Audio Alchemy DDE v3.0 fault

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Mike P

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Joined 2012
I'm trying to fix a fault on a friend's Audio Alchemy DDE v3.0 DAC.

The fault is that although it works fine via a Toslink optical input it doesn't work via the coaxial digital input.

The input jack is PCB mounted so I was hoping it was going to be as simple as reflowing a bad solder joint but all joints look OK.

Any ideas where to start?
 
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batteryman

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Joined 2011
Does the coax led light when the selector is operated?
Looks like the switching is done by U2, the 74hc125 which in turn is driven by U7 a/b.

Check pins 1 and 4 of U2 - pin 1 should be low when the coax input is selected and pin 4 will be low when the optical input is selected.
 
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Mike P

Member
Joined 2012
batteryman said:
Does the coax led light when the selector is operated?
Looks like the switching is done by U2, the 74hc125 which in turn is driven by U7 a/b.

Check pins 1 and 4 of U2 - pin 1 should be low when the coax input is selected and pin 4 will be low when the optical input is selected.
Click to expand...

The LED lights for the three input options all light up cycle through as you'd expect when you press the input selector button but the 'Lock' light doesn't come on when using the coaxial input and there is no audio out.

We've tried it with a variety of different CD players an transports and they all work fine via optical but none work with coaxial.

The coaxial input was working fine up until a few weeks ago before it went faulty.
 
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batteryman

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Joined 2011
Mike P said:
I've had a look with my scope and I can confirm that pin 1 on U2 is low with coaxial selected and pin 4 is low when optical is selected.

What next?
Click to expand...

Feed in a signal and follow it through with your scope. U1 pin 2 = in, 3 = out, U2 pin 2 = in, pin 3 = out.

The fault must be in that path as the output of U2 pin 6 is connected to pin 3 and that has data on it when the optical input is working.
 
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Mike P

Member
Joined 2012
OK, so with the coaxial input selected I can see a nice signal at the following locations:

At the BNC terminal.
At one side of C1
At one side of R1
At pin1 of U1

I also see a slightly different waveform signal at pin 8 of U2.

I don't see the signal on any other pins of U1 or U2.
 
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batteryman

Member
Joined 2011
Mike P said:
OK, so with the coaxial input selected I can see a nice signal at the following locations:

At the BNC terminal.
At one side of C1
At one side of R1
At pin1 of U1

I also see a slightly different waveform signal at pin 8 of U2.

I don't see the signal on any other pins of U1 or U2.
Click to expand...

If there is no signal on pin 3, then U1 must be faulty. (probably been spiked from excess volts on the coax input but might be worth going over the soldering)
 
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