1. Home
  2. Electronics Repair
  3. Unusual keyboard fault

Unusual keyboard fault

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.

Reply to
Gareth Magennis
Loading thread data ...

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!
Reply to
John Tserkezis

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.

Reply to
Cydrome Leader

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.

Reply to
Gareth Magennis

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.