Korg SP-500 repair

[Myoilu]

One of my more ambitious repair projects to date, I was asked to look at the digital keyboard the team I work with uses. Apparently, someone dropped it and it stopped outputting audio. It still turned on and everything else worked, but the output was not. After some work, I found that there was some output, but very very low and very noisy. I checked the output board and it seemed to be fine working with an external input, as well as the volume fader. So my next thought was the DAC. After looking around at it, I was suspicious that it wasn't getting proper power possibly.
-And here's where I goofed-
While checking the power connector I slipped with the probe and shorted one of the rails. And - POOF - the fuse blew. Got a new fuse, but now it just keeps blowing fuses, so I screwed something up.
Separating the power supply board and seeing if there was anything obvious on that I noticed what I think may have been the original problem with the board, broken leads to a transformer in the supply. luckily, I was able to get them re-attached. Nothing else at this point looked suspicious or bad. Powering the board by itself, nothing happens, but when it was connected to the main board it still blew the fuse.
At this point, not having a proper bench power supply (getting one now to hopefully help in the future) I put in a time delay fuse to see if I could find the component getting hot (great method-I know) The system ram chip was getting really hot, so my first thought was great! here's the short. Ordered a new one and replaced it. Still blowing a fuse. Not entirely sure where to go from here, so figured it's about time to ask for help.

Here is a link to the service manual with schematics.
https://www.manualslib.com/download/1096004/Korg-Sp-500.html

[LateLesley]

Ok, I had a quick look, and it looks like the power supply might be separate from the main board. It looks like the power might jumper down on connector CN78. Can you locate that connector, unplug it, replace the fuse, and then meter out the power rails if it powers up with out blowing the fuse?

on the connector pin 1 should be +5V, 2 and 4 are Ground, 3 is +12V, and 5 is -12V.

Lets see if we can verify the power first.

EDIT:and look for IC23, a 78L05 near the DAC. It needs testing, it's possible you popped it.

[Myoilu]

Here is what CN7B is measuring:

1: +14.25
2: gnd
3: +13.6
4: gnd
5: +0.6v

when it had been plugged in to the main board FET F1 had been getting hot, could it be bad/ causing these readings?

[Myoilu]

Okay, so I got a bench power supply to eliminate any possible issues from that board. Connected the rails as marked and now the screen turns on but it does not start up- all I get is a bar that moves across the screen once and then nothing.

Did I completely screw this one up? Is there any hope?

[DimitriP]

Here is what CN7B is measuring:

1: +14.25
2: gnd
3: +13.6
4: gnd
5: +0.6v

when it had been plugged in to the main board FET F1 had been getting hot, could it be bad/ causing these readings?

I hope the power supply was not connected and trying to power up the rest of the instrument.

[LateLesley]

Well, those voltages look baad! Though, I'm tempted to say that you might need to load it down a little with a few resistors to test it properly. Can you put a couple of 1K resistors between each power rail and Gnd, it's just to load it up a little, to sanity check those voltages.

If they stay the same though, than many of the 5V ICs in the machine will be dead, and it would be very difficult to repair. It those voltages are indeed correct I'd expect the ram, flash, Microcontroller and many other parts to have been overvolted, and damaged. In which case you'd be better just getting a new machine, as I doubt you'd get all the chips required, and even if you can, it'll probably add up to more than the cost of the machine.

[Myoilu]

I hope the power supply was not connected and trying to power up the rest of the instrument.

yea, unfortunately, it was...

If they stay the same though, than many of the 5V ICs in the machine will be dead, and it would be very difficult to repair. It those voltages are indeed correct I'd expect the ram, flash, Microcontroller and many other parts to have been overvolted, and damaged. In which case you'd be better just getting a new machine, as I doubt you'd get all the chips required, and even if you can, it'll probably add up to more than the cost of the machine.

given that it won't start up with the bench supply, I guess I have to wait for a used one to show up on ebay for parts, or diy a controller

Thanks for the help.