(..)
Measuring again the UC3842D with the DMM , voltage oscillates between 10 and 16v , no load attached to the output of the power supply , i also made a video with the PSU start and load behaviour.
UC chips are tricky
They were designed to induce the transformer in CCM mode on the secondary
In other words by sensing the primary current UC should be able to
drive an "average" CCM current on the secondary side
But for that to happen some steps should occur first - like soft starting
the primary inductor to leverage the secondary amperage enough
on the sampling FB loop by the "transcondutance" error amp done via TL431
that TL431 is very critical in the UC FB loop because it will induce
the proper CCM mode - or not
So it may help to put some light load at start just in case you have
a wild deviated or even "forged" TL431 chip - which does happen
a LIGHT RESISTIVE LOAD will definitely helps to put stable loop
there are a plethora of forged chips out there which just go wacky
for no reason
the reason to load the regulator is fairly simple as to force a higher
current on the whole FB loop - mostly on the transcondutance error
amp which will induce a higher error current ... if not.. it is crappy
the fb error current should vary with the load... a nice way to check
the loop is to measure the error amp current with the load variation
Some forged chips can be spotted some dont
Paul