Subject: Re: IIgs Sound Question
From: Rubywand <rubywand@swbell.net>
Date: Thu, Dec 17, 1998 09k22(H
Message-id: <36792FDC.EFA1C571@swbell.net>

Paul Grammens writes ...
> 
> I have a IIgs motherbd that causes a continual "frying" noise from the
> speaker. Otherwise the sound seems normal. I noticed a transistor and
> maybe a few other parts added in the area of the sound circuitry. Looks
> like it was a modification done at the factory. Anyone know what the
> purpose of the xtra parts is, and if it might have any relation to the
> noise? I have another board with a video problem (colors are way off)
> that doesn't make this noise, and doesn't have the modification.

     The circuit on the mini-board is a low-gain 1-transistor amplifier
which seems intended mainly as a buffer/voltage-level shifter between
the 'old Apple' sound output of the Mega Chip and Op Amps which drive
the Speaker and Sound jack.

     The circuit is shown in the ROM-03 schematics; but, it does not
appear in the GS 'Hardware Reference ROM-01 schematics (in either the
First or Second Edition). This is odd because the mod was added pretty
early. Any later ROM-01 is going to have the mod via the mini-board
add-on or an updated motherboard.

     If the transistor on the mini-board is very noisy, it would cause
hiss-- the same would go for any of the Op Amps and some other
components in the sound output circuitry. Really, though, Dave's noisy
power supply scenario seems more likely. You could try swapping in the
power supply from the quieter GS.

     By the way, some power supplies make more noise when pulling a
light load. Adding a memory board, SCSI interface board, etc. might
reduce the hiss.

     The quieter GS's video problem sounds a bit like something which
plagued some ROM-00 GS's before getting Apple's ROM-01 upgrade (which
included the new ROM and a new Video Controller IC).



Rubywand