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SX-626 Biasing instructions

tarior

tarior

Dirty pool, old man?
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I'm servicing an SX 626 here, and the service manual seems to have ommited biasing instructions for the amplifier section. I used VR 1 and 2 to set the the collector voltages of Q11 and Q12 to 26 volts as per the schematic, but I'm not really sure where to set the idle current. It's dropping 5mV across R33 and R34, for about 10mA each, 20mA per output pair, if I understand Kirchoff correctly. Suppose I should just leave it at that?
 
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If you're getting 5mv across each emitter resistor, that should be just fine. I would leave it alone.
 
In lieu of bias data, the idea is that bias minimizes crossover distortion, so if you have a scope you could dial up the bias until you minimize the crossover distortion (observe it at lower levels). Don't go higher after hitting the minimum, that's your "knee" point and there is no significant advantage to going higher.

Then measure your emitter resistor voltages and calculate the current usiong ohms law.
 
Glad you said that Mark. Was pondering the same thing about xover/notch dist. as a function of bias when no instructins were avail. DC offset no prob but doing bias wrong could be trouble. Now it's a POC. [ifn ya got a scope]
You made my day.
 
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