"2phar" wrote in message ...

On 5/2/2014 2:24 PM, Gareth Magennis wrote:


"2phar" wrote in message ...

On 5/1/2014 3:50 PM, Gareth Magennis wrote:


Here is the schematic of the main board:
"http://i.imgur.com/hVf4N6i.gif"

IC3=PCM1716E

A quick look at that schematic shows the Op-amp to have a single rail
supply (+9v / 0v) and the output pins will probably normally be at a DC
of 5v, before the DC blocking capacitors.

What I meant to say was a blown op-amp will usually have an output of
either the full +ve supply or the full -ve. In this case +9v or Zero.

With the keyboard idle, using a DVM, the op amp power rails at pins 4
and 8 are at 0 and 9 volts, the outputs pins 1 and 7 are both 2.47 volts
and the + input pins 3 and 5 are both 2.44 volts.

Also touching the 3 and 5 pins is producing hum on the amp output.



Sounds to me like the op-amps and downstream are working fine.

You will need to scope the D/A converter then.
If the Data pin 2 shows a signal stream that varies as you press keys,
then it is being given data.
If it doesn't vary or is non existent, then either there is no data from
the CPU, or the faulty D/A converter is shorting it out. You would need
to cut the track to establish which.



Pins 5,3,1 I think should all be carrying clocks.
There should be 5v on the pins 8,9,15,20.
Rst pin 22 should be 5v, as should the group 23,25,28. (faulty
reset/mute circuitry is not that uncommon)

If all this is happening and VoutL and VoutR are not varying with key
presses, then it is highly likely the D/A is toast.

Gareth.

Yes.. so I have been exploring the DAC pins with the DVM, not having
anything else:

pin 5 (ext clock) = 0.78 v
pin 2 (data) = 0.33 v idle, varies from 0.8 v back to 0.33 when note
played [is a assume there is a data stream here]
pin 3 (bit clock) = 1.66 v
pin 1 (left/right clock) = 1.66v

reset is 3.3 volts - DAC spec says logic high is 2.0 v
all the supply lines are good
zero level output stays at 0 volts
mute/mode lines are hard wired to the correct voltages per diagram

the analog left/right outputs pins 13/16 show 2.44 volts
analog extl/extr on pins 1118 measure 2.42 volts

So yup.. ordering a new DAC.

Any thoughts on desoldering/replacing 28 pin SSOP SMT package?
And why would a DAC within a working board just die? External power surge?





We must have both been typing at the same time there.


There are many ways to desolder/solder SMD stuff, go on youtube and have a
look, there are some great tutorials there.

My preferred method is to put a new blade into a craft knife and cut through
all the pins first, then you can get the main chip body off the board
leaving the pins behind - occasionally they are glued to the PCB really
strongly.




Gareth.