Author Topic: CPC 464: Quiet computer sound, and no sound loading (and no loading)  (Read 751 times)

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Offline llopis

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This is actually a different computer, than the other one I posted about with lots of noise while loading, but maybe they're somewhat related problems.

In this case, this CPC 464 makes a very quiet/faint sound. If I crank up the volume to the max and press DEL, I get maybe about 20% volume of a normal CPC.

Also, there's no loading sound, and tapes don't load at all (don't even detect anything is there).

It's definitely not the sound potentiometer because it's clean and reads resistance of 0 at max volume.

I looked at the tape deck diagram, and it would seem that the LA6324 might be faulty. So I swapped with with the amplifier of another working CPC and... same thing! Here's were it gets weird: I removed the amplifier completely and... same thing!

Sound amplification isn't my strong suit, but I don't understand how there can be any sound with the IC missing. I don't see a direct path from SOUND IN to the speaker. Is it really just going over the C312 capacitor because it's not a DC signal?

What could be causing the amplifiers not to work? I replaced C315 and C318 just in case, but it made no difference. VCC seems fine around here (4.8V, which is pretty normal for the Amstrad from what I can see).

Any thoughts as to what might be going on?

Edit: I should add I already checked for dry joints and even reflowed a few that looked fine just in case. So that's not a problem.

« Last Edit: 20:59, 23 June 19 by llopis »
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Offline Bryce

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You need to trace the signal with a scope. Run some BASIC program that makes repeated beeps. Then measure the signal strength at each stage starting from the connector going to the tapedeck from the AY and then working through the amplifier stages:

Stage 1 input = Pin 12
Stage 1 output = Pin 14
Stage 2 input (LA4140) = Pin 2
Stage 2 output (LA4140) = Pin 6

The level should be rising slightly after each stage, if it drops, you've just passed the problem component.

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