Korg Forums

[denmla]

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

And a few hi-rez images of the PSU:
https://i.ibb.co/Ky6pkVZ/20210108-234403.jpg
https://i.ibb.co/PThHwp9/20210108-234417.jpg
https://i.ibb.co/wzgF8Wr/20210109-002707.jpg

Anyone familliar with the simmilar issue?

Thanks a lot for all the help!

[voip]

Switched mode power supplies can be difficult to troubleshoot without specialist equipment.

There are two outputs from the KLM-1856 PSU. There should be 5V between CN51 pins 3 and 4 and somewhere in excess of 12V between pins 1 and 2. The latter is regulated to 12V by IC13 on the KLM-1846 main board.

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[denmla]

Hi voip, thanks a lot for tuning in!

Definitely a great help here bu I get 15V between 1 & 2 and 5.2V 3 & 4!

So 3 & 4 are OK but 15 seems a little much for 1 & 2 doesn't it?

Thanks again man!

[voip]

15V doesn't seem too far off the mark, since it needs to be regulated to 12V by a 78L12 regulator, which has a dropout input voltage of around 14V, so it has to be at least that. If the regulator board whistles and takes a while to decay on power off, that would suggest that there isn't much current being drawn from it for some reason by the synth circuitry. Investigate that.

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[denmla]

Hey man, I've posted on a few other forums but this is definitely the most constructive help I got. Thanks for that it is really apprechiated!!

If I'm reading you correctly, the PSU KLM-1856 as unit should be fine and the problem would be on the motherboard side?
How could I go about investigating more? Or am I missig the point here?

Also I recorded a "power up at dark" finding out that buttons lit quite a lot and I didn't notice that at first. Could this point to a display circuit?
https://youtu.be/RS2_Z8zB_HQ

This is where regulator board connects to motherboard:


Thanks again!

PS
Can I buy you a beer in form of a small donation over PayPal or something?[/url]

[denmla]

Also I measured voltages again and those voltages are when main board is NOT connected (17V & 5.2V).

When I connect the main board voltages are as follows:

- orange to black 5.9V
- red to black 1.7V

[voip]

So, that would tend to suggest that it is the power supply board that has a problem, and it is not capable of supplying the power required by the main board. Replacement original Prophecy PSU boards seem to be like hen's teeth at the moment, so a repair is the next step, or a replacement by something with similar outputs. The main switching transistor is a 2SK1459 FET, but there's no guarantee that replacing this will fix the issue. As said before, proper diagnosis would require at the very least a decent scope, schematic, and a working knowledge of switched mode PSU operaton.

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[denmla]

Hey man, tank you so much 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!

Also I'm able to solder no probs with that, I'm just having the trouble in diagnosing WHAT I need to replace.

The biggerst worry is wheather the mainboard is OK. For example, if something is shorted there that could also cause such a big voltage drop or not? I mean how likely is this PSU vs Mainboard issue: like 80:20% or more like 51:49%, that is my main concern...

Squeal (amplified):
https://soundcloud.com/ngfkhgfjkhd-dhjztk/psusqueal

is comming from the main transformer here:


Also, this is how startup looks like from front side (buttons light up but LCD not):
https://youtu.be/RS2_Z8zB_HQ

You are giving me some serious help here man... thanks for that

[voip]

A dual output lab type power supply, set to 5V on one output and 16V on the other could be used to try powering up the Prophecy, and would give the answer to the mainboard working or not question. Extra care needed to connect it up correctly. It would probably need to be able to supply 2-3A at 5V and 1-2A at 16V.

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[denmla]

I see, well I'm not sure I'll be able to go that route :/

[voip]

The high pitched squeal is coming from the main power supply transformer. All transformers tend to make some sort of sound. Normally this would be outside the audible range in a switched mode power supply, but this one is presumably not operating normally, so the transformer may be being pulsed at a lower repetition rate than normal. The transformer may be fine, and it is something else that is the problem. As said before, without specialist equipment, all that can be done is component substitution.

.

[denmla]

Thank you very much! You have been of great help.

I will report back once I have something....

[denmla]

My good friend who is great with stuff like this did one really clever thing. He connected the mainboard AFTER turning on the synth...AND everything works!

He also checked for all the Capacitors and they are all good as well as the FET switcher and diodes....

So we are a bit confused but the synth works when the mainboard is connected like 1 second after turning the synth on. That initial squeal becomes fast short ramp "uuuiiippp".

Motherboard is fine now we have to find the culprit that is causing that initial voltage drop that is preventing the rest of the synth to boot up...

[voip]

Wow, that's a result! Possible culprits might include a dry solder joint somewhere, or bad connector, or a component's characteristics changing (with ageing, perhaps) to the point that the power supply doesn't operate in a stable predictable manner, and requires a "jolt" to force it into a normal area of operation.

Good luck.

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[denmla]

Awesome man, we'll go trough these suggestions to check!

I will report back once we have something concreste.

Thanks again for all amazing suggestions and bringing this KORG's little peace of history to life!