Re: help with broken GS

[Rubywand <rubywand@swbell.net>]

Jon Christopher Co writes ...
> 
> I was using my GS and smoke started coming out from the inside.  after
> opening it up my best guess tells me the smoke came from an 8 legged
> chip right under the power supply at the upper leftmost corner (right
> behind the composite video connector and ADB jack).  Right now the
> computer still boots up, but will not respond to any ADB device
> (keyboard).  can anybody identify what this chip is?  i also noticed
> there is a slight bump in the middle.. is this normal or is it > supposed to be perfectly flat?



     My guess is that the chip or module you are talking about is L2
"D-15C". This is an 8-pin thing with 4 inductors (coils). Three
inductors are used. They are in series with the ADB Desktop connector.

     Evidently, one of the inductors burned out. This would explain the
loss of ADB functioning and the bump on the module.

     Repairing the burned out inductor should be fairly easy, although
it will probably be necessary to remove the motherboard. You could take
a length of thin wire-- like wirewrap wire-- make a small coil, and
connect it across the terminals of the module corresponding to the
burned-out inductor.

     The three inductors run ...

     pin 1 to  pin 8
     pin 2 to  pin 7
     pin 3 to  pin 6

You can use an Ohm meter to detect which one is open. (Example: the
correct reading from pin 2 to pin 7 would, probably, be less than an
Ohm.)

     The 'catch' in all this is ... Why did the inductor burn out?!

     Since the pin2-pin7 inductor connects to +5V on the pin7 side, it
appears that pin2 is, some how, shorted to ground.

     Another possible candidate is the pin1-pin8 inductor. Pin8 connects
to +5V through a 470 Ohm resistor. If pin1 should be shorted to ground,
there _might_ be enough current to heat up and burn out the pin1-pin8
coil. However, this does not seem likely.

     With the motherboard removed and KB disconnected, check Resistance
from pin1 to pin8, etc.. When you find the open (high R) inductor, check
R from the low-numbered side (1, 2, or 3) to Ground. (The silvery metal
shield near the ADB socket should be = Ground.)

     If you discover a low R (like a few Ohms) to ground on the low
number side of the blown inductor, my guess is that it will be at pin2
and that the culprit is a shorted capacitor, BC91 and that your GS is a
ROM-03 (since no capacitor is shown on the ROM-01 schematic). If this is
the case, cut the trace to the capacitor and replace it with a compact
0.01 uF capacitor rated at 15V or better.

     Another possible cause of shorting is a bent piece of shielding
near the ADB connector.

     If you discover no short on the low-numbered pin of the blown
inductor, plug in your ADB cable without the KB and measure again. If
you, now, find a short, the indication is a messed up cable.

     If there is, still, no short plug in the KB. A short now would
indicated a messed up connection at the KB socket.

     If there is, still, no short, try the check while wiggling the
cable and plugs on both ends.

     If you can find no short, my suggestion is that you remove the
bottom and back shields and throw them away-- they are a shorting
hazard.

     After repairing the blown coil with a bit of coiled wire, put
everything together and see if your GS is fixed.

   
 
Rubywand