So over a year ago or so, a friend of mine, Kyle, traded me an old, beat up Korg Poly61 (what is my deal with broken korgs?). So it had some weird issues he told me about. Like every 6th voice would be out of tune and the keys would work sometimes. Well I ended up getting the thing home, and it worked perfectly, but would intermittently freak out. I had a ton of other projects at the time so I never really got around to looking at it until last week. And let me tell you, there was a lot to look at. So I powered up the poly61 for the first time in almost a year, and not too much happened. Most of the keys didn't work. I managed to get a couple notes programmed into the chord memory, and then stuck it on the arpeggiator. I didn't have any problems with any voices out of tune, but after running through the parameters, I found that one of the filters wasn't working. This sucked, because the filters were made from OTAs not single chips like in the poly6 or the DSS-1/DW8000 (all SSM stuffs). The Envelopes are SSMs, but not the filters. I have actually heard that it is based on the MS-20 filter, but it sounds too mellow to be an MS-20 filter.
After doing some reading, I couldn't find a lot about missing filters, most people just blabber about the notorious battery issues. These old synths with memory have batteries that have a lovely habit of exploding all over the circuits, and the electrolyte then dissolves everything in its path.
This poly61 did not have this problem. All the chips looked fine with no signs of corrosion.
I did a fairly stupid test though and decided to wiggle around many of the jumper wires going between voices. I noticed that one would intermittently get the filter working. These old jumpers seem to have a tendency of degrading over time. I need to replace the wire still, but I think I might just go through and do all of them. I replaced a couple, and plugged it all back in, but something weird happened!
A voice was out of tune! Just like Kyle had said!
But where the hell did this come from. I was starting to believe that every time I would turn it on would be a new problem. I found it was just a loose connection internally. This connector may be need replaced, but I'll save that for later.
So the next job on the list is to replace all the jumpers in this old guy, I hope it cleans up the "consistency" of the synth.
So tonight, I decided to tackle the next problem: the keyboard not working. The keyboard had worked perfectly before, but it seems that it, as all the other connections, were intermittent. 80s keyboards had many different switch contact types. The DW uses rubber contacts. The Ensoniq Mirage uses these weird, and super-damn-annoying springy things. They give for a nice action, but have many temperamental problems. The poly61 uses similar, classic rubber contacts. The type with the small rubber nipple-things with a conductive silicone underside. Well this conductive silicone doesn't stay conductive for very long. The carbon wears off over time. No deoxit or alcohol or cleaner will fix it. Once they are dead, they are dead. I have seen some pretty nifty solutions, but none have ever been very practical, and most people just resort to completely replacing the contact strips.
Last summer I experimented with a couple ways to fix this. I had a Roland MC 909 I picked up on the cheap on Ebay that was "untested". Well it was certainly. It would boot up, but none of the rubber pads would work, minus pause and a couple random ones. I checked the actual metal contacts on the motherboard with some tin foil, and they were responsive. I had seen this CircuitWriter stuff at RadioShack and though hmmmmm. The stuff is expensive. It is 20 bucks for a little pen with some conductive, silver based ink in it. The nifty thing is, it sticks to silicone rubber really well. It takes a couple days to dry on to the rubber for a semipermanent solution, but I decided to test it out. I applied the stuff, waited 48 hours and put the pads into the MC 909. All the pads were beautifully responsive, almost more so that previous! I was pleased with this finding.
Now I am doing this again. I just coated all 61 rubber contacts, and am letting them bake on a couple days. I tested one, and the keys work beautifully. I hope to seal up the poly61 this weekend, clean up the case and the switches and get it on a keyboard stand.
Wednesday, May 2, 2012
Tuesday, May 1, 2012
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.
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.
Wednesday, March 23, 2011
New blog for all my projects
Anyone who knows me knows me as a maker of things. I have built 3 electric guitars, a Guzheng (chinese harp), synthesizers, theremins and all kinds of other stuff. I have a lot of stuff in production, and should blog about it. Maybe I will have some useful info for other people building similar stuff.
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