Yamaha CS80

Heath Finnie wrote:
> I noticed that if I took TR29 out completely and fired it up, the pitch problem disappeared. 
> (disappeared, although the ribbon and tuning pot still have no effect) Granted, it sounds 
> like crap, but it is a start. I tried 3 different C458 transistors and whenever I put one in, the 
> pitch problem comes BACK and it will be the same pitch across the keyboard.
>
> Heath
>
> --- In yamahacs80@yahoogroups.com, "Heath Finnie" <heathfinnie@...> wrote:
> >
>   
According to the adjustment guide, the TU voltage (input to main DAC on 
KAS board, driven by O output of BA board / TR29) should be at 4 volts 
if the ribbon isn't activated and the pitch knobs are centered. So, if 
you happen to have a variable power supply, you could disconnect the 
wire from TU and instead drive it from the power supply set to 4 volts. 
This should make it play normally, but the ribbon and pitch knobs won't 
affect anything. If you don't have a variable supply, then you can just 
hook up a simple 5volt regulator to the +15v/gnd and use that. 
Everything will be a bit sharp, but otherwise ok.

If this is the case, then the problem is in the BA circuit. Just look on 
the big schematic (upper left corner of 
http://www.therogoffs.com/cs80/manuals/CS80_Service_Manual/cs80_foldout_schematic.jpg) 
to see how the BA works.  Just trace the voltage, starting at the 
ribbon, through the IC1/IC2 S/H, IC3 and tuning knob, and finally 
IC3/TR29 buffer. From you previous description (TU = 13v = IC3 is 
driving to its supply rail, tuning knobs have no effect), it sounds like 
it's the IC3/TR29 part. However, check the input to it (P1/P2 inputs to 
BA board from tuning knob) to see if the voltage looks good there. It 
should also be nominally 4 volts and follow the ribbon, VR3, and the 
tuning knob.  I see you already replaced IC3. Maybe just a bad solder 
joint - try touching up all the nearby ones.

Good luck!

 David

> > I'm back at the CS-80 repair. I ended up removing TU from the KAS board but before I was 
> > getting 'nearly' the same pitch across the whole keybed, and the voltage at TU was 
> > 13.37v. I removed TU and the pitch totally changed on the keys. I was getting much more 
> > variation of pitch across the keybed, just octave issues. I replaced TU back onto the KAS 
> > board and the pitch stayed pretty much the same as this new way now. Here is a clip of me 
> > playing every key up the keybed: http://www.heathfinnie.com/media/cs80issue.mp3
> > 
> > The voltage at TU dropped some as well after removing it and the resoldering it. It's now 
> > at 10.37v and VR3 on the BA board has 0 effect on this voltage like it should. I should also 
> > note that the tuning knob and pitch ribbon still have no effect on the pitch. I tried to do 
> > the adjustment of the KAS board and here are the voltages I'm getting:
> > 
> > TU: 10.37v (should be 4v)
> > Octave Ladder network - should  be 125.0mv at Pin 13 on IC2 I'm getting -4.10v
> > Voltage follower: Pin 6 IC3: Should be 125.0mv, I'm getting 2.55v
> > Note Ladder Network: Pin 38 IC2: Should be +132.4mv, I'm getting 2.55v
> > 
> > On the BA board I've replaced IC1, 2, 3 and 4. TR29 and both Fets. I highly doubt the 
> > problem is on the KAS board as I replaced it entirely from another one and the problem 
> > was exactly the same.
> > 
> > Thanks!
> >