13 Apr
2010
13 Apr
'10
12:19 p.m.
I removed the resistor and it does not try to spin.
Traced things out as
best I could (it's a three-layer board with limited visibility to the
interior plane). The +24V input runs directly and almost solely to the
armature of one SPDT relay contact set. The NO position goes directly to
the motor / armature connector and the NC position goes nowhere. This
gives some support to my theory that the resistor and associated circuitry
is there to limit inrush current. I think this relay is supposed to pull
in rather quickly and feed power directly to the motor.
The downstream leg of the 10-ohm power resistor goes to a 2SD768 power
device and I'm having a lot of trouble following it from there. I think
your guess about a regulator for the 12V circuitry is probably correct.
THis does sound very like the schematic that I was looking at in the 112
manual. The relay there eneables the spindle motor (and maybe the
positioner drivers too), the resistor feeds a power transistor that's the
pass tranasitor for the 12V regualtor.
The fact that the drive fails totally if the resistor is removed would
seem to indicate that the otuput of that 12V regulator is somewhat
present.
I have suggested in my least message that iy would eb worth measuring the
votlages on the leads of that transistor.
What if the transistor was shorted? The regualtor would then become a
simple resistor + zner circuit. The reissotr would ahve to drop 10 or
12V, and woul;d get hot and bothered!. The zener diode wouldn't like it
much either (are you sure _nothing_ else is getting hot?). Anyway, it's
worth checking this.
-tony