Korg Polysix synthesizer

[SteveNielsen]

I was given a Korg Polysix several years ago in very poor condition and am just now starting to look at it. It's basically filthy, dinged up, missing 18 knobs, two potentiometer mounting nuts and one pot has the shaft broken off and a pushbutton switch missing its cover. The person who gave it to me said it worked but had to "warm up" first, so she assumed it had vacuum tubes, which of course it does not, it's all solid state.

I've been smart and have never powered it up and glad I didn't.

On the inside it wasn't too filthy, just some cobwebs and a couple of dead bugs and a few particle board bits from the casing being knocked about for around 30 years by stoned, drunken hippies and hillbillies. It had the typical 34 year old 3.6v NiCad rechargeable battery leaking its chemistry and associated corrosion all over the programmer board so I set about to correct that issue. Removed the board and nasty battery, cleaned a load of nice blue copper oxide and crap off, neutralized any remaining acid with a baking soda scrub and rinse and after drying inspected the board and tested all the circuit traces for continuity and they are all ok. Did the recommended mod to install a 3v lithium coin battery holder and battery in place of the NiCad and disable the charging circuit.

Then I looked at the power supply board thinking I would pull the board and test the caps and saw something wrong right off the bat. Some idiot hot-wired two 1a fuses with copper wire. Well, it must have been blowing the fuses then and this was the moron's way of "fixing" that. I suppose that the copper oxide all over the battery charging ckt could have caused the PSU to pull to much current but I've got the PSU board out now and am going to test parts on it to make sure. Attached is the PSU board top view so you can see the hot-wires wrapped around the fuses.

[stj]

Re: Korg Polysix synthesizer

lol atleast it was copper wire,
i used to find fuses wrapped in the foil from inside cigarette packets!

[SteveNielsen]

Re: Korg Polysix synthesizer

That's true enough. I was a working musician and seen bobby pins, paperclips, staples. etc.

I just pulled the two big caps off the board to test them and used some IPA to loosen the old glue under them and boy howdy! The chemical reaction of the IPA with the glue made a smell that was absolutely UNBEARABLE! It was like burning bakelite only worse. Yuck. I had to leave the room and air out the house for a while. Whew! the caps are not leaking or bulging but I don't think I should trust them. The capacitances measure within tolerance but they both have VERY low esr., .01 and .00 ohms.

[redwire]

Re: Korg Polysix synthesizer

In the middle of a gig, you just short out blown fuses to keep playing...
But those fuses don't blow unless there's trouble or a huge line surge.

I see +/-15VDC and +/-5VDC outputs. I'd hook up the power transformer and just test the power supply board alone.
That filter cap ESR looks too low? If the bung looks ok, I dunno, a multimeter on ohms should be able to charge them up unless they have shorted.

[SteveNielsen]

Re: Korg Polysix synthesizer

The bungs look fine and they behave like normal electrolytics with an ohmmeter, so they aren't shorted. The rectifiers are ok. I will try as you suggest.

[SteveNielsen]

Re: Korg Polysix synthesizer

I'm going to have to clean this board off before I work with it. I think it's from a coating of what I'm sure is flux all over the solder-side of the board. I don't know what Korg used for flux back in 1982 but it smells very strong and bad, like creosote and burning bakelite, and they sure as hell didn't clean it off.

[redwire]

Re: Korg Polysix synthesizer

yeah that Japanese rosin flux/phenolic pcb smell is nasty, especially when you solder it. I use Isopropyl and q-tips.

I had a Cordovox CDX keyboard to rebuild. Found a mouse had lived in the thing. It ate the speakers, chewed on wires and of course had pissed all over the place. I soldered some wires and almost passed out. Worst stench ever.

[stj]

Re: Korg Polysix synthesizer

q-tips?
you must have too much time on your hands.
i just cleaned all that crap off a sega saturn today.
IPA in a spray-bottle, and a medium paintbrush are the tools to use!

[SteveNielsen]

Re: Korg Polysix synthesizer

I used IPA and a toothbrush, warm water rinse, then a baking soda scrub, and another rinse. It's ok now. Can still smell it some but it's tolerable.

[SteveNielsen]

Re: Korg Polysix synthesizer

Well, I got the ckt back together enough to test just the power supply and it isn't peeing a drop. So I checked the stepped down AC from the transformer and nothing. There is ac mains voltage at the fuse (which is ok) and pwr switch. I did some continuity checks and it all seems ok but the way this tranny is wired is weird (Bando TA-005). There doesn't seem to be any path back to the neutral side of the ac mains. I removed and bypassed the noise filter (TDK ZCB2203-11) and no changes. I have the svc manual with schems but it is not clear at all how this transformer is wired. I suspect the transformer has gone bad but I can't find any definitive information about it.

[redwire]

Re: Korg Polysix synthesizer

If the secondary-fuses were jumpered, then the power transformer (primary) becomes the next fuse. That's what I've see happen many times.

The power transformer should have a have a thermal-fuse inside, if you have an open primary. I've done surgery to "replace" them. You can fix it if you have patience and good hands.
Take the transformer apart, pry off the shielding covers until you can see the bobbin. Follow the primary leads, looking for breaks (sometimes the primary winding melts open at the point (hotspot) right where the wire runs into the bobbin) and looking for the "bulge" of the thermal fuse (size like a picofuse). I carefully cut the winding tape until I can get at it.

Otherwise, another shielded transformer about 28VCT gives +/-18VDC for the regulators. I see only a 3-wire secondary?

[stj]

Re: Korg Polysix synthesizer

looks simple enough.

pull the secondary fuses out, and check resistance between the 2 fuse holders and the center tap.
just to make sure they are the same resistance.

then meter across the primary side to see if it's open or not.
finally meter between the primary and secondary for shorts - it should be a split-bobin these days and impossible to short - but you never know with old stuff.

[SteveNielsen]

Re: Korg Polysix synthesizer

If the secondary-fuses were jumpered, then the power transformer (primary) becomes the next fuse. That's what I've see happen many times.

The power transformer should have a have a thermal-fuse inside, if you have an open primary. I've done surgery to "replace" them. You can fix it if you have patience and good hands.
Take the transformer apart, pry off the shielding covers until you can see the bobbin. Follow the primary leads, looking for breaks (sometimes the primary winding melts open at the point (hotspot) right where the wire runs into the bobbin) and looking for the "bulge" of the thermal fuse (size like a picofuse). I carefully cut the winding tape until I can get at it.

Otherwise, another shielded transformer about 28VCT gives +/-18VDC for the regulators. I see only a 3-wire secondary?

Thank you, that is good info but I honestly can't tell for some transformer wires if they are primary or secondary or center taps or what.

Here is a pic with the noise filter removed.

edit: added two pics of the transformer removed.

[stj]

Re: Korg Polysix synthesizer

black & green's are the secondary.

red & white look like the primary.
cant tell what the black is - brown is probably an unused primary for a different mains voltage.

and the yellow supressor - a cap with inline resistor in it is fucked, the resin is cracked.

[redwire]

Re: Korg Polysix synthesizer

The service manual gives transformer wiring colour code:
120VAC - orange
100VAC - brown (should be NC for 120VAC)
Common - white
Electrostatic shield - black (goes to ground)

Secondary 1 - green
Center tap - black
Secondary 2 - green

I'm not sure it has a grounded line cord.
You should get continuity across orange/brown, and much lower reading brown/orange. Nothing to black at all.

[SteveNielsen]

Re: Korg Polysix synthesizer

Ok thanks guys, I was confused by the two different black wires. All the windings seem ok with continuity checks. No the pwr cord is not grounded. You're right STJ the resin has cracks in it. Your eyes are better than mine but I'm 63.

The next thing I'll do is wire up the transformer to AC see if it works alone.

[SteveNielsen]

Re: Korg Polysix synthesizer

Transformer is ok. 120vac in, 46vac out, 22.8 to CT.

[SteveNielsen]

Re: Korg Polysix synthesizer

Both fuses on PUS board blew. Got a short someplace past the filters. Suspect voltage reg IC.

[redwire]

Re: Korg Polysix synthesizer

Bummer. I'd suspect a shorted rectifier, but I think you already tested them.

The IC2 (M5230L) Vreg IC's usually make smoke, with pass transistor Q4/Q3 shorting but none of the parts looked cooked. IC1 (4558 op-amp for +/-5V) might be suspect too.

I would use a variac and see who heats up, or pull each IC to see if shorted.

The M5230L is only rated for a max. +/-35V input and that transformer might give it over +/-31V, not much room for line surges.

[SteveNielsen]

Re: Korg Polysix synthesizer

There's really no telling what this poor device has been through. I wish I had a variac. I've been looking at some cheap low end, low current variacs. I can't afford one right now but I can save up for one.
I did check the rectifiers initially but I am gonna check them again.

I'll keep poking at the PS board and update what I find.

I discovered I put one of the fliters in reverse polarity but apparently it never got enough juice to damage it, it tests the same as before.