I think I am on a good way. ZD202 showed about 5V (that's good so far) but ZD203 did not show 39V and broke during measurement. This looks to me that this diode already has been a problem before. During the measurement (before breaking the diode) the N50 booted up. So that looks to me to a problem at ZD203. Now I ordered some replacement parts and keep you updated. Thank you very muchg so far 
BTW: in post #14 there is a mistake. in the last sentence ZD210 and Z203 is mentioned. This is not correct. We are talking about ZD201 and ZD203. There are only zehner diodes located.
BTW: in post #14 there is a mistake. in the last sentence ZD210 and Z203 is mentioned. This is not correct. We are talking about ZD201 and ZD203. There are only zehner diodes located.
The parts arrived and I changed ZD201 and ZD203 due to the fact that ZD202 has around 5V (seems to be ok). If I'm checking the voltage on ZD202 and ZD203 (should be around 39V) there is nothing to measure. It seems to me that the transformers are not fired with some inputs. Have to try more but seems to be a hard fix and not really fast as I have assumed :-(
Yes, it is strange. Now I'm cooling the IC FM260N with a piece of ice. After that I will put the hole power board back in the fridge, to see if the problem is really on this board. At the moment I'm focussing at the power switching IC. at least as this area was heated again after the fridge the problem occured again. So let's see What I also did: soldered all the parts on the power board again. Did not help so far.
What I also did: soldered all the parts on the power board again. Did not help so far.So, just to let you know if someone else has the same problem. The unit powers on with the following reproducible procedure:
I put the complete device into the fridge and cooled it to about 8°. After that the unit started normal in about 20 seconds. Waiting for some time or heating the STBY board with with a hairdyer the unit failed again. I thought it could be the power switching IC FM260N. I cooled it down and the unit came up again. But I think this was by accident because I did some more tests. I put only the STBY board into the fridge and after that the unit didn't start as at normal temperature. Then I decided to put the complete device into the fridge again and as before as the device was cooled down it started normally. So the conclusion is: it is not the STBY board that causes the problem. Seems to be the main board with the processor on. There are so many ICs that could have a problem so it doesn't make sense to go on investigating. Bad luck. My only workoraound would be to cool down the device, bring it to the location where I want to use it, power on and do not power off via main switch. Operation with standby via remote control is possible after the procedure of cooling. If anybody has a hint for me very appreciated.
I put the complete device into the fridge and cooled it to about 8°. After that the unit started normal in about 20 seconds. Waiting for some time or heating the STBY board with with a hairdyer the unit failed again. I thought it could be the power switching IC FM260N. I cooled it down and the unit came up again. But I think this was by accident because I did some more tests. I put only the STBY board into the fridge and after that the unit didn't start as at normal temperature. Then I decided to put the complete device into the fridge again and as before as the device was cooled down it started normally. So the conclusion is: it is not the STBY board that causes the problem. Seems to be the main board with the processor on. There are so many ICs that could have a problem so it doesn't make sense to go on investigating. Bad luck. My only workoraound would be to cool down the device, bring it to the location where I want to use it, power on and do not power off via main switch. Operation with standby via remote control is possible after the procedure of cooling. If anybody has a hint for me very appreciated.
I could not believe in another board so I gave it another try. Focused again on Fm260n. As the unit was on room temperature it didn't come up.
Cooling the Fm260n with a screw (over night in the fridge/see attachment) the device booted normally. So I really think that it has to do with the power switching IC. As it is really difficult to find Fm260n I was looking for a replacement part and found via a Portuguese forum (https://eletronica2002.forumeiros.com/t178443-resolvidoci-fm260n-alguem-sabe-o-substituto) that Fm260n = dh321. So I bought 5 pieces on Ebay für about 3,50€. So keep the fingers crossed. I will update here
Cooling the Fm260n with a screw (over night in the fridge/see attachment) the device booted normally. So I really think that it has to do with the power switching IC. As it is really difficult to find Fm260n I was looking for a replacement part and found via a Portuguese forum (https://eletronica2002.forumeiros.com/t178443-resolvidoci-fm260n-alguem-sabe-o-substituto) that Fm260n = dh321. So I bought 5 pieces on Ebay für about 3,50€. So keep the fingers crossed. I will update here
Attachments
Thank you. I just replaced the IC with a socket to be prepared for next week when the replacement part arrives. Inserted the Fm260n in the socket and gave it a try at room temperature. Unit did not start. Tried to cool it down with the screw for about 5 minutes and the device came up. So looks good so far