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ronpod 28th May 2011 03:23 PM

Buffalo II not locking
My Buffalo II DAC has been working and the source of joy for many months. It unexpectedly became silent and the lock led is off; the mute led on. The source spdf (Squeezedbox Touch) was checked and is fine. The BII is fitted with Tridents, the outputs were measured; VDD is drawn down to 0.571V. All power supply (incl Tridents) leds are lit. The Arduino fed lcd display is functional, volume control, etc. and the sample rate is shown as 0 with no clock also indicated.

This happened once before about two weeks ago. I probed around the VDD Trident with the multimeter; something clicked when I touch a measurement point with the meter probe and all voltages came back to normal and the lock was restored. Everything was normal again until now; it seems to be repeating but so far no reset. I did try to short across the reset terminals; no affect.

I have the Legato 3.1 and the Placid HD built and ready to connect but, obviously, this problem has priority.

I appreciate any help.

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ronpod 28th May 2011 05:22 PM

Update: The lock led has come on but it is dim. The mute led remains brightly lit. This does not bode well. There is very little voltage drop across the VDD Trident shunt resistor. The VDD remains down at 0.758V. The dac outputs a constantly varying low level hum heard from the speakers.

sangram 28th May 2011 06:45 PM

I had a similar issue and it was traced back to a short around VDD in the power supply (mine is Buffalo 1 with the LCDPS). Removing the short and ramping the voltage up to 5V under load did it. DAC had to remain connected while adjusting power supply voltage - nerve-racking stuff.

It was quite a head-scratcher. Power supply was behaving fine on its own but when connected to the DAC the VDD was dropping off very badly.

What LEDs are you using?

ronpod 28th May 2011 07:59 PM

sangram - thanks for reply. All LEDs came with the kits. The lock LED comes on dimly and then turns off in no discernible pattern. The mute LED is constantly on. weird...

avr300 28th May 2011 08:05 PM

You could take out that Trident and reinsert the ferrite bead. Just to rule out a faulty Trident.

ronpod 11th June 2011 03:41 PM

Good news - my Buffalo II is working again. I have been chipping away at the problem but lately my day job has diverted my attention from focusing on fixing my DAC. I finally did reinsert the ferrite bead and the 1.2V came back indicating that the problem was in the 1.2V Trident. I replaced the opamp on that Trident but not very well because my soldering pencil was too big for surface mount components. But then I placed the circuit under magnification and carefully refreshed all my soldering work with a fine soldering point (better soldering station). I tested the circuit with 5.25V applied from a bench power supply and without load; the output voltage was 1.19V which I thought was good enough. I reinserted the Trident into the BuffaloII board but the Trident output then measured 0.5V. This was purplexing. So I drew up a hand shetch of the Trident circuit schematic and probed more systematically. The measured voltages didn't always make sense but during this probing the output voltage stepped up to 1.19V and everything stablized. I guess I fixed it but it troubles me that I don't know how. Maybe my probing inadvertantly reconnected a cold solder joint; maybe the probe dislodged a piece of swarf that was causing a short; but certainly I am at a loss to explain my success at repair. Mainly because I thought I exercised extreme care during soldering and in cleaning the circuit with isopropyl alcohol.

So I replaced the Legato 2 with a newly built Legato 3.1. The Placid BP remains the power supply until I can expand the casework to accomodate the Placid HD. The headphone output is functional but drives the Senn HD650s at low volume only. Prehaps the increase capability of the Placid HD is required to drive the HD650s.

The sound of the Legato 3.1 is pure, transparent, and musical now but more (or less) may be revealed if burn in is important. My listening is complicated by recent changes to my Orion active crossover circuit component values suggested by the designer, Siegfried Linkwitz.

Russ White 12th June 2011 12:19 PM

It sounds like the CCS resistor was not making good contact. The would cause precisely what you saw. We have seen this occur a few times now.

Russ White 12th June 2011 12:20 PM

As for the Legato 3.1 SE output for headphones that can be adjusted as you see fit.

ronpod 12th June 2011 01:43 PM

Thanks Russ for your reply and for these multiple modular options for the DAC! I will resolder the CCS resistor. I will also consider changing R1-R4 to 187ohm to increase to 2.0 Vrms. I will give the current configuration some listening time to determine if changes are really necessary. I am throughly enjoying the diy interactivity that your kits allow.

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