Recently a friend had a faulty power supply come in for repair. It had the usual problems of blown caps in the secondary, but this one also had issues on the primary side and had blown its FET, a couple of smd resistors and the PWM. The PWM was a LD7575PS. As neither of us had any of these PWM's on hand I decided to go searching for a possible replacement that we may have instead of having to wait for them to arrive from a supplier/ebay.
After a lot of searching on Google, I was unable to find anything except for a post on a forum somewhere that stated there was no replacement that was pin compatible. I did not believe this, so I kept looking. Eventually I decided to search for a datasheet on a PWM that I had, with the markings DAP08. which I had not been able to find anything on. After some searching, for some reason, I ended up searching DAP008 instead, and low and behold, there it was. So now I had both datasheets, I was able to do a quick comparison. To start with, the pinouts were identical, so now it was just the electrical characteristics that were going to determine if it could be used. The frequency of the LD7575 can be set by a resistor between RT (pin 1) and Ground. The datasheet shows that a 100K resistor will give a frequency of 65KHz on the LD7575, and that is exactly what the power supply we were repairing had. So now that I knew the switching frequency, I looked at the DAP008 datasheet and sure enough, the frequency on the DAP008 was also 65KHz.
With this information we fitted the DAP008 to the power supply and fired it up and sure enough, it sprang to life. We did not even remove the 100K resistor between pin 1 and ground. So we put some load onto the outputs and left it running on test for a little while and nothing was getting hot so off it went back to the client, and its been running happily ever since.