Author Topic: Strange Power Supply issue  (Read 261 times)

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Offline Megawilsound

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Strange Power Supply issue
« on: August 01, 2020, 04:47:38 pm »
Hi all

Got a strange one here.

I have the power suply board for a Korg M1 synth, as it failed to power up the other day I removed it to follow Rule Nimber1 (The Shell Test Voltages).

So I probe the out puts (5v DC and +12 -12) got zero.
then out of curiosity I hoocked up the multimeter with thos little hook on things, conected the mains got zero so I unpluged the mains and the multimeter read voltages that droped quickly to zero.

I have never seen this before.

Wondering what could cause this, I have recaped the unit with brand new caps so dont think this would be causing it.

Anyone else come across this before?

Any help in finding the cause would be grate.

Thank

Megawilsound.
 

Offline jmw

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Re: Strange Power Supply issue
« Reply #1 on: August 01, 2020, 06:09:32 pm »
then out of curiosity I hoocked up the multimeter with thos little hook on things, conected the mains got zero so I unpluged the mains and the multimeter read voltages that droped quickly to zero.
If you connected the multimeter in DCV mode to the mains, which supply AC, you should expect to read zero.
 

Offline Megawilsound

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Re: Strange Power Supply issue
« Reply #2 on: August 01, 2020, 06:24:18 pm »
Hi jmw

I used a set of Probe Clips on the DC out puts I did not go near the Mains suply.

If that makes sense.

Mega
 

Offline tunk

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Re: Strange Power Supply issue
« Reply #3 on: August 01, 2020, 07:09:22 pm »
I don't really have a clue, but I think for some SMPSs there's a
bootstrap circuit that should start the switching. It could be that
it's only starting to work when the voltage of the input smoothing
capacitor has declined to a lower level after you disconnected power.
Again, it's just an uneducated guess.

Edit: It may help if you post some photos.
« Last Edit: August 01, 2020, 07:14:08 pm by tunk »
 

Offline Audiorepair

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Re: Strange Power Supply issue
« Reply #4 on: August 01, 2020, 07:31:15 pm »
The rectified mains DC is connected directly to the transformer primary windings.

On switch off, this DC must fall to zero, which presumably is inducing voltage spikes at the secondary.
 

Offline Megawilsound

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  • Country: au
Re: Strange Power Supply issue
« Reply #5 on: August 02, 2020, 03:23:33 pm »
Hi All

here is a cuple of photos of the PCB I am in the prossess of repairing.

It is the Powere suply for a Korg M1 synth bord version KLM-1269 220/240 AC Volt version.

Todate I have removed and tested two components, an NEC transistor NEC C3531 89S and a Diode N9F 5K030, both tested faulty. I have found a new replasment for the C3531 however I am not having any luck in finding a replacment for the DIode. Any advuce on how to locate a replasment would be apreceated.

Links to the photos
https://drive.google.com/file/d/17bdm3b_ZaML5aMnFHbpzsvaCGL9l1bWK/view?usp=sharing
https://drive.google.com/file/d/1uy2EFK7ZzdnuaNiZdKbsZfZ-ByCNLeBq/view?usp=sharing
https://drive.google.com/file/d/1ppkeQKq0VPWzctWdNiEIR54iJqjpuXqB/view?usp=sharing

Thanks

Mega.
 


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