Rodek heats up !

[m-tech]

A few days ago I received Rodek r480i. Amp is working, but it was repaired. Strange to me was idle current, which is about 2A, and after an hour sitting on a bench ( without load ) it gets quite warm. Is this normal ?
All outputs are OK, but I noticed something. When I removed casing, it turns out that two supply fets getting quite hot ( M1 and M3 on simplified schematic ). M2 and M4 are cold. I know it is not normal.
I don't have a scope, but when I measured DC and AC voltages they were same on all four fets. All fets replaced but nothing new happened.
This morning I played some music. It works fine, but heating is problem. Although it has built in fan, I think it will overheat if operate few hours on high volume.
Any ideas ?

[Perry Babin]

Are you sure that the gate voltage for all 4 FETs is the same. Since the heating is for two FETs that are driven by the same drive signal, it's likely that the problem is related to the drive circuit. The drive circuit problem may be with either the ones getting hot or the ones remaining cool.

[m-tech]

DC voltage on M1,M3 gates is 4.51V. On M2, M4 is 4.8V
AC voltage on M1,M3 gates is 16.1V. On M2, M4 is 16.3V
Measured with digital multimeter ( it is not true rms, but I have no better ).
PWM controller is SG3525. Gate resistors are 27ohm ( not 33ohm ). Isn't this a bit low resistance ?

[Perry Babin]

27 ohm gate resistors are sometimes used.

Measure the DC voltage directly on the output terminals of the 3525. Then measure the DC voltage on the gates of the PNP driver transistors. Then on the emitters of the PNP driver transistors. On which points is the voltage identical (outputs of 3525, bases of drivers, emitters of drivers) and which are significantly different?

[m-tech]

SG3525 drives mosfets directly ( trough gate resistors ) - it is common for this controller.
But, problem is solved. Although I thought that mosfets are IRF3205, someone changed them to SUP75N06. 27ohm gate resistors were too much for them. For testing I put 2.2ohm on each gate. I know this is very low value ( this is what I had laying around ) but everything work fine. Nothing heats up now. I was bit afraid about SG3525, but it works happily ( barely warm ).
Today I push it hard. It works great.

Thanks for help Perry.