Crown Micro-Tech 1200 Repair

Another Crown Micro-Tech 1200 repaired.  I was told when someone tried to move it when it was on there was this spark sound and one side went down.  The case isnt straight like it had been dropped or treated a bit rough.

Powered it up and found DC offset of around 500mv on the broken side....

Opened case up for visual, found nice spark soot stain on case and black mark on main board.  Because the case was slightly bent and board not straight, the board made contact with case. .. VCC shorted to ground which normally blows lowside output devices and drivers.  Went through output devices.. low side all shorted, drivers shorted, bias diodes shorted.  Replaced with graded matched output devices and new drivers.  Pre-drivers checked out fine.  Also replaced ODEP current source MM334Z and the MPS8097 as I broke the legs :( Added new thermal grease. Main board seemed to survive just a slight lifted trace. 

Quite confident about the fix... Checked main and output boards over and over again.  Switched on (without dim bulb tester) risky! Came up fine.  Very, very low DC offset, Re-adjusted bias to 320mV.  One side bias pot has to be right at the stop to get 320mv so i'll investigate that.  Left ODEP settings as they are for now.  Load tested and got a very healthy 350watts in 8 ohms!.  CMR adjusted for low noise.

Distortion tested and very very good figures.  Far exceeding specs.  Very low high order distortion and good IMD figures!

Incredible Grounded Bridge Amps invented by the great Gerald Stanley.