Korg DW-8000 - major static issues

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Korg DW-8000 - major static issues

Postby commodorejohn » Fri Feb 02, 2018 3:26 am

So a little while back I took my DW-8000 over to my folks' house for an evening of music. Unfortunately, it was a cold, wet evening and I left it in the trunk of my car for a couple hours before bringing it in and didn't think to let it warm up/dry out before powering it on. It made a nice little pop and I hurriedly switched it off and left it for a while. Unfortunately, while it powers back on normally, it's now completely unusable because of massive noise on the output. The CPU works fine and all the buttons/LEDs respond normally, the keybed works, etc., but there's constant "skipping CD"/"industrial noize"/"glitchcore" junk coming out of the audio outputs. I can even vaguely hear the actual voices somewhere under the noise, but only faintly.

So I'm wondering where to start trying to diagnose this. The noise gets picked up and echoed by the delay circuit, so it's definitely happening before that stage, and since I can still faintly hear the voices I'm assuming that the DAC/VCF/VCA stages are working normally. I'm kind of guessing that maybe something blew in the PSU due to humidity and/or temperature? I'd be a little surprised by that since the digital logic is working totally fine, but it could just be that that's more tolerant of poorly-conditioned power to begin with. And if it is the PSU, where do I start? I'd rather repair it than replace it if possible, since the only spare I can even find is $120. Should I start with just a blanket re-cap? Is there something else I should check first?
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: Korg DW-8000 - major static issues

Postby Rasputin » Fri Feb 02, 2018 7:28 am

commodorejohn wrote:I'm kind of guessing that maybe something blew in the PSU due to humidity and/or temperature? I'd be a little surprised by that since the digital logic is working totally fine, but it could just be that that's more tolerant of poorly-conditioned power to begin with. And if it is the PSU, where do I start? I'd rather repair it than replace it if possible, since the only spare I can even find is $120. Should I start with just a blanket re-cap? Is there something else I should check first?


In my opinion, that's a bad line of reasoning. How do you know that the digital logic is working *totally* fine? Sure, the CPU boots, etc. but digital logic covers a lot of ground.

In other words, there are a lot of pins/lines, gates, inverters, (de)muxers, etc. and any one of those could be bad in a subtle way. Shotgunning stuff is likely to be a waste of time, especially recapping the PSU. Just my opinion.

The good news is that there are lots of test points on the DW-8000 and they're documented. Bad news is that you'll probably have to rig up a logic probe, audio probe, and/or oscilloscope and be slow and methodical about this issue.

My feeling is that it is digital in nature and it's going to be a relatively challenging (but not impossible) fix. Some good news is that the voice and delay are both on daughter boards, so if you have access to another identical machine (fairly unlikely unless you purchase one, I know) then you could take the boards out of your bad machine and put them into the new machine and see which causes the fault.

You could start here: https://syntaur.com/Items.php?Item=4380

That won't really break the bank, and if I had to throw some darts blindfolded, I'd aim for that KLM-775 board.
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Re: Korg DW-8000 - major static issues

Postby commodorejohn » Fri Feb 02, 2018 8:11 am

Well, I'm open to advice, but if I'm gonna drop $55 on a replacement board, it'd be nice to at least know that a bad PSU isn't going to fry it too...
Computers: Amiga 1200, DEC VAXStation 4000/60, DEC MicroPDP-11/73
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Re: Korg DW-8000 - major static issues

Postby Rasputin » Fri Feb 02, 2018 1:56 pm

Testing the PSU is always the best place to start, but assuming the nature of the fault was condensation, it's logical to assume it was a short at some random point in any of the boards and not a PSU specific fault.

And given that the PSU handles the highest voltages and has the highest tolerances (voltage regulators have safety shutoffs on short circuits, etc.), it seems the least likely place to be damaged by a random short. I mean, from your story it would appear this wasn't a power surge event, so the PSU wasn't the "front line" which got abused -- not like a lightning strike.

ICs frequently cannot handle much more than 1.5V over their normal operating voltages, and barely any negative voltage swings, so as far as least robust parts go, I'd look there. And the fault you are describing seems related to the board I suggested.
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