Korg monopoly

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Korg monopoly

Postby sam » Mon Oct 01, 2012 7:26 pm

Anyone had this with a monopoly..

The -15v line goes down when i connect the main board KLM 354 and the 353...

I replaced the 794 on the power board and there are no short circuits on any board.

Any pointers on whats going on..??
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Re: Korg monopoly

Postby synthparts » Tue Oct 02, 2012 1:13 am

Are there tantalum bypass caps on there?
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Re: Korg monopoly

Postby Don T » Tue Oct 02, 2012 2:25 am

synthparts wrote:Are there tantalum bypass caps on there?


I would guess electrolytic, but even then those are probably due for replacement.
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Re: Korg monopoly

Postby sam » Tue Oct 02, 2012 9:27 am

The only tants are on the voice board and they read ok....It's something that fails on power up and i am wondering if a faulty voice chip may cause this..?
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Re: Korg monopoly

Postby sam » Wed Oct 03, 2012 8:03 pm

This is such a strange fault..If i connect only the power to the KLM354 the -ve rail takes around 10 seconds to go down to 0 volts.
The other boards are stable except the KLM355.

Seems like the power supply is failing somehow and i have checked it out and it should work.

However this has once been a 110 v...There is a new transformer but the voltage of the secondary windings are 25v ac even when the -ve rail fails.

I have never seen anything like this before....Any pointers please... :idea:
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Re: Korg monopoly

Postby Don T » Fri Oct 05, 2012 5:00 am

sam wrote:This is such a strange fault..If i connect only the power to the KLM354 the -ve rail takes around 10 seconds to go down to 0 volts.
The other boards are stable except the KLM355.

Seems like the power supply is failing somehow and i have checked it out and it should work.

However this has once been a 110 v...There is a new transformer but the voltage of the secondary windings are 25v ac even when the -ve rail fails.

I have never seen anything like this before....Any pointers please... :idea:


Sounds like the power supply is fine to me. If it is outputting the correct voltages, and is feeding correct voltages to all the other boards with KLM354 disconnected, you definitely have something dead on that circuit board. If you have been checking for shorts with a continuity tester, that won't always find a component that's leaking badly enough to suck the voltage down on one rail.

Do you have a cap checker that can test for DCR and ESR with the caps still in circuit? If not, do you know someone who has one? I know they're not cheap, I bought one a few years ago.
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Re: Korg monopoly

Postby sam » Fri Oct 05, 2012 9:57 pm

Hi Don and thanks for your wise words on this...I can't measure caps with the power on but i will first replace those tants on the voice board, Sort of a starting point as i have know these caps can to go down under stress without total short out...But an elecro cap should not do this but maybe you have experienced this..?
Maybe an op amp could do this but don't forget i get less than 10 seconds to measure things...But it's a challenge and it will add more experience to my fixing skills so far.. :D
Please if you have any more input...
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Re: Korg monopoly

Postby synthparts » Sat Oct 06, 2012 12:37 am

You should defintely replace those tants. If not already dead they can go at any time... I usually replace them with newer electros.
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Re: Korg monopoly

Postby Steve Jones » Sat Oct 06, 2012 10:17 am

After it has been on for a short time do any of the op-amps get hot? If so, replace it.
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Re: Korg monopoly

Postby sam » Sat Oct 06, 2012 10:23 am

Thanks steve....I will try this as well....Now i have stopped obsessing about the power supply..
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Re: Korg monopoly

Postby sam » Sun Oct 07, 2012 8:56 pm

Curiously 2 tants are shorted on the voice board and i have replaced them...The other 2 read ok and are doing the right thing even on power up...I will also replace them.

But the -ve line is still down and no chips are hot...? The freeze spray is not changing anything either.

Could this be a faulty ssm chip..?
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Re: Korg monopoly

Postby sam » Mon Oct 08, 2012 12:09 pm

So Far....SSM chips are not a problem and i replaced all the tantalums..

Nothing is getting hot...

Any ideas....?
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Re: Korg monopoly

Postby Don T » Tue Oct 09, 2012 4:36 am

sam wrote:So Far....SSM chips are not a problem and i replaced all the tantalums..

Nothing is getting hot...

Any ideas....?


Other than reading the schematic and finding every component that goes from the negative rail to ground, and testing them all one by one, I can't think of anything else at the moment. I'll look over the manual if I get a chance soon.

Yes, I've had to do that before, with a really expensive Acurus power amp that came in. I tested every semiconductor, they all tested fine, but it was still blowing fuses. After testing EVERY component on that channel, it turned out to be the very last untested part, a 47pf silver-mica cap had somehow partially shorted, and had become a 47K resistor. Yes, it was a PITA, but the customer was happy, so...
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Re: Korg monopoly

Postby sam » Tue Oct 09, 2012 9:08 am

Thanks Don....I will check everything on the -ve rail...I think i will start with the caps but there are some 4066 chips which are quite prone to fail..?

Sam.
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Re: Korg monopoly

Postby schmidtc » Tue Oct 09, 2012 5:39 pm

What kind of current does the board draw on its own? As last resort, you could socket the chips you have available like op amps and CMOS, then plug it in without these chips installed.
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