Korg SP250 Mainboard no audio output

Is there an audio out/in 1/4 inch socket to 1/4 inch external loop send/ return ? which could have a flakey bypass switch

Thanks for the responses Gareth and N Cook!

Sorry.. I should have been a bit clearer to start with.

The output from the OpAmp on the main board goes to the volume pot, and then on to the separate audio amp board which does all the jacks, line outs, main amp, muting. The audio amp stage is working.. I can feed audio into it ok.

I tested the output off the mainboard that goes to the volume pot, and there is no audio at that point.

There is an on-board self test sequence which I can activate and the memory/logic test passes, and the MIDI test passes when the midi in/out ports are looped back, it fails when the MIDI ports are not looped (as expected).. so I believe the MIDI and basic processor/memory etc part is probably ok.

Here is the schematic of the main board: "

IC3=PCM1716E

So far all I know is there is no audio on CON8-1 and CON8-3 (right of diagram).

Is it typical for both op-amps in the dual op-amp to die together?

I have to figure out how to get the boards running on the bench next so i can get at the mainboard to test it in situ. I will check for DC output on the amp.

Thanks again for the suggestions so far.

On 05/01/2014 09:27 AM, 2phar wrote: there a dark recess with a hidden "reset" button somewhere?

Is ther

Here is the schematic of the main board: "

IC3=PCM1716E

So far all I know is there is no audio on CON8-1 and CON8-3 (right of diagram).

Is it typical for both op-amps in the dual op-amp to die together?

I have to figure out how to get the boards running on the bench next so i can get at the mainboard to test it in situ. I will check for DC output on the amp.

Thanks again for the suggestions so far.

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.

Cheers,

Gareth.

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.

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?

Oops,

the digital circuitry is a mixture of 5v stuff and 3.3v stuff so some "5v readings" I quoted will actually be 3.3v.

In particular the Reset line, which is actually a "Not Reset" line, meaning a Reset happens when 0v is on this pin. Anything higher is ignored.

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.

To follow up this issue.. I replaced the DAC and system is now producing audio again.

Only remaining problem is a few keys on the keyboard not working fully. It appears the switches are those rubber membrane / carbon pad things.. so lots of potential there for flake outs.

yay.

how did you remove the original chip?

I used ChipQuik solder paste along one side and a pretty hot 40 watt iron to get one side molten enough that i could lift it slightly with a pick. then it came off easy enough doing same on the other side. There appeared to be no issue with adhesive holding down the package itself.

Being my first go at SMT, I initially tried to cut the package away from the pins with a knife but that didnt go very well.. some of the pins just bent and broke one track and pulled a few pads off the board. That led to some tedious unbending and repositioning the pads with a 10x magnifier and bridging one track with some 36 swg copper wire, but got there in the end. Live and learn.

I used ChipQuik solder paste along one side and a pretty hot 40 watt iron to get one side molten enough that i could lift it slightly with a pick. then it came off easy enough doing same on the other side. There appeared to be no issue with adhesive holding down the package itself.

Being my first go at SMT, I initially tried to cut the package away from the pins with a knife but that didnt go very well.. some of the pins just bent and broke one track and pulled a few pads off the board. That led to some tedious unbending and repositioning the pads with a 10x magnifier and bridging one track with some 36 swg copper wire, but got there in the end. Live and learn.

You need to put just vertical pressure on the pins as close to the package as you can, so you are not sawing or moving them side to side. The blade will just click straight through them one by one causing no damage.

Gareth.

This works to fix the bad keys.

Thanks.. I have a few cordless phones in bad need of that stuff.

The issue on the SP 250 keyboard turned out to be the PCB the membranes sit on had gotten pushed down a bit a one end and slipped below the plastic retaining tabs on the edge, so the tops of the rubber mouldings were a little too low below the keys.