MA1200 can't get high side bias into spec

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#1 kevvywevvywoo


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Posted 13 December 2011 - 07:26 AM

Hi folks, got a Macrotech MA1200 here and during routine calibration I can't get Ch2 high side bias above 120mV on R02. It adjusts but not enough. I have changed the predrivers (under the clips), the bias transistor (in the heatsink), the bias diode chain and cold checked all component values around the bias adjustment and the ODEP temperature sensor & thermistor. The low side bias is fine at 320mV. All ODEP voltages are fine. I have swapped-over the main pcb connections to prove it isn't ODEP shutting the drive down; Ch1 calibrates perfectly.

The voltage across the emitter resistors shows it is running too cool. The amp otherwise works without noticeable THD.

Any ideas, people?