Hi,
I'm working on this broken farfisa.
As i can't find the schematic i redraw it. (only the power amp part...)
On the power amp board i found a faulty darlington PNP transistor (Q2). I changed it and the keyboard is now alive.
But i've got some questions:
1/ i've replaced the SE9400 with a BDW94C wich seems to be a direct remplacement.
Do i need to adjust something in the circuit according to this? What do i need to check to make a good replacement? Is it better to change the working SE9300 with a new BDW93C?
2/ the part with transistor Q, R12, R10 and R11 seems to be a biasing circuit for the darlington pair.
If i'm correct this configuration is called a "Vbe multiplier".
I've read that the good way to bias is to set current on R7 and R8 (0,22 ohms resistors) between 20mA and 200mA , is that right?
So i need something like between 4mV and 40mV across R7 and R8.
Today I've got 300mV overthere so maybe this is why the darlington get really hot?
Thanks
Morgan