[sdiy] Trying to fix an sh-101 - (more)

[Harry Bissell Jr]

you mean TR9 ?

The collector should go low when the base
(cpu pin - r49) goes high.

If TR9 was shorted, the collector should be
near 0V all the time.

If it was open, it would be near the positive rail.

If it stays at a constant 1.1V... it is probably
connected to something it should NOT be.

I would probably parallel at 10K (temporarily) across
R47 to see if the voltage at the collector of TR9
moves.  

If it moves, there may be an unusual load on the Gate
out line that is way different in resistance than it
should be.  If it stays at 1.1V... maybe this line is
shorted to something ACTIVE like an opamp that is
capable of forcing it to stay at 1.1V

Be wary of any adjacent lines, measure the signals on
either side of this line on the ribbon cable to be
sure that its not shorted to an adjacent signal.

My first bet is still mechanical damage at the gate
out
- gate in jacks

H^) harry



--- Julian <julian at 22host24.com> wrote:

> Some more:
> 
> Around YR9 - 
> 
> Base (0v at leg always) is connected to R49, which
> is then connected to the CPU (4.4v at pin on
> keypress, 0v normally)
> Collector (1.1v at leg always) is connected to R47,
> which is then connected to the positive rail
> (14.16v)
> Emitter (0v at leg always) is connected to ground.
> 
> It seems as though evreything arround YR9 is
> correct, but i cant measure change anywhere, when i
> press a key, except right at the pin of the CPU.
> 
> Would this be a fault with YR9 itself?  Or could it
> be other factors?
> 
> Julian
>