Korg Polysix synthesizer

[stj]

Re: Korg Polysix synthesizer

i thought you had already re-capped it!

what did you meter the cap with anyway?

[SteveNielsen]

Re: Korg Polysix synthesizer

No I didn't recap the board. I tested all the caps with a Chinese knockoff of a Karl-Heinz K ̈ubbeler AVR-based type transistor/esr/etc. tester.

[stj]

Re: Korg Polysix synthesizer

those only test using voltage, not current.
though to be fair, a *real* cap tester costs about 300-400!

[SteveNielsen]

Re: Korg Polysix synthesizer

I know, I can't afford that. I'm lucky to have $30-40 I can spend.

I'll see what I have for replacement cap(s). I know I should have re-capped it first thing considering the age of it alone. I'm learning, sometimes the hard way

[Agent24]

Re: Korg Polysix synthesizer

Sounds like C10 had high leakage current or was partially shorted.

[budm]

Re: Korg Polysix synthesizer

Ii am sure you have Variable DC power supply and DCA to check the leakage current of the suspect bad caps.

[SteveNielsen]

Re: Korg Polysix synthesizer

Well this is interesting, I replaced C10 and am now getting different but still wrong results. The +15v with a load (2.2K ohms, 5w. Lower ohm loads were getting a bit hot, even the light bulbs were warming up) fluctuates between 19-21v and the -15v does the same with negative voltage. +5v fluctuates between 5-6v. Without a load on +15v it goes down to +16-17v, still fluctuating, but no changes on the others. Adjusting the +15 trimpot has no noticeable effect on anything. Putting a load on the other outputs has no effect at all.

Budm, I do have a variable DC supply 1.2v - 27v @ .2A, I don't know if that would suffice and I've never done that before. I looked up some info about it and the sites I've read talk of charging the cap and then measuring the charge decay over time but also say it's not a very good test. How do you suggest to proceed with that?

I know I ask questions like a newbie but I'm not that great of an electronics tech to begin with (most of my work exp has been with computers/networks with board/component work being a very long time ago), my memory is starting to go little by little, and I want to make sure of things, or as sure as I can anyway. Thanks everyone for the patience and help.

Forgot to mention I did pull and test Q4 and Q8, they are ok.

[stj]

Re: Korg Polysix synthesizer

recap the whole mess.
then see how it runs.

[SteveNielsen]

Re: Korg Polysix synthesizer

That's what I'm in the process of doing. Unfortunately I will have to spend money so it's going to wait a little while. My Chinese transistor/etc tester just died too. At least I can still measure capacitance on my DMM.

[stj]

Re: Korg Polysix synthesizer

seriously? there's hardly anything to die on them!
you did discharge the caps you wanted to test i hope - the testers are 3.3v

[SteveNielsen]

Re: Korg Polysix synthesizer

I am very good about discharging caps before measuring them. I may have missed doing so to the last one. It's usually my habit to discharge but I don't know. However, it failed right after I had re-soldered a test lead to it that the strands were breaking on. It's soldered to a length of solid wire that is soldered to a PCB pad, so I didn't overheat anything or that bit of solid wire would have come loose. Regardless, the little guy won't fire up. Oh well. I've gotten a lot of use out of it. I'll get another when I am able to.

Edit: it was also running on a regulated DC power supply not a battery, so I suppose a momentary power fluctuation or spike that didn't get filtered out could have killed it but I've been using it that way for months. *shrug*

[stj]

Re: Korg Polysix synthesizer

post a foto of the parts-side so i can see the design.
it may just be a voltage regulator.

btw, i just ordered this model.
i chose it because it has a 3pin connector and flyleads.
http://www.ebay.co.uk/itm/181861030068

[SteveNielsen]

Re: Korg Polysix synthesizer

post a foto of the parts-side so i can see the design.
it may just be a voltage regulator.

btw, i just ordered this model.
i chose it because it has a 3pin connector and flyleads.
http://www.ebay.co.uk/itm/181861030068

I like that one. I think I want one too.

[Agent24]

Re: Korg Polysix synthesizer

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?

[stj]

Re: Korg Polysix synthesizer

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

[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.

[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.

[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?

[stj]

Re: Korg Polysix synthesizer

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

[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.