Neumann N61 power supply - blows fuses...

Hi All,
For many years my CMV563 has been solid and untroubled.
It recently stopped, due to the power supply blew it's 100mA fuse.
I made visual check and reseated the valve... popped in a new fuse and it stayed on.
With mic connected it stayed on for some days. Once the output was connected to a preamp, the fuse blew almost immediately.
Tried again with the same result.
Can anyone point me in the direction where/what to test?
Feels like my first day at big school as I'm a super newbie, so THANK YOU in advance for any help you can give.
Cheers, Bear

If the fuse always blows when you connect microphone power supply to a preamplifier, most probably the cause is a current leak from somewhere inside the power supply to the ground of a microphone preamplifier. The power transformer can be faulty, also selenium rectifier etc.
If you have a basic knowledge of electronics parts and you have a DMM at hand, you can open the power supply and make a basic continuity tests when the power supply is off.  Then you can switch on the power supply alone and check all test points voltages.
The schematic is here:
https://www.elektroakustika.cz/n61.html
Be very careful when you are testing high voltages.
 

I had a similar problem with an NG psu for a u47. It was the power transformer.  But yes, check the rectifier and caps too. 
Be very careful, and measure to check that the caps have fully discharged their voltage before touching anything in there.  Some very high voltages can be present even when unplugged! 
Also, after running the PSU with no load (mic), turn it off and wait 5 mins for the voltage to bleed out before connecting your cmv.  (Or manually drain the voltage with a bleed resistor ...)