Hifonics Zx7500 problem

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Hello All!
I bought a bad Hifonics Amplifier, with some unsoldered, and missed parts. After i made the Amp to its "original" status, the Amp goes down to protection mode, when i connected it to a low power power supply. Then i started to measure the parts in it ( unfortunately I do not have a schematic), and i found two bad parts on it. One was a zener diode (pos dz4) which sets +15V for the preamp circuits(?), and one switching FET in the protection circuit ( pos Q15, C3198). After replacing the bad components the Amp started normally, but after a while (10-20 mins), it went back to protection mode again, and the new Q15 FET went to bad again. Can anybody tell me what can cause this? I find some strange things during the testing: the Amp has noise in LP filter mode, the Voltage at Dz4, and Dz5 are not exactly 15V (-15,4, and +14,77) and the big resistors which are connected serial with these zeners are very hot in "standby" ( no input signals, and loads).

Thnks for any help!
( a schematic will be the best, but if i knownit well, it is impossible to get it...)
 
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