You're supposed to get a positive voltage but something like a positive 0.6v. This gives two diodes about 1.2v which sets the drive circuit to the correct threshold. With two diodes in series that have 3v across them, the drive circuit will have the bias far too high.
Try this. Install a jumper to short the top of the top diode to the bottom of the second diode. Does the amp power up normally an idle with no voltage across the emitter resistors?
Try this. Install a jumper to short the top of the top diode to the bottom of the second diode. Does the amp power up normally an idle with no voltage across the emitter resistors?
I installed a jumper and it did not drop the voltage across the emitter resistors. I had to reinstall the output transistors on that side of the board. I'm not sure if that had anything to do with it but the two lower diodes now have positive voltage across them. The upper two are still at a negative voltage.
I pulled the two pre-drivers and it did not eliminate the voltage across the emitter resistors but it did drop it. I then pulled the other MPSU07 & MPSU57 and that eliminated the voltage across the emitter resistors. Found that MPSU07 was shorted. Replaced it and voltages across emitter resistors and diodes returned to a normal state.
I then returned the amp back to a normal state step by step checking it as I went.
Having returned the amp completely to a normal state and running under full power, a few minutes later the output fell off. The DC voltage on leg three of the muting resistors was again high, -0.345v. Just after checking them the amp shut down, shutting down my power supply as well. I installed the limiter again and amp powers on for a minute or two and then powers off, then repeats the cycle. DC voltage on pin 4 of the 494 is at +3.5v and rail voltage is +/-0.545v when amp shuts down. Appears I have a component that is intermittently failing.
I then returned the amp back to a normal state step by step checking it as I went.
Having returned the amp completely to a normal state and running under full power, a few minutes later the output fell off. The DC voltage on leg three of the muting resistors was again high, -0.345v. Just after checking them the amp shut down, shutting down my power supply as well. I installed the limiter again and amp powers on for a minute or two and then powers off, then repeats the cycle. DC voltage on pin 4 of the 494 is at +3.5v and rail voltage is +/-0.545v when amp shuts down. Appears I have a component that is intermittently failing.
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