A NOS 192/24 DAC with the PCM1794 (and WaveIO USB input)

Hi Doede and follow DDDac-builders.

Hope you have time for some small troulbeshooting questions :)
Some years ago a got a bag of old style DDDac main- and DAC-boards and some defective newer pcb's from another DDDAc-builder.
I assembled and repaired a secondary DDDac from an old-style MB's and two old-style DAC-boards. This deck have been running fine in my office ever since :)

Now i have got the "solder-itch" again and took a look inside the old scrap-yard bag. It still contains one old MB, one new MB, one old DAC-board and a new one.
I did mark the old DAC-board as "Right channel def." and the new DAC-board as "Both channel def." some years ago and I seems to remember both MB's were tested dead.

Now I started by reheated all solder pads on the two older boards and added a little new solder. I then carefully examined both boards visually and checked the most obviously connections.
All good... I then connected the DAC-board and heard... nothing!!! Now I checked every connections without finding any problems.
All LDO's give correct voltage, BCLK and LRCLK connections are intact from I2S entrypoint up to the DAC-boards. GND is fine everywhere.
DATA is good from I2S entrypoint to first 74VHC164MX-chip (pin 1 and 2) and all connections between these 5 74VHC164MX seems fine as well as the exit-points for R- and L-channels and up to the DAC-boards.
Now I removed all I2S (and GND) wires for both channels from MB and up the the DAC-board and connected new wires (about 5-6cm long) directly from the top connection-points on a functional DDDac and onto the top connection-points of the defective stack...feeding channel separated I2S directly into the old DAC-board bypassing its own MB. Music were now playing from both channels!!!

So... the old DAC-board seems now completely fine (after reheating) and the power distribution on the old MB as well as the the analog side also seems fine.
Conclusion!!! the I2S DATA line is nonfunctional on the old MB. If the problem isn't a defective connection, and I don't think so, it most be one or more 74VHC164MX-chips which have failed.
Since the Right-channel breaks out after the first 74VHC164MX-chip, this chip seems a logical point of failure. Of cause more chips could be defective as well.

Do you agree on this conclusion or have I missed another possibility?
Are there any "clever" ways to check the 74VHC164MX-chips health before replacing them ?

...and sorry for the long post.

Martin Roerup, Denmark.
So you ran the early red board with the new dddac DAC module? That would probably fail
the new MB have a 74 flip flop to delay half A clock cycle. The first generation dac Modules and red boards did not have this. In stead there was a 1kOhm resistor in series with data to create this delay.

so in stead of using a wire from MB to DAC board for the DARa line, install a 1kOhm resistor and see what happens. As said only when the MB is the old red one and the da. Module is of newer generation
No no... ALL boards involved here are old style MB's (Red) and old style DAC-boards (without Tent shunts). Two old DDDacs... one functional deck and one defective deck.
I used my perfectly functional deck with two old-style DAC-boards on top to "inject" a known working L/R channel separated I2S-signals directly into the other problematic decks DAC-board which I'm trying to troubleshoot.
By doing this I bypassed the problematic motherboards own I2S processing completely (from the I2S input socket, through the 74VHC164MX-chips).
Doing this test I still uses the problematic old-style MB's 12V power handling to the DAC-board as well as the analog output from the 1794's through the I/V resistors and out.
This test works fine with music now playing from the defective deck... although with L/R I2S comming from the functional deck. In my logic it proves that this DAC-board also works fine and the problem is the defective decks motherboards I2S logic.
Since these motherboards are rather simple it leaves one or more 74VHC164MX-chips as the only possible failure points, since I have checked all PCB-traces and voltages.
So my questions are...
Is my logic flawed here? ...and can I test the 74VHC164MX-chips health without first replacing them?
It might be both cheaper and easier just to start replacing the 74VHC164MX-chips, but I might learn much more by going for the oscilloscope.
...and to be honest, the learning part of this repair project is actually a large part of the experience. It's not that I really neeed a third (or maybe even a fourth DDDac ;-)
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Hello, my first post here.

I remember a discussion in this tread about possibile benefits by replacing standard Wima 100nF bypass with Kemet 68nF (even if I not remember which positions should be replaced, I'm not able to find the original post).

Today I measured both capacitors ESR at 100KHz using an LRC meter and result is Wima ESR=0.370 ohms and Kemet ESR=0.098 ohms.

Sorry for my question, I'm a Newbye in electronic : is the lower ESR the main reason of reported overall benefits ?

Thank you.


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