When testing an SMPS like this how should you measure output correctly?
Simply meter to + and -? or do you need a load across the output?
Thought of another question for the Guru's of HV. Is there a reason these designs are on small separate PCB's? Is there an issue with combining an SMPS with another circuit on the same board
You can measure the output without load to test it's working, but be aware that it will drop significantly under load.
Read enough info on SMPS design and layout and it will totally put you off building one on it's own, let alone on the same board as the circuit to be powered! ie issues surrounding ground planes, using 'just the right parts' etc. My approach is more ghetto, if it works it works and f##k the science. Here's a 555 SMPS on the same board as a submini tube amp:
Small point - a 555-based SMPS will always be very inefficient - typically 70% at very best, more typically around the 55% mark in my experience.
Also, the comment about sagging under load is simply wrong, so long as you stay within the design parameters of the supply and your design is regulated - your 555 SMPS may well not be regulated - there are many regulated 555 designs out there (it only takes a few extra components - see the files section of the Yahoo neonixie-l group for several examples). The MAX1771 design IS regulated and delivers up to 100mA quite happily at 87% or so and 180VDC. I tested the loading by using non-inductive and then wirewound Al load resistors - its all documented on the original design page.
The MAX1771 design has been built on the same boards as the rest of the circuitry by many users - you just have to be a bit more careful. If you are happy with an unregulated inefficient design, that's fine, but it really is not that difficult to build an MAX 1771 design - I've built loads of different SMPS designs, and if you obey the simple rules, its not a problem.
It all comes down to what you want. If you want an efficient, quiet, stable, predicable & small design then use a specialised SMPS chip (like the MAX1771). If you just want to get something working from bits you might have lying around, then use a 555 design - the two are apples & pears. But don't kid yourself the interference & other general issues about using SMPSs in audio kit will magically go away if you use a 555 - all the the problems will still be there.