Korg Polysix synthesizer

[budm]

Re: Korg Polysix synthesizer

"Okay, Pos lead on anode, neg lead on neg leg of C102 reads 117V AC" That may be because the + has more load than the - supply. The +/- power supplies are tracking power supply, so if one goes up or down in Voltage it will track each other.

[SteveNielsen]

Re: Korg Polysix synthesizer

I think that must be an unintentional paste at the beginning there Budm

Your explanation makes sense though, thanks.

Redwire, remember this was a no load test. I'll be doing some loaded testing and the other suggestions and keep you all updated.

Thanks to everyone for your interest and help so far. You folks are great.

[SteveNielsen]

Re: Korg Polysix synthesizer

No shorts on any of the wires. All the solder joints look ok on the board.

I don't have suitable dummy loads to use. If I shoot for a .5 amp current that will be 7.5 watts and 30 ohms at 15v. Don't have any automotive bulbs.

[Agent24]

Re: Korg Polysix synthesizer

Got any 160-200 ohm or so 5 watt resistors? At 17v, Each one will draw about 100mA and dissipate 1.6 Watts. Start with one and add add more in parallel to increase the load.

[SteveNielsen]

Re: Korg Polysix synthesizer

Nope. I've got a 100ohm 10w and a 560ohm 5w, that's as close as I can get without ganging a boatload of 1/2 watts together, and I'm not sure I've got enough of workable values of those.

[Agent24]

Re: Korg Polysix synthesizer

Just those two? Might be good enough. Put both in parallel and on the 15v rails should draw about 200mA. Should be good enough to load the transistors and warm them up to see if anything weird happens. Unfortunately you can only test one rail at a time with that though.

[SteveNielsen]

Re: Korg Polysix synthesizer

So just load one side at a time? I can do that. Better than nothing eh?

[budm]

Re: Korg Polysix synthesizer

You can use 100W 230V LAMP as the load, the cold resistance is around 30 Ohms, you can easily check the cold resistance with the meter.

[stj]

Re: Korg Polysix synthesizer

dont people get "swat"'ed for owning those now!

[SteveNielsen]

Re: Korg Polysix synthesizer

230v lamps are not that common. I will get it worked out here soon and let you know.

[budm]

Re: Korg Polysix synthesizer

230v lamps are not that common. I will get it worked out here soon and let you know.

I forget that you are in the US. Use 40W lamp then, it has about 30 Ohms cold resistance, you can verify that with the meter.

[SteveNielsen]

Re: Korg Polysix synthesizer

Ok, thanks Budm. Mine measure 21 ohms cold but they're 130v. That will still work and draw around 1/2 amp.

Related and interesting, earlier stj advised me not to use halogen bulbs for current limiting on the ac line because the cold resistance is too low. However I have some 40w 120v halogens that measure 21 ohm cold as well. I think I ought to be able to use them as a load for this as well.

Should I dummy load the 5v outs too?

[stj]

Re: Korg Polysix synthesizer

not at the same time, you will overload it
maybe after the 15v is checked.

[redwire]

Re: Korg Polysix synthesizer

I think you're just making sure the power supply regulates and doesn't collapse under load. I'd just use car lights #194 (~0.3A) as a dummy load for a quick test.

The +/-15VDC regulators have current-limits about 1.2A, and the 5V regulators 0.6A so I wouldn't load heavier than that.
Note +/-15VDC regulator outputs feed the +/-5V regulators input. So a 0.6A load on the +5V and 0.6A load on the +15V is maxxing it out = 1.2A load on the +15V reg.- they add up.

Output voltages running (service manual) some are tight:
+15.000V +/-0.05V
-15.000V +/-0.3V
+5.000V +/-0.01V
-5.000V +/-0.01V

[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.

[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.

[stj]

Re: Korg Polysix synthesizer

Q4 and Q8 are first suspects.

got any freezer?

[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

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

[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