Korg Polysix synthesizer

[SteveNielsen]

Re: Korg Polysix synthesizer

i think your dead tester is just a transistor.
look at the pdf i linked - page11

I did look and have questions but I think I ought to start a new thread about this.

[SteveNielsen]

Re: Korg Polysix synthesizer

Tested the PSU board with a 15K and a 7.5K load on +15v test point with similar results for both. With the 7.5K load measured voltages ranged a volt or so higher than with the 15K load, the +15v trimpot did adjust all the voltages in both cases, however they all fluctuate by 1/2 to 1 volt or so, and unable to get it down to +15v, and -15v runs about one volt lower than +15v, meaning if the +15v reads +16v the -15v reads -17v. +5v never gets down to 5v either.

[redwire]

Re: Korg Polysix synthesizer

OK, so note the +15V is the master regulator. If it is wrong, it messes everything up: the -15V, the +/-5V too. The +/-5V regulator just gives 1/3 of the +15V regulator's output. The -15V part just mirrors the +15V.

So I would first fix the +15V reg. If you can measure IC2 pin voltages, that helps. It's reference voltage should be around 1.8V (pin 8 to gnd) and (pin 5 to gnd).

I would say IC2 (M5230L) is bad, or Q4 is bad. Overvoltage could have also killed IC1 (4558) as max. voltage for it is +/-18V.
The power supply has a design flaw, namely that if one fuse pops (but not both) it can damage the IC's from reverse voltage.

[SteveNielsen]

Re: Korg Polysix synthesizer

Thank you. I have thought of that too. Should I measure IC2 voltages with or without a dummy load? Nevermind, I'll do both anyways.

[SteveNielsen]

Re: Korg Polysix synthesizer

The regulator is working and the trimpot works but the voltages are still too high with or without a load. The balance between +15 and -15 is off by about 1v. Pin 8 (V ref) is not used in this ckt. Pin 5 (V adj) has 2.0v to 2.1v

[SteveNielsen]

Re: Korg Polysix synthesizer

Problem solved. It was the big filter caps on the transformer secondary that I don't have replacements for. I subbed a 6800uF 56v cap for C14 (4700uF 35v) and a couple of caps in parallel;1500uF 50v and 1200uF 50v equaling 2700uf, subbed for C13 (2200uF 35v) and the PSU is operating perfectly now, with or without dummy loading. I've got +15v, -15v, +5v, -5v steady and ready, the v adj pot does it's job (I can get from 13v to 18v). I'll buy some proper replacement caps in a few days for them.

[budm]

Re: Korg Polysix synthesizer

Great job, I wonder the bad reading was due to AC ripple riding on DC due to poorly filter and caused the power supply to be unstable.

[SteveNielsen]

Re: Korg Polysix synthesizer

That's basically what I think too. The caps were unstable and passing too much AC ripple/noise. I wish I could have replaced them off the bat just due to their age. Would have saved some time but in the process I tested just about every other single component on the board so there's really no regrets there

[budm]

Re: Korg Polysix synthesizer

I do not know how good your meter is, but since are not dealing with switching power supply, the AC ripple frequency is only 120Hz so you can set the meter to ACV to see the AC Voltage on the, it should be very low in 100's of mACV range.

[Agent24]

Re: Korg Polysix synthesizer

Problem solved. It was the big filter caps on the transformer secondary that I don't have replacements for. I subbed a 6800uF 56v cap for C14 (4700uF 35v) and a couple of caps in parallel;1500uF 50v and 1200uF 50v equaling 2700uf, subbed for C13 (2200uF 35v) and the PSU is operating perfectly now, with or without dummy loading. I've got +15v, -15v, +5v, -5v steady and ready, the v adj pot does it's job (I can get from 13v to 18v). I'll buy some proper replacement caps in a few days for them.

Good to hear you figured it out

Don't take this the wrong way but this is a good example of why they always say only play with trimpots as a last resort. You will need a very good voltmeter if you want to get the voltage back within that very tight spec in the service manual.

[stj]

Re: Korg Polysix synthesizer

also, this is 5 pages+ because you didnt just recap it first.
anything that old and i wouldnt even bother powering it up without a pre-emptive recap.

[SteveNielsen]

Re: Korg Polysix synthesizer

also, this is 5 pages+ because you didnt just recap it first.
anything that old and i wouldnt even bother powering it up without a pre-emptive recap.

Yeah, you're right, I've wasted so many pages and it's not about something really important like conspiracy fantasies.

[SteveNielsen]

Re: Korg Polysix synthesizer

Good to hear you figured it out

Don't take this the wrong way but this is a good example of why they always say only play with trimpots as a last resort. You will need a very good voltmeter if you want to get the voltage back within that very tight spec in the service manual.

I'll be ok with my DVM. The 20v scale can measure down to .01v so I can get it within specs.

[stj]

Re: Korg Polysix synthesizer

theorys - theorys!
and it's not like you can recap a government - pre-emptively cap a few members maybe, but that wont generally fix the problem!

[SteveNielsen]

Re: Korg Polysix synthesizer

LOL! True. True. Well, we'll see how many more pages it gets to because the PSU of this thing is just the beginning of the project. There are like 9 or 10 other boards in this thing!

[stj]

Re: Korg Polysix synthesizer

yes, i was just thinking about that, all those op-amps and the logic boards.

lets switch to fixing your component tester - go link the red battery wire to the regulator input and see if it turns on when you connect the battery.

[SteveNielsen]

Re: Korg Polysix synthesizer

Yeah, I already miss that little guy.

[SteveNielsen]

Re: Korg Polysix synthesizer

What would be the danger of significantly reducing the capacitance on C13 and C14?

I tried a 1200uF in place of the 2200uF (C13) and a 1500uF in place of the 4700uF (C14). The output is the same with or without dummy loads. I checked AC ripple at the outputs and it is below 5mV, same as using the stacked caps for higher capacitance.

[stj]

Re: Korg Polysix synthesizer

it may not cope well with a varying load.

[SteveNielsen]

Re: Korg Polysix synthesizer

I'll eventually get the right spec caps for it when I can afford to. I'm a poor retired man on a fixed income All I have to use are salvaged parts. In the meantime I'm experimenting