I avoid keyboards whenever I can. They are heavy, have too many screws in,
and take up too much bench space. On this occasion however, I was asked to
look at a Korg LP-350 electric piano by a music store that I have only
recently started doing work for, so had to be 'accommodating' to ...

The shop owner said it was dead, and that he had tried another power supply
already. I figured that this might be something nice and straightforward
like a socket busted out of the board. When it arrived, I was delighted to
see that the socket and on - off switch were located in a largish 'pod'
secured to the bottom of the unit with just 6 screws. So I stood the unit
vertically, leaned against the bench, and removed the pod. In fact, the
entire electronics seems to be on a single board in this enclosure, with
just the keyboard itself and the control switchery being in the main part of
the cabinet. The problem turned out to be some miniscule little sm device in
series with the DC connector centre pin. It is too small to have any value
marked on it, but does have the designation "R" on the board so maybe its a
tiny safety resistor (anyone know ?)

So to get it going initially and check for any other problems, I hung a 1
ohm fusible R across the pads. This restored life to the LEDs on the control
section, so I hooked it to an amp. And this is where it got odd - for me at
least.

Some notes sort of worked, although you had to pound them quite hard. Other
notes didn't work at all. Then when you went back to one that worked a few
seconds ago, now it didn't. Clearly, it's a keyboard with full velocity
sensing, but this seemed very arbitrary as well. My heart was just beginning
to sink when I decided to turn it 'right way up' i.e. horizontal, just in
case. And Lo! Then it all worked. Every note was fine, and the 'touch'
behaved perfectly. Now I don't pretend to understand keyboards, but as far
as I can recall, all the ones that I've previously seen have either been
based on bus bars and springy contacts, or rubber keymats.

So how is this one done such that it won't work when the unit is standing
upright on its end ?

Arfa

I had one like that. I mean I owned it. With the weighted keys. People who play keyboards'll tellya there is a big difference in a piano. Mine was a Previa PX-110. It is a cheaper one actually but plays really nice. And I've gotten on other ones that are also touch sensitive but not weighted, and I can tell you there is a world of difference.

It depends on weight and mass. The mass the key moves is actually partly counterbalanced, which only takes care of gravity. The mass makes it feel like the key is pushing the stirrup or whatever, and hammer in a mechanical piano.

I think they're cool. In fact I like pianos too, and think about how long ago they were invented. Damn them people were smart back then.

On 11/18/2014, 6:30 PM, Arfa Daily wrote:
I avoid keyboards whenever I can. They are heavy, have too many screws
in, and take up too much bench space. On this occasion however, I was
asked to look at a Korg LP-350 electric piano by a music store that I
have only recently started doing work for, so had to be 'accommodating'
to ...

The shop owner said it was dead, and that he had tried another power
supply already. I figured that this might be something nice and
straightforward like a socket busted out of the board. When it arrived,
I was delighted to see that the socket and on - off switch were located
in a largish 'pod' secured to the bottom of the unit with just 6 screws.
So I stood the unit vertically, leaned against the bench, and removed
the pod. In fact, the entire electronics seems to be on a single board
in this enclosure, with just the keyboard itself and the control
switchery being in the main part of the cabinet. The problem turned out
to be some miniscule little sm device in series with the DC connector
centre pin. It is too small to have any value marked on it, but does
have the designation "R" on the board so maybe its a tiny safety
resistor (anyone know ?)

So to get it going initially and check for any other problems, I hung a
1 ohm fusible R across the pads. This restored life to the LEDs on the
control section, so I hooked it to an amp. And this is where it got odd
- for me at least.

Some notes sort of worked, although you had to pound them quite hard.
Other notes didn't work at all. Then when you went back to one that
worked a few seconds ago, now it didn't. Clearly, it's a keyboard with
full velocity sensing, but this seemed very arbitrary as well. My heart
was just beginning to sink when I decided to turn it 'right way up' i.e.
horizontal, just in case. And Lo! Then it all worked. Every note was
fine, and the 'touch' behaved perfectly. Now I don't pretend to
understand keyboards, but as far as I can recall, all the ones that I've
previously seen have either been based on bus bars and springy contacts,
or rubber keymats.

So how is this one done such that it won't work when the unit is
standing upright on its end ?

Arfa

Mercury switches?

John :-#)#

--
(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

"Arfa Daily" wrote in message
...
I avoid keyboards whenever I can. They are heavy, have too many screws in,
and take up too much bench space. On this occasion however, I was asked to
look at a Korg LP-350 electric piano by a music store that I have only
recently started doing work for, so had to be 'accommodating' to ...

The shop owner said it was dead, and that he had tried another power
supply already. I figured that this might be something nice and
straightforward like a socket busted out of the board. When it arrived, I
was delighted to see that the socket and on - off switch were located in a
largish 'pod' secured to the bottom of the unit with just 6 screws. So I
stood the unit vertically, leaned against the bench, and removed the pod.
In fact, the entire electronics seems to be on a single board in this
enclosure, with just the keyboard itself and the control switchery being
in the main part of the cabinet. The problem turned out to be some
miniscule little sm device in series with the DC connector centre pin. It
is too small to have any value marked on it, but does have the designation
"R" on the board so maybe its a tiny safety resistor (anyone know ?)


Its probably a diode to prevent the wrong polarity PSU breaking stuff.
(probably shottky for low Vf).

I have a SP-250 schematic that shows a diode between DC centre pin and
regulators, marked S8540, but didn't find anything in Google.
Sometimes these things will have a reverse diode shunt to ground instead of
a series one. Whatever, there should be reverse polarity protection there
somewhere, not a fuse or resistor.


As for the key shennanigans, the key contacts are probably operated by the
hammers that the key moves, rather than the key itself. Up-ending the
keyboard causes all these hammers to hang loose, probably close or even
touching the contact strips, so you will get very unreliable results.
The hammers give a realsitic impression of how a real piano feels to play,
something a lot of pianists like.

Trouble is it makes the keyboard the weight of a small car.




Gareth.

"Gareth Magennis" wrote in message
...

"Arfa Daily" wrote in message
...
I avoid keyboards whenever I can. They are heavy, have too many screws in,
and take up too much bench space. On this occasion however, I was asked to
look at a Korg LP-350 electric piano by a music store that I have only
recently started doing work for, so had to be 'accommodating' to ...

The shop owner said it was dead, and that he had tried another power
supply already. I figured that this might be something nice and
straightforward like a socket busted out of the board. When it arrived, I
was delighted to see that the socket and on - off switch were located in
a largish 'pod' secured to the bottom of the unit with just 6 screws. So
I stood the unit vertically, leaned against the bench, and removed the
pod. In fact, the entire electronics seems to be on a single board in
this enclosure, with just the keyboard itself and the control switchery
being in the main part of the cabinet. The problem turned out to be some
miniscule little sm device in series with the DC connector centre pin. It
is too small to have any value marked on it, but does have the
designation "R" on the board so maybe its a tiny safety resistor (anyone
know ?)


Its probably a diode to prevent the wrong polarity PSU breaking stuff.
(probably shottky for low Vf).

I have a SP-250 schematic that shows a diode between DC centre pin and
regulators, marked S8540, but didn't find anything in Google.
Sometimes these things will have a reverse diode shunt to ground instead
of a series one. Whatever, there should be reverse polarity protection
there somewhere, not a fuse or resistor.

There is both. Centre pin goes straight to this tiny device. It is
designated R4. Off the back side of it, there is a small ceramic decoupler
to deck, and a series choke, L2, the back side of which trundles off to the
single pole on / off switch at the front edge of the board, as well as
having another little ceramic decoupler to deck. The return from the switch
comes all the way back to the rear of the board again, where it fetches up
at the arse end of a series protection diode, D3, marked 348A. The cathode
of this diode is then the main power distribution point into the 3v3
regulator etc. So the device that has failed is definitely some very low
value series element, and I'm still thinking resistor, as it is called "R4".
I too tried Googling without finding anything. The unit is needed for
Friday, so not finding anything definitive on what the device is, or its
value / type, I think I am going to stick with a low value fusible R. The
volts drop is very small across the 1 ohm that I hung in there. I might go
down to 0.47 ohms, just in case it draws enough to increase that drop under
some circumstances. Can't see anything that looks particularly
current-thirsty on the board, though.


As for the key shennanigans, the key contacts are probably operated by the
hammers that the key moves, rather than the key itself. Up-ending the
keyboard causes all these hammers to hang loose, probably close or even
touching the contact strips, so you will get very unreliable results.
The hammers give a realsitic impression of how a real piano feels to play,
something a lot of pianists like.

Trouble is it makes the keyboard the weight of a small car.

Yes, I think you are probably right. The keys do have that mechanically
'weighted' feel of inertia, and it *is* the weight of a small car ... :-)

Arfa





Gareth.

"Arfa Daily" wrote in message
...


"Gareth Magennis" wrote in message
...

"Arfa Daily" wrote in message
...
I avoid keyboards whenever I can. They are heavy, have too many screws
in, and take up too much bench space. On this occasion however, I was
asked to look at a Korg LP-350 electric piano by a music store that I
have only recently started doing work for, so had to be 'accommodating'
to ...

The shop owner said it was dead, and that he had tried another power
supply already. I figured that this might be something nice and
straightforward like a socket busted out of the board. When it arrived,
I was delighted to see that the socket and on - off switch were located
in a largish 'pod' secured to the bottom of the unit with just 6 screws.
So I stood the unit vertically, leaned against the bench, and removed
the pod. In fact, the entire electronics seems to be on a single board
in this enclosure, with just the keyboard itself and the control
switchery being in the main part of the cabinet. The problem turned out
to be some miniscule little sm device in series with the DC connector
centre pin. It is too small to have any value marked on it, but does
have the designation "R" on the board so maybe its a tiny safety
resistor (anyone know ?)


Its probably a diode to prevent the wrong polarity PSU breaking stuff.
(probably shottky for low Vf).

I have a SP-250 schematic that shows a diode between DC centre pin and
regulators, marked S8540, but didn't find anything in Google.
Sometimes these things will have a reverse diode shunt to ground instead
of a series one. Whatever, there should be reverse polarity protection
there somewhere, not a fuse or resistor.

There is both. Centre pin goes straight to this tiny device. It is
designated R4. Off the back side of it, there is a small ceramic
decoupler to deck, and a series choke, L2, the back side of which trundles
off to the single pole on / off switch at the front edge of the board, as
well as having another little ceramic decoupler to deck. The return from
the switch comes all the way back to the rear of the board again, where it
fetches up at the arse end of a series protection diode, D3, marked 348A.
The cathode of this diode is then the main power distribution point into
the 3v3 regulator etc. So the device that has failed is definitely some
very low value series element, and I'm still thinking resistor, as it is
called "R4". I too tried Googling without finding anything. The unit is
needed for Friday, so not finding anything definitive on what the device
is, or its value / type, I think I am going to stick with a low value
fusible R. The volts drop is very small across the 1 ohm that I hung in
there. I might go down to 0.47 ohms, just in case it draws enough to
increase that drop under some circumstances. Can't see anything that looks
particularly current-thirsty on the board, though.



Yes, I guess I can't see how anything called R4 in this circuit can be
anything other than a resistor really.

I have 2 other Korg piano schematics, 1 uses a series 1N4002, and the other
a series SMD shottky:
http://pdf1.alldatasheet.net/datashe...SX301L-30.html




Gareth.