I have 2 mono Power Amps that were des...

I have 2 mono Power Amps that were designed and built for me about 30 years ago. They have been in storage for about 12 years and I just got them out to test them. They were rated at 400 Watts into 8 ohms. When I tested them, one worked perfectly bot the other one produces no sound at all. I tried each one through the same channel of the preamp with the same wires to elliminate that as a cause. I read the voltage just before it goes into the circuit boards and both amps mathched, so the power supply seems to be working fine. I checked all fuses and all have continuity. What might cause no sound at all? These were really nice sounding amps and I'd love to have them both working again as one without the other makes them useless. They were originally designed and built by my uncle (a DIY audiofile), but he is no longer alive to help. I remember that the transisters he used were called Mosfets. They are very large and each amp has 12. They say Motorola on them and I remember him waiting a long time to get them. Thanks