Dark_AleX, Mathieulh, and Yoshi have released their firmware 2.71 to 1.50 downgrader. This DOES NOT work on TA-082/TA-086 motherboards! Check here to find out if you have a TA-082/TA-086 motherboard.

As with any downgrader, there is a chance that you will brick your PSP! Use at your own risk!

Instructions:

Make sure you have your PSP charger in at all times! Read carefully through all of the instructions first.

1. To lower your chances of bricking, reformat your memory stick and reset your PSP to default settings. Make sure you have at least 75% battery power.
2. Extract the downgrader files to your computer.
3. Copy the contents of the MS_ROOT folder to the root of your PSP’s memory stick.
4. Exit USB Mode and go to the Photo section on your PSP.
5. Make sure your charger is still plugged in! The downgrader should begin once you enter the photo section. If it doesn’t, select the “hw.tif” file.
6. The screen will turn red and the memory stick light will be flashing a lot if the downgrader has started.
7. This may take a few tries. If the PSP freezes without the red screen and the memory stick light flashing, turn off your PSP and try again.
8. When the downgrader is running, don’t turn off your PSP!
9. The PSP should turn off by itself after the downgrader has finished.
10. Wait for a while then turn your PSP back on. A blue screen with many words and languages should pop up if successful. Press O (Circle) to reboot. If you get the red screen with many words and languages, turn off your PSP and turn it back on.
11. Now you should have a Firmware 1.50 PSP!

Download from PSP-Hacks