I did repair a few of UC3844 a few years ago. Usually the 3844 is powered, on startup via cap/resistor network. This is only required for a few pulses until the transformer switches and one of the secondries supplies power to the chip. It sounds like what you are seeing is simply the startup switching, and not the proper running. A few things to try; 1 - it sounds like one of the outputs may be overloaded, so it cannot startup. To test, remove mains and apply the correct output voltage(+5? +12..) with a PSU after the output bridge and see if there is excessive current draw 2. - or the startup circuit may be faulty. To test, power the unit up and use a PSU with a diode- in-line to supply 12V directly to the 3844, and see if it runs smoothly If something goes bang, at least you'll know where to look (:-o Regards Roland ----- Original Message ----- From: "Alessandro Queri" To: Sent: Thursday, December 01, 2005 3:15 PM Subject: [ee]: UC3844 switching PS repair > 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 > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist