[sdiy] MG-1 How to get -12v from TIP30 and 741?

[WeAreAs1 at aol.com]

In a message dated 9/15/04 8:25:51 PM, c0r3dump23 at yahoo.com writes:

<< The MG-1 is still limping along. As I had mentioned last week, the
filter is not working because there is no -12v supply from the keyboard
PCB. >>

It is possible that you have a short or bad component somewhere that is 
"pulling down" your -12v supply.  When tis happens, the bad component or short 
would be drawing all of the current that the supply can provide, and then the 
supply will basically give up and drop to near zero volts.  A bad opamp could 
definitely do this -- or a shorted wire or solder bridge somewhere.

With the power turned OFF, try taking a resistance reading between the -12v 
supply output and ground.  If the reading is very low, then you have a physical 
short circuit somewhere.  If that test checks OK, then try isolating the -12v 
supply from everything else (you could simply cut the supply output circuit 
trace with an X-acto knife to do this -- you can fix the trace cut later with a 
bit of wire and solder) and see what kind of voltage it puts out.  If it 
reads -12v when it's isolated, then you likely have a bad part somewhere.

Michael Bacich