13 Apr
2010
13 Apr
'10
7:05 p.m.
On Tue, 13 Apr 2010, Tony Duell wrote:
As you surmised, the problem was on the 12V rail, which measured 0.5 ohms
to ground. It runs to quite a number of chips, but was surprising easy to
follow around and through the various via holes. I dragged out the dremel
tool and did a binary search by slicing traces. Took the full logN tries,
but on the last cut I nailed a 4.7ufd / 16V tantalum cap with a dead short
:-).
I could _really_ learn to start hating the blasted things. It seems like
almost every failure of this type has a tantalum cap behind it - on both
audio and computer gear. Just finished resurrecting a KLH/Burwen phono
noise suppressor in which EVERY blasted one of the things was shorted.
My eyes are crossed from staring through the board at a bright light...
Will patch traces and reassemble tomorrow. I'll let you know if things
cooperate.
Thanks again for getting me headed in the right direction!
Steve
--
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.