Sudden Death - KORG Prophecy

[Yekuku]

If you want you can use electronic devices that you have at home instead of resistors , for applying load.
Lets say an old cellphone on 5V and a 12V computer fan on 15V( some 12v fans can work up to 15V, but others fail, you have to test first with another psu that the fan works on 15V) .
Aprox 0.5A on the 5V and 100mA on the 15V is good, in order to test if the psu is working ok.
When we rule out the PSU , I am thinking of connecting only the 5v line to the mainboard and check if the unit is powering up.

[denmla]

Hi Yekuku, I've got some great news!

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 wel 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".

Still investigating... but the little silver bugger is alive

[Yekuku]

WOW ! I am speechless and clueless! watch out when hot plugging power connectors, if the ground pins make contact after the voltage pins , there is risk of damaging components.
Did you measure the voltages when it booted up ?

[denmla]

Sure enough! Lot of potential hazard there...

Yeah, once booted all the voltages are good (15V, 5V), synth plays works in a usual manner.

So something is causing that initial voltage drain and it's not Capacitors (all tested and good) diodes (all tested and good) or FET switcher (tested and good).

More investigation will follow and I will report back!

[Yekuku]

Do you have the schematics of the power supply ?
It looks like there is a component in the PSU that doesn't like the large peak of current .

[denmla]

This is the service manual but I cant find PSU schematic there:
https://www.synthxl.com/wp-content/uplo ... manual.pdf

Part is called "KLM-1856" and there is no schematics of it in the service manual....crasy

[Yekuku]

I have noticed it , thats why I have asked
The bridge rectifier could be a suspect, also the optocoupler and all diodes.
I doubt that you can find the faulty component, just by removing it from the pcb and testing it.
It is the inrush current that is making it fail, personally I would replace all suspicious components.

[denmla]

I see, thanks a lot. We'll check/replace and I'll report back!

[Yekuku]

What about a replacement psu like this :

https://www.meanwell-web.com/en-gb/ac-d ... ut-pt--45c
this one has an extra -15v power line, a small resistor as a load might be needed if the -15v is not used, otherwise the voltage outputs could be unbalanced.

[denmla]

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

Cheers

[Yekuku]

Well congratulations Denmla, this was a tough one !
Out of curiosity , near all prophecies that I had used had low output volume, are you experiencing this issue ?

[denmla]

No, the volume level is good but one channel (Right) has a slight (-50dB) hum to it.

It's aaaallive

[madtheory]

Great news!

This "control resistor" is that a 10 ohm fusible resistor?

[denmla]

No it is 560 kOHM one in place of the blue one I'm showing in the image.

[madtheory]

Are you sure it's a control resistor? Usually that means a voltage divider, so it should have a partner. I'm no expert, just curious and hoping to learn