Q*Bert sound board repair log

Problem:

No sound (has the daughter board mod for the sound amp)

Verified voltages at the daughter card were correct, the wet finger test at
the inputs didn't produce anything.

Swapped out the TDA2002, with no change.

Replaced C36 (470uf 35v cap), sound restored. Cap was open (though it was a
recent cap kit done to the sound board)

Fix:

Replace bad cap at C36

Problem:

Not all sounds (or wrong sounds) show up in self-test.

Probed the sound select lines entering the board. S1,2,4,8,16 are all
toggling during sound select.

S32 is toggling at the connector (pin 4) and up to pin 3 of the LM193
driver. But nothing is coming out on pin 1 of that chip.

Replaced LM393 at U24, signal is now coming out of pin 1 and all sounds
selected as expected.

NOTE: Schematic lists U24 as a LM193 but all the sound boards I have on hand
(6) have LM393 @ U24

Fix:

Replace LM393 @ U24

Problem:

Speech sounds way too fast R6 has no effect

>From the boot up sound to all the other speech sounds they are all at a very
fast rate with no change during adjustment of R6.

U19 1408 DAC pin 4 is reading about 4vdc, a good board reads 2vdc.

Substituted U19 from a working board with no change

U21 (LM741) pin 6 is reading right close to 12v with no change from R6.

Replaced U21, speech is now much slower and rate can be adjusted with R6

Note: scopeing U14 pin 15/16 should show a constant saw tooth signal with
about 3v swing. Adjustment of R6 should adjust the frequency of that signal
even if there is nothing playing in the sound circuit.

Fix:

Replace LM741 @ U21

Always fun when you have multi problems on the same board.

David

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Received on Sun May 12 19:42:22 2013