I received an UC3843 based SPMS (driving an audio amplifier) for repair, and it turned out slightly more complicated than anticipated, so would be happy about some help and inspiration...
According to the user, the amp kept blowing fuses as soon as it was switched on. The PCB looked fine without any visual damage of the components. I started checking components and found out that R521 (gate resistor between UC3843 and MOSFET) was open, also U506 and Q513 were gone, replaced all of these. I also noticed that R509 had a tiny hole, so i also replaced R508 and R509 (current sensing).
There is one inconsistency in the (manufacturer supplied) circuit diagram, R508 in situ was actually 1K2 (diagram 1K) and R509 was 10? (diagram .1?). The soldering of both looked quite original, not sure if the 10? was a design change to handle current spikes?
For a first test i put a 1K load on the +22VDC output and (bulb in line) powered up the unit. The SPMS did not fully power up as it apparently was stuck in the UC3843 startup. I then removed the 1K load and tried again and interestingly now it seemed to power up properly. I then applied the 1K load while it was powered up and it still seems to run fine (and it supplied all external voltages), so i assumed it is just a startup problem.
I checked the UC3843 VCC supply and replaced C510 (support cap) as well as C512 (VR support cap for Q515). That did not change anything, unfortunately. R518/R519 for the initial (boot up) voltage support to UC3843 were also ok, and i could measure 165V before R519 but only 8V at VCC with the external load. Without an external load, the startup seems to work as the initial voltage support came up to 11V.
The primary voltage suppy measured after C517/C518 delivers a clean and ripple free 165V DC, so this seems to be fine.
When i lower R519 (limiting resistor between 165V supply and UC3843/11V Zener) from 47K to 22K, the startup seems to work as expected with the unit powering on and supplying secondary voltages as expected. I can also see that the UC3843 now takes up 1-2mA as required for the initial power up sequence.
Then there is the secondary voltage supply from the transformer winding 1-8 to take over after the startup has happened. With a 1K load in place, i just measure 8V after D532 (rectifier between transformers 22V AC and UC3843) and before R521 (limiting resistor before 11V Zener), without the load it's 16V should be enough to supply 11V after the Zener and run properly.
I then removed the transformer and measured the windings with my LCR set at 1 KHz. The values all seemed to be healthy (4-6: 2 mH/660 Ohm, 1-7: 10uH/0.2 Ohm, 2-7: 1.97mH/418 Ohm, 10-9 and 10-11: 40uH/0.8 Ohm, 15-14 and 15-16: 9,9uH/0.2 Ohm).
So... what i don't really understand is how the 1K load at any of the secondary outputs can destabilize the startup process, any idea? It seems to be both a problem with the startup voltage as well as with the transformer supply voltage, but D507 seems to be ok and C510 was replaced.
Anything else i'm missing?
Thanks!