Hello everybody,

 http://www.fairchildsemi.com/ds/UC/UC3844.pdf


 I 'm trying to repair myself a switching Power Supply made by ZyXEL. The PS 
suffers from 5V line oscillating (actually the PS starts, shuts down, 
starts...). I checked against the output filters and CAPS and they seem ok, 
so I took the Datasheet for the UC3844 and went decoupling the AC line with 
a 2:1 transformer. As many of you know, here in Italy the line is 220, so 
I'd like to decouple and lower line to get 110 and save my oscilloscope ;-) 
. First the IC doesn't start even if the PS is rated 100-240VAC, so I had to 
ramp up the AC line to 184 (strange, isn't it?). The supply on the IC 
(capacitive reactance supply) seems to be a bit noisy floating from 14 to 
16.5. On the other hand, when externally supplied, the IC starts over 16V 
and stops running below 9, so that's ok. Now I read from the datasheet: 
typical osc freq 52 KHz but I cannot understand where to test it: on pin 4 I 
have 92 Khz while on pin 6 I have 46 Khz and a really UGLY waveform (seems 
chopped, as being amplitude modulated, not as in fig. 5). Is the former FAR 
away from the working point or the latter a little below? Has any of you 
clues about usually suspected components in faulty PSs? Many thanks in 
advance for your help.

 Alessandro Queri

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