(Topic ID: 197276)

help troubleshooting sound on sys11 MPU

By miatawnt2b

6 years ago

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104 posts
7 Pinsiders participating
Latest reply 4 years ago by GRUMPY
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So looking at your list. what is not working is your DAC chip on the MPU board. Of the sounds that are working 3,4,6,7,8 are speech and sound samples produced by the CVSD. 0,1, and 5 are digital sound effects produced by the DAC chip (1408) at U2.
The op amps should Be Ok as U4 will take out only the speech/samples while U5 will take out sounds and speech/samples.

So, most likley bad, U2, bad Q1 or bad C8 cap 10uf.

looks like you've replaced 2 of these, so I would replace the cap at C8 and then maybe verify that you have an known good chip at U2 (rather than an unknown new chip, I've occasionally gotten bad new chips and it totally throws a curveball at troubleshooting)

If those are all good search the trace from the sound effects from pin 2 on U5 back towards the output of the 1408 at pin 4 as well as the traces between the U5 DAC and the U9 PIA.

Good luck

Quoted from kbliznick:
So looking at your list. what is not working is your DAC chip on the MPU board. Of the sounds that are working 3,4,6,7,8 are speech and sound samples produced by the CVSD. 0,1, and 5 are digital sound effects produced by the DAC chip (1408) at U2.
The op amps should Be Ok as U4 will take out only the speech/samples while U5 will take out sounds and speech/samples.
So, most likley bad, U2, bad Q1 or bad C8 cap 10uf.
looks like you've replaced 2 of these, so I would replace the cap at C8 and then maybe verify that you have an known good chip at U2 (rather than an unknown new chip, I've occasionally gotten bad new chips and it totally throws a curveball at troubleshooting)
If those are all good search the trace from the sound effects from pin 2 on U5 back towards the output of the 1408 at pin 4 as well as the traces between the U5 DAC and the U9 PIA.
Good luck

Thank You for the help! I traced everything from U2 to U9. All good there. Traced everything on U5. Appears to be good there. The 10uf cap is new, so I'm pretty sure we're good there. The 1408 did come from a questionable source (meaning it's a local shop that I've gotten bad chips from in the past.) I did pull the 1408 completely and my sound tests are the same so you've definitely put me on the right path.

All traces though appear to be right on. I think i probably need another 1408 and a Q1 to test out.

Fixed.
Bad DAC at U2
Thank you GRUMPY & kbliznick for your help.

This is a bittersweet fix. The very first components I replaced on this board was Q1, U2 and C8. They were sourced from a local electronics supply here in Tucson. I have gotten bad components from them in the past; needless to say I won't be going back there.

Ordered a DAC0808 from digikey and all is working.

Nothing worse then chasing your tail in circles from a bad part. Just sucks. Great job getting it fixed.

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