Yamaha P71 four C and four F# Keys not working

Synth Repair
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I’m really glad I found this community while trying to gather resources to repair my son’s Yamaha P71. I’m hoping to get some advice here to help me feel more confident about ordering the right replacement parts.

The issue I’m dealing with is that all the C and F# keys on the LOW and MID key contact boards aren’t working. This is my first attempt at repairing a keyboard, so any guidance would be greatly appreciated.

Here’s what I’ve tested so far:

  1. I powered on the keyboard and used the C and F# contact strips to touch the electrode areas of other keys, and they worked. So, it seems like the contact strips themselves might not be the problem.
  2. I measured the resistance at the electrode regions (interdigitated electrodes on the key contact boards) for both the non-working and working keys, and the readings were very similar.
  3. I noticed that the voltage readings for the non-working keys are different from the working ones while pressing the contact strip. However, when the keyboard is powered off, the diodes read about ~0.58V for both working and non-working keys.

Any suggestions on what to check next or which parts might need replacement would be a huge help. Thanks in advance!

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I’d try to get the service manual if you can, or something similar. I couldn’t find the P71, but I’ve downloaded the P80 manual which may be similar on the keyboard contacts.
If so, the keyboard scanning chip may be at fault, or the traces from it because all C and F# are processed on the same line.
Alternatively, there’s some spare parts on Syntaur for around $35. It’s how you value your time.

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I have the same problem with Yamaha DGX660 that all the C and F# keys on the LOW and MID and HIGH key contact boards aren’t working. In addition, the lower B key is not working. Please advise. Thank you in advance.

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Thank you so much! I really appreciate your time and suggestion. I’d like to go ahead and replace the key contact board using the spare part you mentioned. Out of curiosity, is the keyboard scanning chip located on the key contact board itself, or is it part of the mainboard?

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I don’t honestly know. You’d have to locate it.
I’ve seen it on one of the contact boards on other pianos and it’s been faulty, but you’ll have to look at yours to find it.
Post a picture if you do find it!
BTW, check all traces from the chip to the contacts C and F#

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Thanks so much for the helpful insight! I’ll inspect the contact boards closely and trace the lines from the chip to the C and F# contacts. Fingers crossed I spot something :smiley:

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