

I also have a totally dead MK1, and I have a few questions.

1. Can I do fault finding with the PCB3 completely out of the unit. Only supplying the 12V to the board, and no other connections ?

2. So far, I have tested it outside the unit. And I have 12V on all LM3578 pin 8's, but they all also supply 12v back on pin 7. Is this normal ?

4. I cannot find 5v anywhere at all, is seems the 5V supply is totally dead.

5. I dont know much about the LM3578, how are they supposed to work, do they need any oscillation input or something I should look at on a scope ?

5. I have the service manual for the MK1, but it is missing the circuit diagram for power supply. There is a block diagram, but it is not of much help. Anyone have the missing diagram ?

6. It states in the service manual, that if 100/120hz is missing, the product will not operate. I would assume that somewhere, even with the board out, there must be 5v to power the std.by / CPU and so on ?

Any helpful tips on where to start is much appreciated. :)

/Weebyx

You should feed the board with 11-20V AC and not DC. You won't measure all the voltages all the time. The processor controls each line.