[sdiy] x0x repair questions

Pete Hartman pete.hartman at gmail.com
Fri Dec 20 07:25:17 CET 2013

These may be more generally applicable too....

I picked up a non-functional x0xb0x for a good price, with the intent
to repair and resell.

The good news is, the repair is 99% done.  A number of interesting
problems, but nothing too frustrating.

I have one repair related question and one general question.  The
repair-related one is: does anyone have a suggestion for a good way to
match LED colors?  Particularly BLUE.  The 13 note LEDs are blue, but
it's not the piercing bright blinding blue I'm accustomed to :-).
Unfortunately, I don't have any stock of ANY 5mm blue LEDs, so I will
have to source some.  I'd ask the builder, except apparently this
board has been through several hands before it came to me, so I don't
know who that is even....and that was a couple years ago, so I think
it's fairly unlikely they'd even remember if I did track them down.

The ground plane in this beast is pretty hard to work with when
desoldering, even with my temp controlled iron, so I really don't
relish the idea of replacing all 13 LEDs....

The general question is: one of the problems was a significant
low-frequency (100Hz?) hum on the output.  It did not vary with the
volume setting, was about 1Vpp, and was a periodic but irregular
waveform.  Kind of like a folded sine with some resonance added.  But
the sound to my ears was just a barely buzzy low hum.  It did not show
up on the "mix out" jack, only the headphone jack.

As I tracked this down I found three classes of problems.  1) the 5V
regulator had been bent back and forth and was barely attached, so
bumping it reset the x0x.  2) the 6V regulator was putting out 12V
(this feeds power for the headphone amp only)  3) there were a
significant number of what appeared to me to be cold solders in the
vicinity.  I repaired all these things and the hum is now gone.

I'm curious what my culprit was likely to be for the hum.  My
immediate reaction was that the 5V regulator was unlikely, it was just
causing overall instability because it would reset the processor and
doesn't directly feed the output.  I figured it was the 12V instead of
6V supply for the headphone amp, but as I think about it I'm not sure
that it wasn't possibly the cold solders.  Anyone with more experience
care to hazard a likely guess? If it was the 12V supply for the
headphone amp, I'm curious why that might be.



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