crown xls2500

[mellowamoya]

hi
im working on a crown xls2500 amp, one channel works fine but the other no sound at all i have the service manual, when i check the supply voltages to the power amps they both have +84 and -84 volts on the rails but the faulty amp has +84 volts on the output line which would indicate that one of the diodes or output transistors is on or short, i have tested all the components and cant find one that is faulty. is anyone familiar with these amp and can give me some advice.
thanks

[Jazz P Bass]

How have you 'tested' the output MosFets?

If they are not shorted, I would suspect U102.

[Enzo]

So stop looking to find a "bad part", and start troubleshooting it. The output has +84? Is -84 present right at Q104? What voltage do you find on the gates of those MOSFETs? If the gate is at 84v so will the output be.

This is a digital amp, so I agree with Jazz, check the IC, does the output look the same from it as the good channel?

[km6xz]

Agree with Enzo, unless the positive rail switch is Q103 is shorted, it is going to be a dynamic problem in that the output is being told to turn on that transistor. But why? It is a very complex amp on the control side but a VERY simple power output section. Very few options, but most lead to U102 or shorted D101.
Random passive testing of parts, in circuit does not tell you much, unless one of those two devices, D101 or Q103 are shorted. What is the power device Q103 being commanded to do?