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'[OT]: Korg Digital Piano'
2003\02\23@121855 by Sean H. Breheny

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Hi all,

Sorry for the way-OT mail. I've looked all over for info on this and have
not been able to find it, even on the relevant newsgroups. I once again
approach the amazing piclist for help from its vast store of knowledge ;-)

I have a friend who has a Korg C-505 Digital Piano. A couple of weeks ago,
it started occasionally shutting off while he was playing it. It has since
gotten worse. It does not matter whether you are manually playing or
playing a tune from its internal memory, but it does seem more likely to
quit when notes are actually being played as opposed to sitting idle. When
it is on, it seems to work fine.

When you turn it on, there is a brief period of about 3 seconds before it
can be played. After this brief delay, you hear a relay click and then it
is ready to play. It has always done this. When it quits, you hear the same
relay click sound, and sometimes the power LED remains on, sometimes it
doesn't.

He asked me to take a look at it. We opened it up and I looked over both
the digital board and the amp/power supply board for anything obviously
wrong and I couldn't find anything. No burnt components or anything similar.

Since these things run a good $1000 and the only shop in our town that
fixes them is run by a couple of idiots (from what I hear), I'd like to
have a go at fixing this for him. We looked around the net for a service
manual, schematic, or other info and found nothing for this particular Korg
model, probably because they now focus on synths.

Finally the question: does anyone here know of a source for additional info
on this machine or know someone who is familiar with it and might be able
to suggest some things for me to check?

Thanks,

Sean

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2003\02\23@124934 by Joe Farr

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Having repaired a few electric pianos in the past (don't know this
model), I'd start by looking at the power supply section. Try and find
the output from the power supply section and watch the volts when you
start playing notes and see if the voltage starts to fall off - you
could have a faulty voltage regulator or this could be a sign of
excessive load from another port of the circuit.
There might be a fault in the audio drive section. After it's been
switched on for a while and starts to fail, 'very carefully' test the
temperature of components - especially those bolted to heat sinks and
see if things are getting hot.
I've seen overheating audio output IC's cause similar problems to those
you describe.
I've seen a case where the relay used to prevent loudspeaker thump when
the power is applied fail - but in that case there was no audio at all.



{Original Message removed}

2003\02\23@124940 by Robert Rolf

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Sounds like a cold solder joint or thermally sensitive regulator or other
component.

When played, the unit draws more more current so regulator heats up faster and
joint gap/short happens sooner. Could also be a thermally sensitive regulator
(joint problem with chip bonding).
Proof is simple. Cold spray or ice cube on regulator heat sink when it quits.
Keeping regulator cold prolongs time to failure.
Also look for sudden drop in in reg voltage due to shorting tantalum
or Al cap. Again freeze spray/hair dryer is your friend..

Robert


"Sean H. Breheny" wrote:
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2003\02\23@140711 by Vern Jones

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In addition to the other answers you have received, check for failing
electrolytic capacitors. These become leaky with age, or intermittently
leaky and when in a power supply or audio amplifier section can cause
the symptoms you describe.

Vern Jones, Sound Research  http://www.foothill.net/~soundres

"Sean H. Breheny" wrote:
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2003\02\23@151357 by jim barchuk

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Hi Sean!

> When you turn it on, there is a brief period of about 3 seconds before it
> can be played. After this brief delay, you hear a relay click and then it
> is ready to play. It has always done this. When it quits, you hear the same
> relay click sound, and sometimes the power LED remains on, sometimes it
> doesn't.

Related to the electrolytic cap suggestion, 'Gotten worse' indicates that
the cap is degrading further.

The fix, more specifically...

Power is still on, part of it, but the delayed power to the power amp has
shut off.

Look near the relay for an RC that turns on the relay. Start at the relay
power pins and trace backwards. Probe with a meter to find the cap that
turns on, then later turns off. Change the cap and upgrade the specs too
so it lasts longer next time.

Possible solder joint too but that should cause far more
intermittent/irregular problem.

I had an old Rat Shack tuner/amplifier that, about every five years or so,
exhibited almost exactly the same symptoms as you desribe. The difference
was that it wouldn't power -up-, then would stay up, and over time the
usual delay got longer and longer until it was maybe half an hour. :).
Changed the cap and it was good for another five years. Unfortunately the
last time it happened something -else- also failed and I was too annoyed
to spend the time to deal with it further so I tossed it.

Have a :) day!

jb

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2003\02\24@101335 by Sean H. Breheny

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Thanks to all who responded,

Last night, after a marathon session, we fixed it! It appears that the
problem was indeed a cold solder joint in the power supply.

We found that we could cause the piano to shut down and restart by pressing
on various parts of the power supply board with a pen. When I inspected the
bottom of the board for cracks or bad solder joints, I couldn't find
anything visually. I then went to check conductivity with my meter and
found that the probe had broken off the meter lead (!) while it was sitting
on the floor when we were working. So, I ended up just reheating all of the
solder joints in that section of the board. When we reassembled it we could
not reproduce the problem, even by pressing on the board. We played it for
about an hour and no problems.  Since, before the repair, we had always
seen the problem at least within 10 minutes, it does indeed seem that it is
fixed. Whew, at least several hundred dollars saved! My friend was VERY happy.

Thanks again!

Sean


At 03:17 PM 2/23/2003 -0500, you wrote:
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2003\02\24@112310 by John Ferrell

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Great!
The "shotgun" approach wins again!!!

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2003\02\24@124607 by Andre Abelian

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Sean,

A few things I will recommend.

1. since on power on you have delay and relay it sounds to me
  that relay works as a mute to cut the power on pop sound
  take the relay cover out and spray WD40 in it.

2. press and hold a note and with any thing gently click on relay see
  if the sound cuts out if it is then it is relay problem.

3. press and hold a note and with none conductor pen or something
  tap on whole board see if the sound cuts out if it is happening
  in relay area then it is relay contact problem or loose connection.

4. use line out connector left and right connect it to other amp
  for test if it does the same thing then internal amp is ok if
  it sounds fine then your problem is in the amp section.


Andre Abelian




{Original Message removed}

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