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Old 01-08-2021, 02:43 PM   #1
denmla
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Default Anyone into hardware synth service? - KORG Prophecy just died

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.

- 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 wire on the port which goes to motherboard

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 captured the sound on video. When I disconnect the motherboard the sound is not heard:
https://youtu.be/gBf0GMobK74

Anyone familliar with the simmilar issue?

Thanks a lot for all the help!

PS

Last edited by denmla; 01-08-2021 at 05:05 PM. Reason: Video added of the "squeal" sound
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Old 01-08-2021, 03:19 PM   #2
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It could Be your Power Supply!

Check and confirm if it is, you may be able to get a replacement.

My Peavey StudioMix died about a year ago and it was the power supply.

Got some Part#/model# off it and did a search on the internet found it and purchased it, installed it and now everything works.
It only cost me about $60.00


Robert
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Old 01-08-2021, 05:02 PM   #3
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Hey, thanks for tuning in rncwalker!

Yeah, no doubt, I did that first! The problem is the PSU is internal (KLM-1856) an listed everywhere as out of stock!

Tried few "famous" synth part shops, ebay etc.

Looks like I'm going to need to fix my own...

Thanks
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Old 01-09-2021, 12:37 PM   #4
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The Peavey StudioMix has the PSU inside the unit, I Opened the StudioMix and verified that the PSU was bad. Took out the PSU and found the S# and Model#'s and did a search on the internet for it.

Don't search for the Prophecy parts, but just the PSU.
The same PSU is most likely in a 100+ other products.

I found mine not on Peavey's or Cakewalks Websites but from a company that makes powers supplies.

When I find where I got it (info that is) I will post the information.

Robert
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Old 01-09-2021, 03:43 PM   #5
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Hey man thanks again.

The board seems unique for this synth unfortunately an the part number is
KLM-1856

I know how to search (or at least I believe so) and wasn't able to find a shop with the part on stock...

Thanks and have a good one
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Old 01-10-2021, 10:53 AM   #6
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I cannot find the information where I got my "PSU" yet (In the middle of moving right now).

If you are adventurous and willing to get your hands dirty, try this:
Remove the PSU from your synth and find someone local that could check and possibly repair your PSU or setup as noted below.

If that cannot be repaired, find out the voltage that the PSU supply's to your synth (Both volts and amps).

With that information you may be able to use a external PSU (or power brick) and wire up a connector port on the synth for the power to connect too.


I did some looking searching on the internet as well for you and found nothing.


Hope what I suggested may help!



Robert
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Old 01-10-2021, 12:05 PM   #7
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Hey man thank you very much for all the help!

I'll be repairing the power unit, and report back what went wong for others that might have the simillar issue.

You did all you could and I wish you a pleasent stay in your new home
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Old 01-10-2021, 12:09 PM   #8
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Is this the right service manual?
https://www.synthxl.com/korg-prophecy/
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Old 01-10-2021, 12:20 PM   #9
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Quote:
Originally Posted by denmla View Post
I'll be repairing the power unit, and report back what went wong for others that might have the simillar issue.

Could be the "Cap's", if any of them have a bulging top, it's bad!

Hope you get it fixed.

Stay safe


Robert
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Old 01-10-2021, 12:23 PM   #10
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Yes it is... particulary ths one from that site:
https://www.synthxl.com/wp-content/u...ice-manual.pdf
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Old 01-10-2021, 12:51 PM   #11
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Cool. N.B. According to the block diagram, the psu seems to have +5 & +12 ?

I guess the +5 is for the digital stuff & the other one for the analog side.
Shouldn't be too hard to throw a new psu together.

Altho the PCB seems to say 15v ???




Anyhoo, it's something to test.
Attached Images
File Type: png korg_psu.png (40.4 KB, 274 views)
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Old 01-10-2021, 01:11 PM   #12
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Chances are if it has electrolytic capacitors, which it probably will as a PSU, changing those out will most likely fix it (the whine is also a clue) - as they die over time just sitting there. And I would not trust them by looking or basic measurement, just change them if the unit is old enough. I've repaired a number of pieces of gear where the caps looked fine and measured fine with a cheap meter but were still bad.

Measuring the PSU voltage "might" help but it could read right and the filtering caps (as above) are allowing too much AC ripple into the DC output which may or many not be noticeable on the meter.
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Old 01-10-2021, 02:32 PM   #13
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Quote:
Originally Posted by karbomusic View Post
Measuring the PSU voltage "might" help but it could read right and the filtering caps (as above) are allowing too much AC ripple into the DC output which may or many not be noticeable on the meter.
It's not always the caps None of them seem to be bulging. You'd have to take the pcb out to see if any of them have lifted off the board. But perfectly good-looking caps can be defective. If you have (or know someone who has) an oscilloscope, you could measure the ripple (with a dummy load).

My first step would always be to check the volts on the psu. Both o/p rails. But that's me.

I agree tho' that failure of electrolytic capacitors is most likely. It's a common fault in older gear. That said, I've got some ancient stuff that's absolutely fine. As well as some (pump motor) caps that last about 18 mo. It's a temperature thing.

They only cost pennies, so (if you're handy with a soldering iron) it's probably worth replacing them anyway. It may just fix it.

If you don't fancy having a go yourself, there must be a synth doctor near you who can have a look?

Or I'll buy it off you, spares or repair. £20?
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Old 01-10-2021, 02:44 PM   #14
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Quote:
Originally Posted by jrk View Post
It's not always the caps None of them seem to be bulging. You'd have to take the pcb out to see if any of them have lifted off the board. If you have (or know someone who has) an oscilloscope, you could measure the ripple (with a dummy load).
Of course it isn't always but they are the number one failure point with age, often the easiest and cheapest to fix, and the last 3 devices I've repaired, the caps "looked" perfect...

Fireface UFX, Fireface 800, Egnator Rebel 30.

In other words bulging is even less of an "always". The point there is it's cheaper to just replace if you are going to take it a part anyway- unless it is overly troublesome or there is some piece of evidence that is greater than it being caps or just completely out of budget - if the unit has age though, they should probably be replaced cuz their gonna degrade most likely - especially since the PSU is failing.
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Old 01-10-2021, 02:55 PM   #15
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You're quite right - I edited my post to include "But perfectly good-looking caps can be defective", but you read it first.
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Old 01-10-2021, 02:59 PM   #16
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Hey guys that's some seriously great and constructive suggestions going!

Thanks karbomusic & jrk!

I will report after further investigation...
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Old 01-10-2021, 03:02 PM   #17
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Quote:
Originally Posted by jrk View Post
You're quite right - I edited my post to include "But perfectly good-looking caps can be defective", but you read it first.
All good, I agree with your points.
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Old 01-12-2021, 01:30 PM   #18
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Prophecy is alive!

Control resistor in place of this little blue R2 resistor went and synth would go to sort of safety mode. Replaced with new and all works.



Thanks everyone for helping out and hopefully somebody in the future will learn something from this thread!

Cheers Reaperites
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Old 01-12-2021, 01:32 PM   #19
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Good job!!!
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Old 01-12-2021, 02:15 PM   #20
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Cheap fix! Nice one!
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