Ahh yes diodes, i will desolder and check. Most of the small caps i have replaced. I have not checked the values of all the resistors, except in immediate areas to where i have been checking.
Hi, good luck with replacing components, have you the tools to test capacitors? Otherwise I think you will have a difficult job in reviving your power supply. I would suggest a capacitor meter, an ohmmeter and an ESR meter (for electrolytics).
The best guide I have found for SMPS repair is by Sencore, TT205 archived here :
https://web.archive.org/web/20061230062617/http://www.sencore.com/custsup/techtips.htm
If you suspect the protection circuit, I would suggest you check for poor solder joints. Then check how the comparators are performing, to do this you will need a couple of DC power supplies.
Set up (as per Sencore guide) would be as follows:
Disconnect SMPS from mains - for this test don't apply any voltage to the SMPS input.
Power up the SMPS control and comparator circuitry - this needs a DC supply of around 10 to 15V (check datasheets for ICs). Connect DC supply via a current limiting resistor (eg 680 ohms) to SMPS IC - check voltage on each IC to determine that supply is within range, increase DC voltage until all ICs are powered up, check voltage drop across current limiting resistor for current drain.
This would be a good point to use an oscilloscope (as you don't have any dangerous mains voltages / earth loops etc to worry about) to check the control IC is generating a suitable output at say 20kHz to 40kHz. Then check the comparator ICs, each probably contains two or more comparators, each has two inputs and one output. If the + input is higher voltage than the - input the output would be high (or low if inverting), and vice versa.
If all comparators are OK, then use the second DC supply to 'power up' the SMPS 5V output - ie feed a voltage back in to simulate a working PSU. One of the comparators will be for over voltage, eg increase output from 4.5V to 5.5V, one of the comparators should flip - tracing the circuit should show which!
HTH,
John