However, the schematic also documents an engineering failure: Ampeg issued several service bulletins and a "Rev B" power supply board, but the damage to the PF-500’s reputation was done.
The Class D driver IC requires a "bootstrap" capacitor to supply voltage to the high-side gate driver. In early PF-500s, the specified capacitor (a standard electrolytic) dried out under the chassis heat. When it fails, the high-side MOSFET stops switching, causing DC offset at the output. Result: Loud pop, then smoke. ampeg pf500 schematic
If you own a working PF-500, keep it. If you are repairing one, replace all small electrolytic capacitors (especially the bootstrap cap), upgrade the main NTC thermistor, and add a small 12V fan to the rear panel. With these schematic-informed modifications, the PF-500 transforms from a ticking time bomb into a reliable, lightweight monster. Disclaimer: Working on switch-mode power supplies and Class D amplifiers is dangerous. The primary side of the PF-500 contains lethal voltages (300V+ DC) that persist even after unplugging. Do not open the chassis unless you are a qualified technician. When it fails, the high-side MOSFET stops switching,