Voltage issue on a Yamaha Tyros 2 keys matrix scanning board

[dariodario]

Hi, I'm trying to repair a Yamaha tyros 2 keyboard.

The problem is that all the couples the D# - A keys sound only the first time I press them. The second time I press them they don't sound anymore. The D# and A are on the same wires in the matrix scan as you can see from the attached schematics.

There is no interruption in the circuit D#-A (in fact the first time they sound) but some oxidation points.

I noticed (with reference to the attached schematic) that the pull-up 5V voltage in the EMKS-F board is present on all scan lines except on the D#-A line. In the D#-A line are present 1.1 V instead of 5 V. So this could be the reason of the D#-A not working keys.

With the board disconnected, I also noticed that measuring the resistance from the 5V line to the scan lines, all are about 32-33 Kohm (OK, as per schematic R30..R41), except on the D#-A line in which I measure about 20 Kohm. Unfortunately I cannot access to the hide layer of the board due to 2 connector that are difficult to desolder. That 33K smd resistors are on the hidden layer of the board.

Shouldn't I have 5 volt even if the SMD resistor changed its value from 33K to 20K, since it is connected as pull-up resistor?

Some suggestions please for desoldering this difficult connectors whitout damage the two layer board? The problem is that I need to remove the two at the same time to push out the board...

Thanks a lot.
Regards.

[Audiorepair]

Hi, I'm trying to repair a Yamaha tyros 2 keyboard.

Shouldn't I have 5 volt even if the SMD resistor changed its value from 33K to 20K, since it is connected as pull-up resistor?

Well yes you should.

The most common problem with key PCBs is someone spilt something onto the keys and it leaked onto the PCB.

It sounds like you have a partial short to ground on this line, so look very carefully for any traces of liquid damage by following the line as best you can.

[dariodario]

Audiorepair, you are right.

I cutted the main trace (D#-A) near the connector, and now the line is generating 5V as expected!

The problem now is finding the point of the partial short to ground, since 1/3 of the circuit is covered by the scanning board...
and just this 1/3 has the most number of vias. I think that the partial short is on one of that vias due to the nearness with the ground (shaped ground layer) and the oxidation I saw on that.

Regards.