Korg Polysix synthesizer

[stj]

Re: Korg Polysix synthesizer

blowing BOTH fuses??

[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

are you looking to keep it original?

[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.

[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

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?

[stj]

Re: Korg Polysix synthesizer

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

[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

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

[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

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

[SteveNielsen]

Re: Korg Polysix synthesizer

Do you have any specific suggestions?

[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?

[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.

[Agent24]

Re: Korg Polysix synthesizer

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?

Because some people are silly and they were probably running it at maximum volume for long enough to blow the fuses.

Would still be smart though to put some dummy load on it and run it for a while.

[SteveNielsen]

Re: Korg Polysix synthesizer

Well, I do have a heat gun That'd be overkill though I think.

I don't think I've got any auto bulbs around, but I'll have to look. I've got several power resistors of various values though up to about 5k I think.

The wires are all routed clear of anything that could pinch them but I have not checked them for shorts. I could measure at the test points on the PSU and plug each cable in and check that way, if there's a shorted wire I should be able to narrow it down to a specific bunch that way. Thanks for the good suggestions.

[SteveNielsen]

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.

Nothing I see indicates slo-blo but that brings up a question I have in mind. Why is there a 4700uF on the + input and a 2200uF on the - input of the regulator chip (IC2)?

Because some people are silly and they were probably running it at maximum volume for long enough to blow the fuses.

Would still be smart though to put some dummy load on it and run it for a while.

I would not doubt that fuse blowing scenario at all. Looking at the poor thing it has obviously been through hell. Knowing who it came from only casts more suspicion of abuse. Ironically, her father was an electronics tech.

I agree on the dummy load thinking. I just have to see what I've got around here for dummy loads that could work. I think I'll need 3 loads, one for each; -15v, +15v, and +5v. I don't know how much current draw each voltage line should be tested at.

[Agent24]

Re: Korg Polysix synthesizer

Since the original fuses are 1A, don't go beyond that. Half that at 500mA per rail or so is probably a good start.

[redwire]

Re: Korg Polysix synthesizer

... I measured +17vdc, -17vdc and 5vdc at the output test points...

Wait, it's a +/-15VDC power supply, not 17V? Time to turn the trimpot?
There's also +/-5VDC outputs; four outputs in total.

[Agent24]

Re: Korg Polysix synthesizer

Playing with trimmers of any kind is a bad move until you rule out other problems.

R13 or R14 or R17 could have drifted. The thermistor TH1 could be bad. Check values and solder joints on all of them.