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[natere2ster]

Hello fellow circuit benders,

First post here, I hope its in the correct location!

I have a korg poly 800 on the bench here and its got an odd issue problem.

On batteries, it powers on fine.

On ac wall wart, it does not power on.

The wall wart is verified to be giving out 13V to the PCB, where I can measure it on the DC jack as well as on pin 11 on the CN06A connector (its tied to the hot of the DC jack)

I suspect the voltage regulator transistors might be at fault but am not an electronics wiz (or even very knowledgeable about some things!)

Since these are not expensive parts I am considering replacing both Q1 and Q3 (both 2sb744) but would like to know anyones advice and guidance on how to check them...what should the voltage be on both "sides" of Q1 & Q2?

From the schemos I see that I need to get +5V out?  Is that correct?  Drop and regulate a +/_5V for transistor logic?
Again everything is fine off the batterys just the DC in does not power the unit on at all....

Cheers!

Nathan

[natere2ster]

thanks for moving it to the correct place!  DOH!

 

Cheers!

Nate

[Gordonjcp]

13V seems a little high; I'd be looking for about 10-11V with the power supply loaded.  Check it's the correct polarity and that it puts out at least 750mA - the Poly 800 is a hungry beast.  More current is fine, less will give you problems.

[natere2ster]

how can i measure the current?

i agree 13V unloaded seems high but i have tried 2 different korg ac adapters, the original ones, and they both are outputting ~13V.   when loaded its still around that!

how do i measure the current?  I dont have an ammeter...
 

I will Google and see if i cant plug it into some circuit on the breadboard....

I setup a "transistor tester" and both Q1 & Q3 seem fine (at least they work in common emitter mode)

Can you explain how the power supply works on the Korg?  Or do you know of a handy resource that explains it?

[druzz]

one of my friends add a problem with the power input on his poly -800  and we couldnt figure it out so he made a new power input ( a 1/8'' jack) and i think he connected it to the battery poles  . 

[Gordonjcp]

http://www.oldcrows.net/~patchell/poly800/poly800.html

On sheet one, about half way down the left hand side.  The PSU input regulator consists of Q1 and friends, a 2SB744 PNP tranny and a Zener diode in a fairly ordinary configuration.  Note that the regulator actually sits in the *negative* supply rail so you'd expect to see a difference between PSU ground and chassis ground!

It's also worth mentioning (and I'm sure you know this) that Zener diodes work "backwards", that is to say that they work as a normal diode with about Vf about 0.5V but when wired up with positive to the cathode they break down at a specific voltage.  So for the Zener regulator to work you stick the "wrong" end to ground and feed it through a resistor.  Now in this case "ground" with respect to the voltage regulator is actually the positive input so the cathode of D4 goes to positive, R2 goes between D4 and PSU negative, and you should see about 9V across D4.  Measured from PSU ground you should see whatever the input voltage is minus whatever you see across D4 - let's say 12V and 9V so you should see 9V across R2 and about the same across the collector and emitter of Q1.

Hope this makes sense, I'm barely an inch into my first coffee of the morning.

[natere2ster]

When I take the voltage measurements, do I need to have any of the connectors on the board (CN6, etc.) plugged in? 

Will that cause any change in the measurements? 

Right now I only have the DC jack plugged in for the voltage supply but nothing else...

Cheers,

Nate

[Gordonjcp]

It's probably a good idea to have the board hooked up exactly as it would be normally, yes.

[natere2ster]

Ok, I have an update that might or might not be useful.

I put Q1 & Q3 back on the board, soldered them in, and am still getting voltage in from the DC jack, across D2, and can measure V(in) on the collector of Q1...but thats where it stops.

Caveat:  Yesterday, after I put both transistors back in, plugged the wall wart in and got some coffee...

I came back to a burning smell and instantly ran over to the wart to make sure it was not on fire! 

It was not smoking or anything but was VERY hot to the touch and smelt of burnt plastic...

After airing out the shop I noted that the 9V supply I was using, with a switchable polarity, was only rated at 500mA not the 700mA like the korg adapter....not sure what the wattage was, on the korg its rated at 15W on the input...

I guess the Korg was pulling some juice!

Anyway, I took a break and built my Thingamagoop 2....and now I am back the the Korg
 

I still suspect the power regulators....

Anyone know of a suitable replacement?  Q1 is 2SB731 and Q3 is 2SB744

I would like to swap them both out and re test....

Even though they seemed to work fine on the breadboard!

 

[Gordonjcp]

Have you checked that the power supply is the correct polarity?  A 500mA power supply should just about run the Poly-800 but might hum a bit - it should *not* overheat.  That sounds like something has shorted across the output of the PSU, which suggests the protection diode in the Poly-800!

If the power supply will run a Casio or a Roland synth, it'll run the Korg.  Yamaha ones are the opposite polarity.

[natere2ster]

Yes I looked at the protection diode and could read a voltage across it in both directions.  One was +13V the other -13V...
Perhaps this is an issue?!  If that is bad and it was not protecting what else could it have fried?!?!  AHHH!

As far my my power supply its one of those switchable polarity / switchable voltage adapters.

I will swap it out and see if that helps any...

Can I use a 1N4001 for the D2 diode?

Thanks!

Nate

[Gordonjcp]

How exactly did you measure the voltage across it "in both directions"?  Measuring the voltage won't be helpful but measuring it with the diode check range of your multimeter might - obviously with the PSU disconnected.

What it does is short across the PSU if you plug one with the wrong polarity in.  A 1N4001 would be just about perfect.

So, to recap - measure the voltage across D4 which will tell you what the regulated input voltage is - IIRC about 9V.  Put the negative probe of your meter on battery negative, and measure between there and the negative terminal of your power supply.  With 13V input you should see about 4V across Q1's emitter and collector, and about 9V across C3.

[natere2ster]

actually i AM getting something across C3, its around 0.8V...

 

Also, like I am getting around 4V across Q1's EC...

I turned off the synth and it doesn't read anything if nothing is "pulling" at it!

The collector is the middle pin, and when i put my black probe (GND) on the middle pin and measure 1 pin i get 3.7V...when i measure the other pin, with the collector still connected to my black probe (GND), I get 13V...

[natere2ster]

I get for D2, the "protection diode", V=13
R2 I get 3.7V across (should be 5V?!)
D3 I get .413V
D4 I get 10.3V

Across Q1, emitter to collector, 12V
Across Q1, Collector to base, 3.72V  (should be more that 5V no?)
C3 0.8V
C4=0.85V
On CN6A, ping 6, still getting 0.4V (as measured from pins 3 & 4 on CN9A)

Looks like i am not getting the full 5V anywhere...

I found a place that has the replacement transistors so I am gonna pick those up and swap out and see if that does anyhting to improve the output

[natere2ster]

replaced Q1, ot works!

thanks for the help.