Sudden Death - KORG Prophecy

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denmla
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Sudden Death - KORG Prophecy

Post by denmla » Fri Jan 08, 2021 10:25 pm

Hi to all, I’m new here!

Few moments ago my beloved Prophecy died - it wont turn on.
Pressing the power button only makes a small light change in button color but LCD does not power up.

Very very quiet but high pitched squeal/noise is heard From the PSU area while turned on. The sound keeps going until the device is shut down. Then, the sound decays slowly…

- I checked the fuse -> its good
- device was working normally yesterday and was in the studio so no moisture etc.
- measured 5.2V betveen red and black vire on the port which goes to motherboard

I captured the sound on video. When I disconnect the motherboard the sound is not heard:


Anyone familliar with the simmilar issue?

Thanks a lot for all the help!

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meatballfulton
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Re: Sudden Death - KORG Prophecy

Post by meatballfulton » Sat Jan 09, 2021 1:45 am

Might be tripping here, but I seem to recall hearing about noises like from bad LCD displays.
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sat Jan 09, 2021 8:44 am

Hi meatball fulton, thanks for tuning in!

No you are not tripping, KOG Wavestation has that problem of simillar buzzing sounds comming from display area. However, as far as I'm able to accoustically locate the sound source it is definitely comming from the PSU, and ONLY when PSU is connected to the rest of the synth (only one 4 pin connector going to mobo).

Thanks

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Re: Sudden Death - KORG Prophecy

Post by Ashe37 » Sat Jan 09, 2021 10:19 am

transformer whine? i couldnt hear the sound in the video.

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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sat Jan 09, 2021 10:35 am

what about the voltage on the orange and black wires, is it 12V ?
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sat Jan 09, 2021 1:48 pm

Guys, thanks a lot for tuning in!

@Ashe37 - it's quiet, turn up the loudnes or put it to 720p on YT, maybe its filtering the top end. Its igh up 15kHz+

@Yekuku - Orange to black (both) is 15V exact (15.007 V)! What would that point to?

Thanks, I really apprechiate the help!

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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sun Jan 10, 2021 10:08 am

The 5.2V that you have measured is a bit on the high side .
The 15V should be good , if i remember correct there is an error in the service manual, showing 12V.
How did you take measurements, with the psu connected to the mainboard or disconnected ?
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sun Jan 10, 2021 10:25 am

Hi Yekuku, thanks again for helping out!

I did measure in both cases, WITH and WITHOUT mainboard connected:

1. Mainoard DISCONNECTED:
- orange to black == 15V
- red to black == 5.2V

2. Mainboard CONNECTED:
- orange to black == 5.9V
- red to black == 1.7V

Also, I recorded the video of the powerup and buttons lit up (more than I expected at least):


I mean, I would be happy if it was power supply, but the fears started that it could be the mainboard that went :cry: Can I know that somehow?

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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sun Jan 10, 2021 10:30 am

DO NOT POWER UP AGAIN!
I will reply as soon as I can
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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sun Jan 10, 2021 10:52 am

Denmia unfortunately I have bad news.
The PSU without load looks good, there is only a small chance that when load is connected the PSU collapses, but I doubt it.
From what it looks like you might have a short on the analog voltage lines ( +15v -15V) thus dragging the 15V down to 1.7V and unbalancing the the voltage outputs (5v,15v) of the PSU .
5.9V for the digital power line is no good, there is a chance that some digital components might already have fried.
I have a few thoughts about how to continue but there is risk of doing further damage to the unit, so lets wait to hear what other people have to say.
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sun Jan 10, 2021 11:07 am

Thank you so much. Bad news still helps as I'll be hunting for main board insteand of PSU if determine that is the way to go....

Generally speaking, we can continue even if that means further damage to the unit as it is better than "status quo" but surely can wait a bit more.

So connecting the mainboard drags 15V to 5.9V orange to black and drags 5.2V to 1.7V red to black :(

Also, here is a "squeal" amplified and it is comming from the PSU transformer:


Thank you for your time

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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sun Jan 10, 2021 11:42 am

The squeal is produced because the psu is stressed due to the short , it is probably one of the transformers or the coils producing it.
When mainboard is disconnected , the squel stops, right ?
I ll have a look on the schematics and I will reply back.
Last edited by Yekuku on Sun Jan 10, 2021 1:48 pm, edited 1 time in total.
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sun Jan 10, 2021 1:45 pm

Yes man that is correct!

Squeal is produced by the transformer:
Image

When I disconnect the mainboard it goes away.

Here is the KORG Prophecy service manual:
https://www.synthxl.com/wp-content/uplo ... manual.pdf

Thanks you so much man for taking the time.
I'm ready to go as far as needed and invest as needed to bring this little bugger back to life!

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Re: Sudden Death - KORG Prophecy

Post by Yekuku » Sun Jan 10, 2021 2:58 pm

Even if I think that the problem is not in the PSU , we have to rule this out first.
In order to understand if it is working right , you have to test it , without the mainboard connected, but with a load on it.
You can use resistors as a load.
I did a bit of calculations, but please feel free to calculate these too, just to be sure.
Connecting a 10 Ohm/10 watt resistor to the 5V power line should pull 0.5A from the PSU
Connecting a 150 Ohm/5 watt resistor to the 15V power line should pull 0.1A from the PSU
With the resistor connected to the 5V/15V lines power up the psu and take voltage readings, you should have 5V and 15V exactly and this should rule out the PSU.

I ll be studying the schematics , and I ll continue my post later.
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Re: Sudden Death - KORG Prophecy

Post by denmla » Sun Jan 10, 2021 4:42 pm

Awesome Idea.

Sure enough I'll get to electro shop to get loads and to measure current draw from PSU.

I=U/R cant be simpler than that!

Thanks man, will report back once I got the results.

:)

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