Thanks guys, I really appreciate the input.
@fzabkar
I think the bottom note might explain why the 'equivalents's are not an exact match...
Note2) Electrical characteristics and pin arrangements differ among equivalent. The inductor... I had the same thought last night, what if it was not generating the required field during the off cycle... but when measured it's 150 uH, correct as per it's marking (151).
and I can't easily disconnect the 12v, but I can prove otherwise that there is no significant load on the converter.
For example, voltage drop is practically zero across a 5cm strip of narrow PCB track, and I can apply my own 12v to the load and there is very low current drawn.
@DaJMasta
No, D1 is fine, it measures good, it was just a price of flux.
better image below...
I agreed its odd that the converter is generating 50kHz, so it could be ok.
The other possibility is a leaky Transistor I guess, It tests ok ar DC but there might be a problem at 50kHz.
Im going to do some digging on the parametric tables over on Digikey, looking to find a similar device.
and I might just replace both the Convertor and Transistor if I can find equivents.
Cheers!