This made me laugh.

I have a Korg SP200 digital piano in for repair that had a block of 8 keys
not functioning. Took the offending keyboard PCB off to find the notes
would work if the scanning lines were manual shorted, but the conductive
pads refused to trigger them. (They have a resistance of around 60 ohms,
less if you press hard).

Couldn't work it out at all at first, the scan circuitry looked fine, the
pads just wouldn't trigger those notes.

On closer inspection, some of the '1' s of the scan datastream were low -
around 0.5 volts lower than the correct 3.3v.
It turned out that the scan circuity also scans the top panel switches and
LED's, and there was a partial short on one of the LED's, compromising that
one scan line that was also responsible for the non functioning block of 8
notes.


The machine is a single microcontroller, hence the use of the same port for
scanning both keyboard and switches/LED's, normally you would find 2
completely separate systems in use.



Gareth.

Gareth Magennis wrote:

I'll never looked at LED failures the same again.

Some time back, we had a piece of comms equipment come through the
workshop with a critical led not lit. This was not good. If that led
wasn't lit, it meant very, VERY bad things.

The guy who was working on it was in a panic, speculating multiple
track breaks (the boards were cheap multi layer with very thin copper.
They were so fragile, we never did trust anyone else handling them.

Turned out it was just a led failure. Had it been plugged in and
turned on, it would have worked.. Unfortunately, we didn't have a live
data feed it needed in the workshop at that time, which would have saved
us a bit of time.
Only reason it was pulled out of service, was the field guys had seen
the missing led, and although it appeared to be working, they quite
rightly brought it in anyway.
--
Karma, Let me guess, The Toyota Factory!

John Tserkezis wrote:
Gareth Magennis wrote:

I'll never looked at LED failures the same again.

Some time back, we had a piece of comms equipment come through the
workshop with a critical led not lit. This was not good. If that led
wasn't lit, it meant very, VERY bad things.

The guy who was working on it was in a panic, speculating multiple
track breaks (the boards were cheap multi layer with very thin copper.
They were so fragile, we never did trust anyone else handling them.

Turned out it was just a led failure. Had it been plugged in and
turned on, it would have worked.. Unfortunately, we didn't have a live
data feed it needed in the workshop at that time, which would have saved
us a bit of time.
Only reason it was pulled out of service, was the field guys had seen
the missing led, and although it appeared to be working, they quite
rightly brought it in anyway.

I have a power amp that still works except the signal and peaking LEDs on
one channel are dead. It makes no sense at all, I don't even run the thing
warm. It's got to be something as stupid as a burned out 2n2222. I just
don't feel like removing all 18 screws to take the chassis apart to check
it out.

I mentioned this to Korg yesterday, and apparently their service engineers
are aware of this.

Incidentally, the fault was not with the LED itself - it was the bridge
between their solder pads that had maybe grown some whiskers or other
contamination. I measured something around 100 ohms IIRC, dunno it was a
quick check that I took to be a dead LED.

Others of these LED pads had been manually resoldered at some point, either
as a post sale repair or during manufacture.



Gareth.