Jim Yanik wrote in message
4...
"N_Cook" wrote in
:

One tool I don't have in my toolkit - a sniffometer. Nothing smelled
while in use or close handling of this power supply. But on removing
the electros , that distinctive fishy smell of leaked elecrolyte, with
soldering iron temperature on the pins.
Tiny space available to fit some capacitance so they used 2x squat
format 22uF,35V in parallel. One leg pulled out of one cap on
desoldering and the other a telltale drop of brown gloop immediately
under the cap but not enough to be visible over the pcb.
Both bright green nichicon 105 deg C. Found space on the hot side for
replacement and powered up long enough at about 3KHz instead of 40KHz
to give full V on all o/p ( unloaded) with very audible 3KHz coming
off the transformer. As , presumably, the pair of switching power fets
were operating at least partially linearly at an undesigned 3 KHz they
heated up quite quickly. 15 V over that sensing zener . Removed the
27nF and now operating correctly with substantial test loading on all
rails.

at TEK,I used to replace a lot of the green nichicon 100uF/25v
electrolytics in the 1700 series waveform monitors and vectorscopes.ESR
problems,probably aggravated by the long operating hours in hot racks at
TV
stations.



One mystery remains , the original tick-tick was about 1 a second, on
adding the 27nF to the 555 timing capacitor the tick-tick rate
increased to about 10 a second. I've not traced out the path from the
"goodness of oscillation" circuit to the 555 but what sort of
intermediary cct would lead to an increase in that tick-tick rate ?




I think you answered your own question;you ADDED capacitance to the 555
timing capacitor.

what sort of SMPS uses a 555 for a controller?

--
Jim Yanik
jyanik
at
localnet
dot com

Adding C usually decreases the f

UC3842 PWM controller , instead of simple RtCt pin , it uses the o/p of a
555 so that there can be diode steering for different timings