Yamaha CS80

Are you sure the velocity is OK for that key ? Check this first.

I'll suppose so. (if not, ignore the lines below as the issue might not be the aftertouch).

With a broken wire, you would most certainly get no aftertouch at all for that key. As only one key is concerned, and as a TSB rebuild was done, you could probably check the TSB2 board, and more precisely closely look at the area where the D#5 AT signal arrives (yellow cable at terminal 511(25)). Check the voltage at that terminal. It should be zero when no key is pressed. You can compare with voltage at terminal 505(28, red wire) which is AT for C#5.

Thanks Joachim, 

At the terminal 511 I read -0.49V with no key and +6.9V fully pressed....

At 505 I read -0.38V and it does not change pressed or not....

Ok, double checked -you´re right. They are the same. So I have AT there but can´t hear it work - it seems to be fully open all the time.

How should I check velocity? 

I read at T2 -512 terminal +4.9V and it drops to 0 when pressed. Just like other keys. I don´t know is that it..

Hitting the velocity on that key does nothing. It´s always at full volume/brilliance.

This board was fully recapped and new IC´s were installed in sockets. I will check my soldering following your advice.

In general I have a velocity problem on my keyboard. Some keys are very silent and and some very brutal. But it was like that before I rebuild TSB1, 2 and TKC. I´ve noticed sometimes it is linked to the connectors on the back of each key. Adjusting them a little sometimes helps but in other cases it did not. I am still puzzled about it. 

OK. So there is some coherence in this issue. The first thing is to check the NMJ4558 (or what you changed them for) and the sockets. Also try swapping IC4 and IC5.

Concerning the general velocity issue, are you sure that it is linked to the keys themselves ? That is, if you hit the same key a few times in a row, is it always silent (or always "brutal") ?

Concerning the contacts of the keys, they have a frail part easily overlooked. There is a thin (silver ?) wire on each metal strip that is used to make the electric contact. On my unit, a couple of them were broken at one of the tiny holes where they go down. One was up and acted like a spring. These keys were difficult to play. You could check all them with a strong magnifying lens.

It´s a key related of course - cycling through the voices makes same effect on that key. Always high like aftertouch is maxed all the time...

I swapped the IC and it changes nothing. Soldering seems ok. Contact is also looking good.

I traced the signal to the IC4 and it´s there. No idea where it should come out before going to TKC tough. Could be one of the new caps on the way - I don´t know.

Wow. Crazy. If I touch with my multimeter pin 3 or 15  of IC4 the key, AT, IT works! But the multimeter has to be turned on....I read there 0.14V to 6,5V fully pressed. When I remove the probe it key works few times and it´s back to screaming.

You write that you track signal to IC4. Do you mean that there is a voltage at pin 15 (D#5) as long as the key is pressed ? Normally, you should first measure the decreasing voltage corresponding to the velocity and soon after, the voltage of the aftertouch. Which means that if you press D#5 very slowly to produce a slow velocity and if you keep the key slightly pressed to hold the note on without producing aftertouch, the signal should go very fast to 0V. Is this what you get at pin 15? If so, also check pin 3 (out) under the same condition.

Resoldered  1M, now it works perfectly! Thanks everyone and especially Joachim for your patience.

I´ve learned a lot today!