Hello to everyone here.
Although we didn't have the right schedule at hand (in the meantime, Martin from Neumann has send me one), we think we managed to get the PSU to work as it should.
We first measured all the stages in the circuit and found out that the transformer put out a hefty 227V. With a load in place, the polarisation voltage was a staggering 299V! Firstly we tried to attach the 240V tap of the transformer (it is an older model, so 220V only on the circuit board, but there actually is a 240V tap located on the transformer. This brought down the voltage to about 280V, but the filament went down to about 4,8V and we were unable to bring that back up to the required 6,15V. Eventually we went back to the 220V tap and replaced the diode bridge. We subsequently augmented the dropping resistors (R1 and R2 on the newer schematic, but R2 and R3 on our circuit board) to 18 Kohm and 10 Kohm respectively. We also saw that there was no 160 Kohm resistor around R7 (R8 on our circuit) and put a 100 Kohm in there (as per an other, slightly different schematic we found).
In the end, we managed to get a polarisation voltage of 209Vdc. We also discovered the variable resistor on the filament rail was 'stuck' at 26 ohm and there we strapped a fixed value in parallel (actually 3 x 8 ohm resistors) untill we reached 6,18V under load. The microphone has been switched on and in use since three days and it performs flawlessly.
In the meantime I received a schematic for the NU67 with the 03 suffix on the circuit board. It seems no one had tampered with the PSU as I suspected at first but surely the transformer must have been way out of spec. Anyway, it now works as it should. Many thanks for your detailed input Klaus, Kai, JJ Blair and Uwe!
I'll try to attach the early schematic here, it's interesting, with the extra small filter caps etc. compared to the later ones. (I hope I don't break Neumann's forum rules here). Hope it works...
Greetings from Belgium,
Koen.