Yamaha CS80 group photo

Yahoo Groups archive

Yamaha CS80

Archive for yamahacs80.

Index last updated: 2026-03-30 01:20 UTC

Thread

Repairing faulty AT sensor

Repairing faulty AT sensor

2013-12-04 by <pyjamagroove@...>

I´ve tracked my G3 AT problem to the sensor itself. After the keys were removed I´ve seen that somebody was there before me and placed a 67K resistor between -15 and +15 terminal. It was kind of repair as in Joachim´s topic about his faulty AT sensor.


After I took the rubbers and cover out I started to measure. At working AT sensors it starts at around 14V and the current decreases at each terminal about 2V finally reaching 4V. At T3 I read -0.3V depressed and around 6v pressed.
At my faulty AT sensor 14V is there but the current does not descend at any terminal....I read 14V at the end and -15 at T3.... If any of them not seem to work, does that mean that I have to place 8 resistors in their place?...

RE: Repairing faulty AT sensor

2013-12-04 by <pyjamagroove@...>

Ive just noticed ,that the little carbon track is broken ( the one between -15 and T3). What is it´s function? Resistance?

RE: Repairing faulty AT sensor

2013-12-04 by <pyjamagroove@...>

So after placing the 6.7K resistor between -15 and T3 in place of the broken carbon track it came alive a little. I read 0.7V drop. So I guess I need a bigger resistor as I have to go from 15V to around 4V.

RE: Repairing faulty AT sensor

2013-12-05 by <pyjamagroove@...>

Well, I´m back at the beginning.

Adding resistors does not seem to change anything. The outgoing voltage from the AT sensor is 14.2V. It should be 4 (depressed). Printed resistors seem to work but the voltage drops very little at each one of them. Not sure what to try next. 

RE: Repairing faulty AT sensor

2013-12-05 by <joachim.milson@...>


> After the keys were removed I´ve seen that somebody was there before me and placed a 67K resistor between -15 and +15 terminal.

That's rather strange. Why would someone do that? This would only produce an increased current consumption. Are you sure that the 67k is between -15 and +15 terminals?


>It was kind of repair as in Joachim´s topic about his faulty AT sensor.

Not exactly. I added a resistor in parallel to an R1 carbon track to reduce its excessive resistance value.


>After I took the rubbers and cover out I started to measure.

>At working AT sensors it starts at around 14V and the current decreases at each terminal about 2V finally reaching 4V.

You do not give precise information of what you're measuring.


>I've just noticed ,that the little carbon track is broken ( the one between -15 and T3).

>What is it´s function? Resistance?

Yes it is. It's part of a simple voltage divider. Look at the photo I posted where I overlayed an excerpt of the diagram and a few labels.


>So after placing the 6.7K resistor between -15 and T3 in place of the broken carbon track it came alive a little.

You're not easy to follow. Is this 6.7k the 67k you mentioned above?


In my case, the issue was an excessive R1 value (No idea why it became excessive). The thing is that it was easy to fix without directly touching the printed board.

In your case, it seems this R1 value was removed and replaced by a resistor by a technician. Perhaps the R1 had a too low value and in this case, there was no other solution than removing R1 and replacing it.


Could you post a photo of the defective part? A picture being worth a thousand words...


RE: Repairing faulty AT sensor

2013-12-05 by <pyjamagroove@...>

http://groups.yahoo.com/neo/groups/yamahacs80/photos/albums/1557574004/lightbox/1720453728

There it is. You can see it´s pretty banged up there. And the sensor on the right was also already repaired.

Yellow cable is the bridge for the broken -15V track.


Sorry if my messages were confusing- I want to start from the scratch.

On the working sensors I read around 14V on the large track ( like that next to LC letters on the photo),

and 4V on the left one (next to HA letters).

On this one it´s 14.8V and 14.2V.....Seems that resistance is way to small there.


You´re probably right- the 67k resistor was placed to keep AT settings low.


RE: Repairing faulty AT sensor

2013-12-06 by <joachim.milson@...>

Ouch ! There is glue here and there and especially on carbon tracks! I don't know if that's a recommended thing. But don't remove it for now as the result might be worse. You should try and find how this glue reacts with carbon tracks in time.

I reposted your photo with a few labels to make descriptions easier:
http://groups.yahoo.com/neo/groups/yamahacs80/photos/albums/1557574004/lightbox/1723503384

So you have two damaged AT circuits, I call them 1 and 2.
- The one on the left (1) has a defective R1 that was fixed with a parallel resistor RA.
- For the one on the right (2), it's less easy to analyse because of the glue, but two resistors are used: RB and RC. RB seems connected directly to R4 through a white wire, which suggests that both R3 and R4 are damaged. It's not clear where RC is connected to, nor where the other end of RB. Could you check this?

Which circuit corresponds to the defective key, 1 or 2 ?

RE: Repairing faulty AT sensor

2013-12-06 by <pyjamagroove@...>

Number 2 works rather well. I think we should not worry about 2 (or should we? they - 15 is in series).

Anyway, on number 2 RC is connected to -15 and probably to T3. RB is connected to T3 and R4 (to the second printed track from the left). But as I said - that circuit works. The glue is more like a silicon stuff to keep everything in place and protect the joints.


The circuit 1 is defective.R2 has very little overall resistance - around 0.7V as mentioned before.


RE: Repairing faulty AT sensor

2013-12-06 by <joachim.milson@...>

For resistor RA, the colors seem to be blue-gray-brown. Can you confirm this, especially the brown?
If this is the case, its value is only 680 ohms, which is very little. This small value was used to compensate for the low resistance of the damaged R2 and keep the voltage divider overall range.

Now, your measures show that the value of the resistor R2 is now significantly lower than 680 ohms.
To recover the voltage divider range, I can see three ways:
1. Reduce the value of RA. But doing so will increase the current through RA and R2. I don't know how the PSU will support this. This seems risky. I wouldn't do it.
2. Increase the value of R2. I don't know how to do this nor whether it is possible to recover a printed resistor. You'll need the advice of a "printed track + silicone glue" guru. Then you'll also have to adjust R2 for an appropriate value.
3. Recreate the R2 resistor with a series of 10 resistors, one for each step. That's a bit of work, but this seems to be the most accessible way and realistic way.

RE: Repairing faulty AT sensor

2013-12-06 by <joachim.milson@...>

PS at the end of step 2, I meant :
Then you'll also have to adjust RA for an appropriate value.(not R2)

RE: Repairing faulty AT sensor

2013-12-06 by <pyjamagroove@...>

RA is blue, grey, red. So it´s X100. 67K.


1. Is it really a risk? Now it´s 67K.. I could try 44k for example.
2. The "Guru" is unknown. Might be dead as well...:/ But that would be the best solution - Increase the R2 value.
3. That´s what the "Guru" did. He baypassed second printed resistor of the divider with RB. To replace all of them seems like a crazy task. Not sure do I have skills or the iron that small....

RE: Repairing faulty AT sensor

2013-12-06 by <joachim.milson@...>

blue, grey, red give 68 x100 = 6.8k, not 67K.

This is better than 680, but certainly lower than the initial value of R1 (when I had to fix my own AT issue, Kent Spong suggested a 82k value for R1).


I fear that reducing RA from 6.8k to 4.4k won't be enough. Remember that you wrote 14.8V and 14.2V at both sides of R2. This is not much.

You could perhaps try with a potentiometer instead of the RA to determine an adequate value for RA. Starting with say 5k and reducing progressively and cautiously.


(By "printed track + silicone glue" guru I meant someone who could help you with his knowledge of how these materials interact, not the one who bungled the board ;-)



RE: Repairing faulty AT sensor

2013-12-06 by <pyjamagroove@...>

I know what you meant by "The Guru" ;) - I liked the term.


I would try to go with 4.4k and when it works better, down to 2.2k. But right now I´m now scared about the PSU. 

RE: Repairing faulty AT sensor

2013-12-07 by <pyjamagroove@...>

So I´ve changed RA from 6.7k to 3.8k and nothing. I still read 14.2v where it should be around 4V.

RE: Repairing faulty AT sensor

2013-12-07 by <joachim.milson@...>

I fear that the value of resistor R2 has become too low now to let you compensate by a reasonable value of RA.

RE: Repairing faulty AT sensor

2013-12-07 by <pyjamagroove@...>

That would explain why the "glue guy" left it alone and repaired the other one. Anyway - I´ve switched the pcb´s and it´s in the place that is less bothering when I play. Someday maybe I will try to do the 10-resistor repair. Thanks Joachim.

RE: Repairing faulty AT sensor

2013-12-08 by <joachim.milson@...>

One final thing you could probably do while waiting for the final repair to be done.

Currently, whenever you play the key with faulty AT sensor, it plays as if with maximum AT. Right?
So, you could unsolder the white wire at T3 and solder it to the T3 of the next key to get the right default voltage (-0.4V).
An other solution would be to use two 100k resistors soldered in series and forming a bridge between -15 and +15 and then solder the white wire between the two resistors (to recreate a voltage divider).