Boss Riot R735 in protection mode

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I had this Boss Riot R735 working for a couple years and yesterday it was working for 1 hour in a row and stopped working. It went into Protection Mode (Red LED). Every 10-15 seconds it tries to work, the Green LED turns on for a fraction of seconds and returns to Protection mode. What could have burnt, a transistor, a relay, what? any help will be appreciated. Here are the spects and picture of this amp:

Boss Riot R735 amplifier:

Tri-mode operation
Max Power 2 Ohm 250 Watts X 4
RMS Power 4 Ohm 100 Watts X 4
Bridged Power 500 Watts X 2
Signal-To-Noise Ratio: >105dB
THD: 0.01%
Variable high &low pass crossovers
Remote subwoofer level control
High pass crossovers
Line and speaker level inputs
0-+18dB Bass Boost
Line outputs
Gain control
Power and Protection LEDs
MOSFET power supply
Plexiglass top cover
Input voltage switch 100mV-2V/2V-8V
11 3/4" W x 2 3/16"H x 11" L
1-year SonicElectronix.com warranty