Korg C-4000 digital piano, no sounds

[Alex R]

This is a very well made old digital piano, early 90s I think. It has MIDI in/out and it will work fine as a MIDI keyboard. I have the service manual but it is too big to attach, it's an 8.5meg pdf; I can send screenshots of bits of it if needed I guess.

I ran it through the self test procedure which checks quite a lot of functions and it's all fine. But it makes no noise from the internals.

The +12v/-12v and 5v supplies are fine and are getting to the chips as they should.

There are no waveforms coming out of the DAC.

The internal audio amp and speakers work fine.

It has a plug-in 32 pin EEPROM with a paper sticker on it. I suspect this has become deprogrammed, though the problem might I guess also be in the Korg-labelled processor chip.

Korg don't supply either the board or the chip - discontinued, no stocks left.

Any ideas or potential ways forward before I skip the poor old thing? I don't know much about digital sound production and/or EEPROM reprogramming but am fine with all kinds of other electronic repairs and have a good range of analogue test equipment.

[Alex R]

OK it seems no-one has any more idea than I do! Well I decided the eprom probably got wiped, tried and failed to find one (Korg can't help, model too old), got hold of an old sound module and it worked just fine. So that was the solution - use it as a midi kepboard and connect a sound module. There are some good pure piano modules out there s/h.

[Enzo]

I have never seen a EPROM that wiped itself, but it could happen. More likely the chip fails than wipes itself.

But your unit works other than sound. Why assume it is a ROM of all things? I'd suspect that DAC before a ROM any day. In fact ROM is about the last thing I'd suspect. Explore the master controls section. DO you have "LOCAL" on or off? Look at other parameters like output assigning if it is present. Also any other level related parameters.

Does this thing have a memory battery? Is it flat? ANything with memory has to have a reinitialization, so if so, reinit the unit. If it has a wrong bit in memory that has an attack paramemter set for 10 minutes, you'll never hear anything unless you hold a key down for 10 minutes.

Niothing comes out the DAC? So inject some noise at thqat point to see if the rest of the signal path works, your outputs could be stuck in mute.

[Alex R]

Thanks Enzo. I did try a few of those things, before I added the sound module, cleaned the piano up and installed it in my daughter's flat in London. I was pretty pleased, the whole lot including module cost under £100 and it's a very good weighted keyboard. With a proper piano sound module like a Kurtzweil Micropiano or the Yamaha one it would be much better than it could have been new.

I guess I assumed the wave ROM had got wiped because I have had to replace a few switching ROMs in Hughes and Kettner three-channel amps because they got wiped somehow. Or just failed, maybe. Dodgy logic I know.

I'd still like to fix it though! I did try injecting noise at the DAC output and it was amplified through the rest of the stages no problem. I ran it through all the test and initialising and resetting procedures in the service manual. It passed all the tests. All the power supply rails are at the right voltage. The amp side of things is just fine. I dunno how to test it further so maybe replacing the DAC might be the next step? It's surface mount dammit, a millipede. I wonder if it's still available. I bet the Korg processor chip isn't.

[Enzo]

Don't just start replacing stuff, what if it isn't a bad part at all, but an open control line to an enable pin?

Can you link us the schematic?

Look at a block digram for most any synth, and you will see a circuit to handle the keyboard input, probably dual switching for velocity. That then goes to the CPU, wwhich responds. It generates control signals, not least of which are MIDI, and also asks the sound generation circuits to make noise. That will require addressing ROM to decide what to do, then multiplexing/demultiplexing and other stuff to put the data on the bus that the DAC reads. That data bus is shared by a lot of stuff, so you can't just look at it and see individual notes going by. But many times you can see that changes are happening, and they go with the signal. Presence of such a picture on the scope is usually good enough to tell me hte signal is getting that far. All chips more complex than simple gates will tend to have control lines, and a stuck enable pin is all it takes to prevent anything coming out of an IC. Or stuck clock pin or stuck reset pin, or...or...or...

I also look at data and address lines for contention - two different forces, one trying to pull it high the other trying to go low, and the poor line winds up with a state halfway up in the middle - which is invalid to a logic circuit.

The thing to do is systematically follow evidence of signal through the system to isolate where it fails.

SOmetimes you can force the issue. Just as you fed signal in at the DAC output, assuming the DAC control lines are healthy, you could probably kick a couple funny states at its input pins. It doesn't matter if it is a real sound, if you can short a couple pins together for a second and get a noise, that means the DAC is capable of making noise. You symptom is NO noise, not wrong noise. This is the equivalent of touching the tip of a guitar cord to see if the amp works. Doesnl;t matter that it is just a loud hum rather than music, it serves to tell us the amp works.

And you could be right all along, bad ROM, bad DAC, we just shouldn't guess, we should KNOW.