Yamaha 503II Keyboard some buttons issues

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

19-06-2022, 08:22

I started to have problems with button on the same circuit as Enter, ESC, BS, Tab, F5, F4 , Select and Stop. The problem also will happen with the same buttons while using shift.

The buttons will work but with other characters coming up. I can attach pictures of what I got but here is the enter:

7;bjrgoto
4.

That is what I get. I assume goto is one of the Fs.

I tried to check things around as per the schematics and cleaned the membrane but none worked.

Any suggestion?

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By Wierzbowsky

Guardian (3440)

Wierzbowsky's picture

19-06-2022, 22:08

Check the edge of the ribbon cable. Sometimes the flat tracks come off and bend. So they can touch other tracks. If so, carefully return the track to its original position and fix with instant glue (just a little bit of it under the track).

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

21-06-2022, 01:08

Alexey wrote:

Check the edge of the ribbon cable. Sometimes the flat tracks come off and bend. So they can touch other tracks. If so, carefully return the track to its original position and fix with instant glue (just a little bit of it under the track).

I no longer have a ribbon cable, I had to replace it with something else because I have one corroded pin on the cable and on the connector. But, the keyboard problem started before I changed it, it also continued with the new cable. All the other keys function without problem. Only the one I mentioned.

The following picture is for the ENTER button:

https://www.msx.pics/image/yTjgY

The following picture is for the STOP button:

https://www.msx.pics/image/yTlbs

I noticed that those buttons when pressed, the machine post all the characters that are on the same line from up to down as you can see in the following schematic. I highlighted the direction with red.

https://www.msx.pics/image/yTVWe

The only effected keys as I mentioned are:

ENTER
BS
STOP
SELECT
F4
F5
ESC
TAB

The other keys work without problem if used. Only the ones above. I tried to check resistance measurements but I didn't see anything out of the ordinary except the bussed network resistor has 2 pins reading 2.54 k ohm instead of 4.7K ohm as supposed to compared to the other pins (8 pins network resistor) RA305.

I am out of ideas at the moment.

By sdsnatcher73

Prophet (3405)

sdsnatcher73's picture

21-06-2022, 04:36

So all the keys affected are on Y7 and when pressed all keys in that X row are activated. For example STOP is on Y7/X4, if you press stop X4 becomes active and only Y7 should be active but it seems Y0-Y8 all become active (these signals are passive high apparently so the actual thing in the MSX will be that the go to 0V).

Seems like a short circuit somewhere. Was your “corroded” pin On the flat cable also Y7? It may have been caused by the short sending too many power through the line.

You have to find what is causing the short on Y7 I think.

By Wierzbowsky

Guardian (3440)

Wierzbowsky's picture

21-06-2022, 10:06

According to the schematics, the keyboard is connected directly to MSX Engine chip. So, check that the chip doesn't have any shorts between its pins. If nothing is found that could short the pins or tracks, then I would replace the chip. RepairBas has those in stock.

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

21-06-2022, 18:27

sdsnatcher73 wrote:

So all the keys affected are on Y7 and when pressed all keys in that X row are activated. For example STOP is on Y7/X4, if you press stop X4 becomes active and only Y7 should be active but it seems Y0-Y8 all become active (these signals are passive high apparently so the actual thing in the MSX will be that the go to 0V).

Seems like a short circuit somewhere. Was your “corroded” pin On the flat cable also Y7? It may have been caused by the short sending too many power through the line.

You have to find what is causing the short on Y7 I think.

The corroded pin was the last pin 24 which is for 5v to power the LED for power on. The pin on the connector was corroded and part of the pin on the ribbon cable. but even when It was corroded, I didn't have problem with the keyboard, it just started recently.

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

21-06-2022, 18:29

Alexey wrote:

According to the schematics, the keyboard is connected directly to MSX Engine chip. So, check that the chip doesn't have any shorts between its pins. If nothing is found that could short the pins or tracks, then I would replace the chip. RepairBas has those in stock.

That came to my mind so I cleaned the around the pins but it didn't help. I will have to have closer look at that and see.

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

23-06-2022, 23:13

Here is how I fixed the problem:

Took short wire and connected Y7 to the ground and pressed the failed buttons, then press the ENTER button while the short wire connected between Y7 and pin 22 which is 5V if I remember.

As if it was stuck on certain position and what I did reset it.

By Wierzbowsky

Guardian (3440)

Wierzbowsky's picture

24-06-2022, 16:06

That is a weird way to fix things, but if it worked, then good! Check the continuity from connector to MSX engine and the pins of MSX engine. I've seen that sometimes soldering of these pins gives up.

By alali.ahmed.t

Resident (52)

alali.ahmed.t's picture

01-07-2022, 02:26

Alexey wrote:

That is a weird way to fix things, but if it worked, then good! Check the continuity from connector to MSX engine and the pins of MSX engine. I've seen that sometimes soldering of these pins gives up.

I will check it next time I open the machine for Keyboard maintenance, I have some rust on the metal part that I need to take care of. Thanx Alexey