Korg Polysix synthesizer

[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

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.

[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.

[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.

[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.

[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.

[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

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

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.

[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

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.

[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

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.

[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

I wouldn't connect anything up at this point, in case it doesn't regulate and things get damaged.
Since the trimpot does something now, I suppose it's improved. I think a dummy load would be a good idea, just to see what happens. Better than blowing the rest of the circuitry if it doesn't work.

Can you post a photo of the track side of the power supply PCB so we can check it?

Ok, here's a couple of pics. It's a single-sided board. As you can see there are a few spots where a solder pad came off or loose, not from heat initially but from moving the soldered part. The bond between the traces and board is pretty brittle, probably from age, eh? Please excuse the less than perfect soldering, I've run out of thin enough solder and I'm not a perfect solderer anyway. There aren't any solder-bridge shorts though. There's one that on Q4 looks like it might be but it isn't. I cleaned that up since I took the pics anyway.

I agree with you to err on the side of safety, that's my usual mode of operation so thanks for steering me back to that. I am going to increase the resistance of the dummy load further though, I just want to test it for basic operation, not put it through the wringer.

[stj]

Re: Korg Polysix synthesizer

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

[Agent24]

Re: Korg Polysix synthesizer

I wouldn't connect anything up at this point, in case it doesn't regulate and things get damaged.
Since the trimpot does something now, I suppose it's improved. I think a dummy load would be a good idea, just to see what happens. Better than blowing the rest of the circuitry if it doesn’t work.

Can you post a photo of the track side of the power supply PCB so we can check it?

[SteveNielsen]

Re: Korg Polysix synthesizer

Update

I finished replacing the smaller electrolytic caps. I don't have replacements for the two big filter caps in the rectifier section but I've tested them and think they may be ok. For now at least.

The first cap I used to replace C1 with also looks like it leaked when I had a load on +15v and I thought I heard a little sound from it when powered up at first with the dummy load, so I removed it, cleaned the board again and used another replacement cap. This time I did not have a load connected and under these conditions the +15v trimpot now works, even though it still reads a volt too high set lowest. I haven't tried it with a dummy load though. I'm kind of afraid to. I'm thinking I should just connect up all the wires to the other boards and see what happens.

[SteveNielsen]

Re: Korg Polysix synthesizer

see if that regulator near the battery wires has 9v going in and 5v coming out.
btw.

Nope, input on the reg is only 2.4v, output is about 1v.

That thing looks like it has no input protection, you could well have blown the uC with a charged capacitor.

In any case, for your Synth... that wrong and nonadjustable voltage behavior sounds similar to the PSU in my DSA524. Do the output wires and load on yours somehow complete the ground connection for the power supply circuit, too?

Yes, it's a known fact that a charged cap can damage the chip. The manual warns of it.

Synth:
I honestly don't know if there is a ground return path from another board or not. Not sure how to test that other than connecting all the boards to the PSU and firing it up. Do you guys think I should do that at this point? The svc man says to adjust the voltage all the connecting wires should be made. I suppose that would be the next step but I'm going to replace 3 more caps on the PSU first.

Update: Replacing caps; C1 bung blew and spewed some fluid. It was hidden under the part but it definitely cut loose and leaked electrolyte.

[stj]

Re: Korg Polysix synthesizer

see if that regulator near the battery wires has 9v going in and 5v coming out.
btw.