"Gareth Magennis" wrote in message
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"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.