Another LC475 with no sound output (solved)

VeryVon

New Tinkerer
Jul 2, 2024
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Actually this was a Quadra 605, but mainboards are the same.

On this board 4 of the VIA's around C137 were toast:
  1. SPKR- feed to the TDA7052AT Amp
  2. 12V supply to the TDA7052AT Amp
  3. VIA's that connect U30-24 to Sys_Reset
SPRK- was bodged to a trace underneath the board that was scraped away so I could solder it. It goes to the speaker jack J17-2 (see 2nd picture)
The 12V supply can be picked up as shown in the picture on R85.
U30-24 was bodged to U19 as shown in the picture (this is where the broken VIA's were going.)
There was also some repair and replacement of the TDA7052AT Amp.

VIAsBetter.jpg



SPKR- Bodge underneath the board which goes to the speaker jack J17-2
1784465602658.png


Now the speaker was working, but just giving static.

After slaving over U30 / DFAC-II for a while hahah, I found out that something ugly must have happened under the chip, because U30-12 & U30-21 were not connected to QVCC!

So a couple bodges later...
1784465861122.png


Bodge U30-12 and U30-21 to U30-1. And now it's working.

DFAC-II Pin Reference
1784466002981.png


Schematics (page 13)