[Mark Zacharias]

"Billy Goat" wrote in message
...
Hi All,

I have this broken Theta ds pro prime dac I am trying to fix. I know
just enough to do some basic trouble shooting but don't really know
enough to totally figure out what's wrong with it so I was thinking
someone might be able to point me in the right direction. I don't mind
spending a few bucks to fix this thing but I don't really want to
throw good money after bad. Schematics are not available so I am
checking this blind.

Symptom, DAC will not lock onto input signal ether coax or fiber. If I
power cycle the unit a few time it will lock on the coax signal and
hold the lock un till the signal or power is removed (although lately
its been dropping the lock on the coax side but not the fiber.) I can
switch between the two when locked as long as I flip the switch fast
enough.

What I have found so far poking around inside with my scope.

It uses a Yamaha receiver ym3623b chip
http://pdf1.alldatasheet.com/datashe...M3623B.htmland
on pin 28 I see a signal when locked and none when unlocked. I believe
this signal is from the PLL circuit. it looks like there may be 3
chips that are part of the PLL circuit but Theta removed the part
numbers so I don't know what they are. So the PLL circuit is not
producing a signal when unlocked. don't know if this is normal or not?
I was thinking maybe the input buffer/s but I don't know for sure what
chips they are, maybe part number mc74hcu04a
http://pdf1.alldatasheet.com/datashe...C74HCU04A.html
hex unbuffered inverter. These are near the input section. Putting the
scope on the pins shows not change with or with out lock but one of
the square waves on one of the pins on one of these chips seems to
have a very low voltage compared to the other pins. don't know if that
means anything.

I really don't know if it something upstream of the PLL or YM chip or
down stream.

I don't mind buying new input buffers or the receiver chip but I
really don't want to buy parts only to find out its the PLL circuit is
bad and I cant buy these parts as the part numbers are removed.

Any Idea as to what might be broken?? If I get a high confidence level
that the receiver chip is bad I will take the chance and just swap it
out as well as the input buffers if someone thinks that is the
problem.

Any thing else I could check and report back.

Thanks

Bill


Always start with the power supply. There could be some hum (ripple) or a
low voltage.
Sometimes an oscilloscope will reveal something not obvious with a
multimeter, such as oscillation at the output of a voltage regulator.

Mark Z.