Korg Polysix synthesizer

[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?

[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.

[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

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

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

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

recap the whole mess.
then see how it runs.

[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.

[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.

[Agent24]

Re: Korg Polysix synthesizer

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

[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

[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

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

i thought you had already re-capped it!

what did you meter the cap with anyway?

[SteveNielsen]

Re: Korg Polysix synthesizer

Just the +15v went up.

I may have found the culprit, C10, and I was suspicious of it earlier but didn't mention it because out of circuit it tested good.

I ran the board with a 40 ohm load just on the +15v for about ten minutes but voltage gradually increased to +26v, so I thought I'd heat up an area with the heat gun on low and watch the voltage. I was focusing around IC1 area where +15v originates and as it warmed up the voltage began to drop slightly, which I attributed to the thermistor responding, it got down to about +24v. After a very short time (25-35 seconds) with the heat gun on low about 3-4" away from the board C10 popped and fizzled and released a bit of magic smoke, and the +15v output remained at +25.2v.

The reason why I was suspicious of C10 was when checking for shorts from the outputs to GND the +15v behaved differently than the others, on a higher resistance scale it showed a cap charging up (as normal) but it reached a certain point and remained and a fixed resistance about 1K or less while the others eventually read out of range. Like I said though I pulled it out and it tested fine. I attributed the behavior to it being in the thermistor circuit. It's obviously not fine now anyway. I'll replace it and see what it gets me

[stj]

Re: Korg Polysix synthesizer

before you do, was it only the +15v that went full-on, or the -15v too?

[SteveNielsen]

Re: Korg Polysix synthesizer

That's not good! Something wrong there definitely. Though if the -15v rail is OK, it may not be the regulator, just some component on the +15v rail side that is bad, like a capacitor gone open causing oscillations or something, perhaps. Maybe the pass transistors are bad...

Could even be a bad solder\connection issue.

I have tested all the electrolytics and they're ok. I've checked the thermistor and all the resistors around the regulator chip and +15v output and they're ok. I've looked at solder joints in both areas and others, no evidence of bad joints.

Q4 and Q8 are first suspects.

got any freezer?

No but I've got the next best thing, some canned air that is useless for anything else besides scaring cats away and freezing hands and parts

I chilled out Q4 and Q8, no significant changes, I also froze all the other transistors and both chips in turn with no significant changes.

Now I am thinking of a part that fails when cold but works when heated and using heat to find that. As I mentioned in the 1st post the person who gave it to me said it had to warm up before it would work. I have a heat gun but no nozzles to confine that heat to a specific part. I have a temp controlled soldering iron but the lowest temp is 200c/392f. Should I try the iron at 200c?

[stj]

Re: Korg Polysix synthesizer

Q4 and Q8 are first suspects.

got any freezer?

[Agent24]

Re: Korg Polysix synthesizer

That's not good! Something wrong there definitely. Though if the -15v rail is OK, it may not be the regulator, just some component on the +15v rail side that is bad, like a capacitor gone open causing oscillations or something, perhaps. Maybe the pass transistors are bad...

Could even be a bad solder\connection issue.

[SteveNielsen]

Re: Korg Polysix synthesizer

Yes, and that's good info. I'd have found it in the manual when I got to that point but you saved me the trouble

+15v output shoots up to 23v with a load. I think that indicates the regulator has a fault. The other voltages remain ok when loaded. I didn't load the 5v lines at the same time as the 15v lines, and the same results when loading one output at a time. I used both the 20 ohm and 30 ohm loads on the +15, same result.