Oscilloscope / Troubleshooting Question

[Brentd27]

So I'm a bit of a beginner when it comes to using an oscilloscope, beyond the absolute basics.  My comfort zone is digital circuits.

That said, I'm working on troubleshooting a sound board for a pinball machine.  The board is producing sounds, but they are incredibly muted and very staticky even with the volume turned all the way up.  I pulled the board and connected it up on my workbench to verify that it wasn't something in the game cabinet wiring, and same result.  The board uses a 6503 CPU to drive a DAC1408P (Datasheet attached) to produce the sounds.  I've attached a partial board schematic that shows how the output of the dac1408 connects to the LM380N amplifier to produce sounds.

I have verified that the amplifier is working properly by lifting the leg of C15 that connects to the junction of R6, R10, & C@, and injected signal first with a signal generator, then audio source.  Sound was clear and amplified properly.

This is where my question comes in.  How do I look at the output of the DAC with my oscilloscope?  So far, all I've been able to see is the +5V offset, even when I trigger the sounds.  Can't get the oscilloscope to register anything.  Reconnecting C15 to the junction of R10, etc, and lifting the connection to R3 and attaching the oscilloscope probe to that leg of C15 then triggering sounds will show a little bit of movement if I have the gain turned way up, but the signal seems to be only a couple of millivolts above the noise.  I wish I had a way to force the board to play a constant tone rather than the game sounds, but unfortunately, I don't have access to the game code, or enough knowledge of 6503 programming to cobble something up.

Now, given the schematic I attached, and the simplified internal schematic of the dac1408 from page 1 of the attached datasheet, my current thinking is that something has gone wrong with the current amplifier in the output stage of the DAC1408.  Does this seem likely?

[alsetalokin4017]

1. Where you say "R3" do you mean R13 or R5, or are my glasses fogged up again?

2. What happens when you disconnect Pin 4 of the DAC and just scope that, without the coupling and amplifier stage connected?

I'd guess that if you still have a low quality signal here, your chip is fried.

I'd also look carefully at the two capacitors and two resistors as you could be losing signal integrity there. You might have to lift legs and look at their values and mechanical condition and solder joints and board traces. It seems that you have verified that the amplifier stage itself is working OK, so that doesn't leave much other than the chip output itself and the coupling components.

[Brentd27]

Sorry for the delay in reply. 

 alsetalokin4017, Yes, I did mean R5.

Also, I ended up swapping out the DAC1408P and the board now playing sounds perfectly.  I'm guessing that the internal output amplifier stage of the DAC was fried in such a way that enough signal was leaking out that I could hear it faintly through the noise with the LM380 at full blast.

Thanks for the help.  I have a bit more confidence now.  At least I know that with this circuit I wasn't completely out in left field with my troubleshooting methodology.

[ambrosia heart]

Just several words

Every service manual should include a block diagram. Make a block diagram analysis.  Find out the signal flow.
probe the pin from  signal input to signal output that means from one stage to next stage.   Any signal loss is
a clue to you.   But first make sure every stage should have a proper voltage supply.

You device may have a DSP.  I saw "A" in your schematic.  A means address signal.

[cheater]

You could try injecting logic values into the DAC input if it has one.