DW 8000 Restoration

So last week I was lucky to score a non-working Korg DW-8000 off a guy on craigslist for free!  It seems like a really great synth; from my favorite era of synthesizers, the mid-80s.  I like synths like this one because there were in some sort of weird transition phase between digital and analog.  I have a DSS-1, which is like the bigger, sampler version of the DW-8000, and uses these damn-awesome SSM filters giving a nice buttery texture to what I think are fairly high quality samples.

This DW-8000 has some "power" issues.  I thought it might have been a blown fuse. 

I generally approach synth restorations, and general electronics repair in the same way, sort of a "simplest to most complex" method.  I do first what I call the stupid test.  Plug it in, turn it on, fiddle the buttons, check the contrast.

I learned people don't like to adjust things.  I picked up a DX100 that the screen was "not working".  It was working fine, but the contrast knob hadn't been adjusted.

I think people just immediately default to "I don't think it works, so I'm not going to try at all."

It's too bad.


So, upon completion of the "Stupid Test," the DW didn't respond.  I then opened it up, and it wasn't as easy as many other synths.  A lot of synths have this whole "car hood" thing going on where you remove a few screws from the front and the hole top half comes up like a car hood.
Well the DW wasn't like this; you have to take off the entire bottom, which makes working on it a bit of a pain in the ass.

So I looked at the fuse, and it was blown, replaced it, and still not powering on.  But things were getting really hot on the power supply board, and the transformer was giving very strange voltages, which made me begin to suspect it having issues.  I would later find out this wasn't the case. 


After doing some reading on a few forums, apparently the two bigass capacitors found on the board are prone to failure.  Luckily, I have access to a cap checker and all kinds of other equipment as I am a tech for the chem department at my university.  I checked the largest cap, C11, a 6800 uF cap, and it was bad.  But I didnt have any spares around, so I replaced it with a 4800 uF instead.  This seemed to work, but one of the test points was weird.  The power supply outputs +/- 5, +7 +.6 +11.5 and +11 Volts.  all of them checked out, except the 11.5 and 11 volt lines. 

I put the board back into the DW8000, and viola it turned on fine.  But I wanted to let it cook a while and see if that 15 volts would ever come down.  It never came down, and things really started cooking.  All the board on the synth were hot, and so were the chips.  I didn't want to potentially damage anything, so I disconnected everything, and took the board back to work to look at those caps and such. 

That damn 11 volt line was still bothering me though.

Looking at the schematic it is easy to see that this is literally taken straight from the output of the transformer after passing through a diode bridge to rectify the voltages.  The odd thing was, the output on this rectifier was right on, almost exactly 11.5 volts, so There has to be some bleed.

I was getting about 16 volts, which suspiciously looked like the +5 volt lines + the +11.5 volt lines.  There had to be a short somewhere, but the fact that all the damn caps were so old, I decided just to replace everything.  I have currently desoldered everything, and am not just working on a parts list.  When I get a BOM from mouser or something, I'll post it.