Apple IIgs -old Apple sound- mod

[ToasterKing <toasterking@SPLATbigfoot.com>]

For some reason, the message I originally posted never appeared in the
group on my news server, so maybe it never posted correctly.  Anyway,
I've found the answer, and it is stated below for the archives.  But
first, my original post:

-=-

I have a ROM 3 Apple IIgs with an Applied Engineering Sonic Blaster
card for stereo audio output and input sampling, connected to external
amplified speakers.  The Sonic Blaster only processes the digital audio
coming from the J-25 connector on the Ensoniq 5503 DOC and doesn't play
the legacy sounds from 8-bit Apple software.  The problem is, if I have
the GS's internal speaker on for those sounds, the internal speaker
also plays the GS sound that comes from my external speakers, but with
a lesser acoustic quality and monophonically.

I want to be able to do one of two things:
1. Modify the GS so that the legacy Apple sound also plays through the
external speakers, and disconnect the internal speaker.  I think this
would require some decoupling and possibly an op-amp.  I only want to
do this if it won't introduce noise into the external speakers, or
degrade the sound quality.
2. Modify the GS so that ONLY the legacy sound comes from the internal
speaker (and only the Ensoniq output through the external speakers).  I
would think this would be as simple as clipping a motherboard trace,
but I don't know which one.

If anyone has pinouts of the Mega II (for old Apple sound output)
and/or the mixing and amplification circuitry for the headphone output
and internal speaker, it would be most appreciated.  And otherwise,
does anyone have any tips for me on this?

Thanks,
TK

-=-

Whether I could have had help or not, I've discovered a solution for
scenario #2.  This was annoying without schematics or a scope, but I
did what I could with a home stereo and multimeter.  Keep in mind that
this applies only to a ROM 3 IIgs board; the ROM 01 will most likely be
different.

The analog output from the Ensoniq 5503 DOC sound processor is
premaplified by the TL072 dual op-amp at board location UL11.  "Output
#1" of this IC (on pin 1, or the bottom-left pin if you're facing the
chip with the white bar on your left) carries the premaplified analog
audio from the Ensoniq.  The signal goes through some resistors, where
it merges with the legacy sound output signal from pin 1 of the "Mega
II" at UI2 (and buffered through a bipolar transistor).  The combined
signals are then decoupled through an electrolytic capacitor and fed
into pin 1 of the MC3340P at UK11, which is a signal attenuator, and
functions as the hardware volume control.  The signal eventually makes
its way into pin 2 of the LM380N-8 (2.5-Watt power amplifier) at UK10
for amplification.

All one needs to do to remove the audio from the Ensoniq DOC from the
internal speaker, but leave the legacy beeps and sounds from 8-bit
software, is to desolder the surface-mount resistor SR53 from the
bottom side of the board.  This is a 10K resistor, with the code "103"
stamped on it.  Removing this resistor affects the headphone output on
the rear of the GS case too.

So now, my GS plays the "GS audio" through my AE Sonic Blaster to my
stereo speakers, and only plays the "legacy Apple sound" through the
internal speaker.  I like it much better that way.  In my case, I
wanted to have the option of getting the Ensoniq sound back to the
internal speaker later if I change my mind, so I installed a switch. 
Where the SR53 resistor used to be, there are now leads to an SPST
slide switch with a 10K 1/4-watt resistor inline.  I used hotglue to
affix this switch to the top of the Ensoniq IC.

If anyone wants to try this mod, this is how to do it on a ROM 3.  Good
luck!  Let me know of any inconsistencies or anomalies in my
documentation.

TK

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