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Macintosh Plus screeching sound
Posted by: bibilit on 2022-08-09 23:45:26
I have a Macintosh making a screeching sound, but working ok otherwise.

The issue is in the LB (tested with another and working ok, so the AB is fine)

i have done a swap on both ROMs and those are ok, have not been any further, but probably a chip bad on the LB.

How is generated sound on the Plus ? (any particular chip ?)
Posted by: joshc on 2022-08-10 01:59:07
See attached Apple's schematics for the Plus (they are bad scans, so a bit rough).

Looks like sound is shown on page 3, here:

1660121934186.png
Posted by: bibilit on 2022-08-10 02:31:46
Thanks, yes i was able to find that same picture, but the quality was poor and i was unable to see much. should have a look at my board, probably not a chip freely available.
Posted by: joshc on 2022-08-10 03:23:50
It looks like a Sony sound chip with an Apple part number, from what I can tell on this pic (couldn't find a high res enough pic and don't have a Plus logicboard to check myself).

1660127022402.png
Posted by: Phipli on 2022-08-10 03:43:47
Is the sound chip part of the reset circuit on the Plus? On the Mac II at least the sound chip is used to reset other hardware. If this is the case on the Plus, then that hopefully means the sound chip itself is likely working.
Posted by: Phipli on 2022-08-10 03:45:38
Yup, the reset switch is connected to the Plus sound chip and the rest of the computer is reset from there. So the chip is doing some or all of its job at least.
Posted by: bibilit on 2022-08-10 04:12:57
Yes i agree, the Plus is working except for the noise, so probably the chip is half working.
That Plus had a lot of issues ( the AB board had several issues, bad transistor, bad diode bridge, burnt resistors...still not working though) LB had a bad ram and that noise issue, missing mouse rubber ball... but was not too expensive either.
Posted by: Phipli on 2022-08-10 04:31:13
Yes i agree, the Plus is working except for the noise, so probably the chip is half working.
That Plus had a lot of issues ( the AB board had several issues, bad transistor, bad diode bridge, burnt resistors...still not working though) LB had a bad ram and that noise issue, missing mouse rubber ball... but was not too expensive either.
I'd start by beeping out pins 2,3,4 (to the 6522) and 16 (to U4D and onwards?).
Posted by: bibilit on 2022-08-11 04:35:09
nothing obvious, i will remove the chip and solder a CI support first.
Posted by: techknight on 2022-08-15 09:07:25
nothing obvious, i will remove the chip and solder a CI support first.
Grab a scope and check for oscillation. That sound IC looks like an amplifier, etc. the sound is generated elsewhere as indicated by the sound input pin. Check the output, AND the input for oscillation/noise.
Posted by: bibilit on 2022-08-16 06:08:37
So managed to remove the old Chip, and test with another unit (had no other Plus at hand but a bunch of bombed SE/30s, and reference is very close... moreover the SE/30 had two of them)

Didn't worked, and can't use the switch to try a reset (the Plus suffered from a battery leak as well, both switches are corroded) so i will go ahead @techknight and follow your advise
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