Korg Polysix synthesizer

[budm]

Re: Korg Polysix synthesizer

I wonder if the fuses should be slo-blo type due to that 4700uF filter cap has lost of inrush current.

[stj]

Re: Korg Polysix synthesizer

warm it up?

seriously - connect some small lights to the +/-15v outputs.
maybe 2w or 5w car interior lamps or similar.

did you check the wires the psu connects to for a short someplace forward of it?

[SteveNielsen]

Re: Korg Polysix synthesizer

Do you have any specific suggestions?

[stj]

Re: Korg Polysix synthesizer

my point is when it warms up the regulator may short out!

[SteveNielsen]

Re: Korg Polysix synthesizer

it may do something unpleasant when it warms up - you only did a quick no-load test.

The no load test shows there is no direct short on the PSU board, which was the entire point of doing so. Thanks for citing the obvious.

[stj]

Re: Korg Polysix synthesizer

it may do something unpleasant when it warms up - you only did a quick no-load test.

[SteveNielsen]

Re: Korg Polysix synthesizer

Thank you, I won't use the halogen. I found a suitable 25w bulb and wired it in series the hot 120vac line to the transformer primary. I put 3a fuses on the secondary lines (all I have available at the time) and fired it up. The lamp glowed barely for a half-second. No fuses blew and I measured +17vdc, -17vdc and 5vdc at the output test points. Subsequent power cycles the bulb does not glow at all and the same output voltages.

Edit: I think I blew the fuses before from having one of the caps reversed polarity. If the PSU is ok then the question in my mind is why did someone jumper the fuses?

The power transistors are not shorted to the heatsink.

[stj]

Re: Korg Polysix synthesizer

try it. - dont use the halogen though because the cold resistance is too low.

[SteveNielsen]

Re: Korg Polysix synthesizer

Least wattage bulb I have is a 40W from the fridge or a 40W halogen bulb. I'll figure that out somehow and try your suggestions. I don't want to keep wasting fuses in troubleshooting this though. If I used a 40W bulb will it blow the fuses or should I try it?

[redwire]

Re: Korg Polysix synthesizer

I would say this beast is so analog, the PSU must be low noise and not drift.
Tempted to put in three terminal regulators like 7815/7915 etc. but I notice this PSU is temperature compensated and has a balance trimpot. It needs a very good PSU.

With no variac, I would put a light bulb (<25W) in series (across power switch). Not enough to keep popping fuses, but will light up if the short is still there.

I would first pull IC1 (4558 +/-5V) and see if it comes up.
Then pull IC2 and see if if it comes up.
Also check the power transistors are not shorted to the heatsink.

[SteveNielsen]

Re: Korg Polysix synthesizer

Not necessarily. Are you thinking a different PSU? The thought occurred to me. I just like to try to fix things.

[stj]

Re: Korg Polysix synthesizer

are you looking to keep it original?

[SteveNielsen]

Re: Korg Polysix synthesizer

Yes, the two fuses on the power board. 1 amp fuses originally blown, someone wrapped wire around them to jumper them. I replaced for testing with two 2 amp fuses and they blew as well on the first test with just the transformer and the power board connected.

The rectifiers I checked again and they are still good. I'm still checking parts downstream and connections.

[stj]

Re: Korg Polysix synthesizer

blowing BOTH fuses??

[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.

[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

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

[SteveNielsen]

Re: Korg Polysix synthesizer

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

[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.

[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.