Mac Pro 4,1 won't start

Hi,

I have a Mac Pro 4,1 dual-CPU 2.26 that won't start. Initially it would power up with no chime, and now it won't power up at all. Any advice?

In detail:

1) Initial problem:

Mac Pro powered up, initial brief red LED flash on the CPU tray, normal fans and a little bit of hard drive activity. No chime and no boot. Apple support initially suggested it was the graphics card. Resetting the SMC had no effect. I changed the card from the Radeon 4870 to a GT120, but this had no effect.

Diag LED:

- flashed red—when connected to power.

- orange 5V STBY when diag button pressed—when connected to power but switched off.

- green EFI, green GPU, orange 5V STBY—when connected to power and switched on, and idling without chime.

2) Later problem:

I removed all but two RAM sticks in slots 1 and 5, removed the single hard drive, removed the GPU. The problem described in (1) continued. Then I popped the GPU back in and restarted. The GPU fan ran really fast for ~10 seconds, then the whole Mac clicked off. This happened two more times.

Now, the Mac Pro won't start up at all. I get a red flash on the Diag LED on connecting the power cord, and the Diag button gives me an orange light on the 5V STBY and that's it.

Still no boot without the GPU.

—

This sounds to me like a power supply problem. Any thoughts?

Thanks!