After 7 years of correct functioning I've got a problem with my DAC Buffalo II.
The fist days after purchasing I used SPDIF -- it produced a good sound but then I turned to I2S-bus which sounded better. Two days ago I decided to refresh my old impressions of SPDIF input of Buffalo II. As the previous time I connected cable to the GND and SPDIF terminals on Buffalo board and set K1 switch to ON position. But no LOCK indication had appeared and hence no sound. I restarted both devices twice or thrice and got the same result. I checked my SPDIF source with another DAC -- it was all right (and still is!). Then I turned back to I2S-source again, set K1 to position 1 and found no LOCK like it happened with SPDIF. Restarting brought me the same result as the SPDIF case.
All the reconnection procedures were done with all commutated devices switched off.
But there appeared one detail that seems I haven't seen before -- when Buffalo was starting the first and I2S-source the second, the LEDs on AVCC board were dimming for a few seconds and D1 was dimmed deeper and longer than D2. I measured voltage on power supply terminals (Placide) and found a short-time voltage drop from standard 5,25V to 3.75V (approx.) while the I2S device was starting but then the voltage restored to 5,25V. To say honestly I cannot state that there was no such an effect earlier and I have noticed it just now. May be it already existed but it didn't attract my attention.
All the next tries to resurrect functionality of Buffalo such as a shortening of "reset" pins and multiple power on-off cycles didn't change the situation. NO LOCK -- no sound. I'm not sure that reset was done correctly because I haven't found any description of this procedure in Buffalo II user guide and I had to look for the necessary information on different DIY sites.

Now the questions are -- is there any ways to cure the DAC which I had not used yet and what are the check methods and measure points (for oscilloscope and multimeter) that could clarify is the patient dead or still alive?

Thanks in advance!

Thank you, Brian, for your quick response!
I use no Tridents, but AVCC module and on-board regulators only and Placid as a main regulator.
The voltages I've measured (neither I2S or SPDIF connected) are shown on the picture attached
2018-11-02 21.27.29.jpg (140kb) downloaded 5 time(s).
U(in)= +5,25V
GND
U(out1)= +3,46V
U(out2)= +3,25V
Voltages are the same on both sides of AVCC board.

Edited by user Sunday, November 4, 2018 6:37:26 PM(UTC)  | Reason: Not specified

Sorry for the long delay. Day job is keeping me quite busy these days.

Did you check the voltages of the other regulators? This can be measured between Pin 1 of each regulator and GND.

If all the voltages are okay, I am going to guess it's a wiring issue (bad connection) or possibly something has gone wrong with the DAC chip itself. The voltage sag you mentioned is concerning.