Re: Help! Rom3 goes silent.

[rubywand@aol.com]

In article <5d1bpf$hjk@gandalf.Advent.COM>, smcgroga@gandalf.Advent.COM
(Stephen McGrogan) writes:

>
>While inserting a mono remote speaker plug into my son's new
>(to us) IIgs Rom3, the sound was permanently reduced in volume.
>I suspect that the mono plug shorted out one of the two stereo
>rings in the jack, which must have caused some damage in the
>audio driver circuitry.
>
>The computer was producing fine sound before, and great sound with
>the external speaker, but now, with or without the remote speaker,
>only produces faint sounds with the control panel at absolute
>maximum volume.  The change occurred when I was attempting to
>exactly position the mono plug while the computer was generating
>sounds.
>
>I see that the IIgs has a much different audio system that the
>IIe and II+ (for which I have schematics).
>Apparently the sound synthesizer drives a LM380-N-8 2-watt amplifier
>that then is fed thru a MC3340P electronic fader.  This latter
>drives the speaker or headphone.  There also seems to be a number of
>small electrolytics in the circuit.
>
>Has anyone had this experience?  Could someone describe the circuit
>in terms of what could fail when the output is shorted?  I am an
>EE and can replace components if needed;  the lack of a schematic
>is the limiting factor.  The fader seems like it might be okay since
>the computer is still able to control the volume;  just from inaudible
>to very faint.
>
>

     Although the GS output is mono, it does go through a stereo jack;
and, both 'hot' wipers in the jack do carry the signal. Plugging a mono
plug into the GS's stereo jack would, naturally, short the output to
ground.

     There does not seem to be any easy way to blow the LM380; so,
probably, it is okay. My guess is that you blew the 22uF capacitor (SC6)
which couples the LM380's output to the jack. If so, the cap could be
fried and acting like a high resistance. Try removing the cap and
soldering on a replacement.


Rubywand