Yamaha CS80

Last installment on chip/cap replacement!  I can hang up my
desoldering station (p.s. - I am now selling the desoldering station.
 If anyone is interested, here's the link to the eBay auction where I
bought it a couple of months ago:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3841768300
As you can see, I paid $100 + $20 shipping.  I'll let it go to anyone
in the group for the same price.  If I don't get an offer in the next
week, I'll put it up on eBay). 

The final board was the KAS (pics under Photos > CS80 renovation >
KAS).  This has the custom Yamaha chip that scans the keyboard and the
custom Yamaha exponential DAC that generates the 8 control voltages
for the VCOs and VCFs.  The desoldering, socket installation, and chip
replacement went smoothly.  I also replaced the two electrolytic
bypass caps on the +/-15 supply.  

This was the last of the rework, so I turn on the power and ... voice
3 is a constant, high pitch.  Ok, don't panic.  I 'scopeed the output
of the DAC and see a high voltage with some modulation on it.  Try
replacing the 4016 CMOS part that this voltage runs through.  No effect.  

I looked over the KAS theory pages at
http://home.debitel.net/user/jhaible/jh_cs50.html.  Look at some
waveforms between the keyboard scanner and the DAC.  Everything looks
fine.  Did I blow the DAC?  Ok, time to panic.  

Well, the same DAC is used on each of the KBC boards to implement the
high/low volume and brilliance controls.  So, I (really carefully)
desolder the DACs from the KAS and KBC2, put in sockets, clean up the
chip pins, and swap the chips.  No difference!  Good news is that it
doesn't look like I blew the DAC.  

So, I start tracing the net on the PC board with a light and a
magnifying glass.  Turns out to be a tiny solder bridge on the 4016
chips, pulling the line to a high voltage.  Two seconds with a
soldering iron and everything works!  Yay!  Blood pressure returns to
normal.  

Nothing like working on old, expensive, huge, heavy, complicated gear
that you can't get most of the spare parts for!  I wonder why Kevin
stopped working on these :^)

Now on to tuning/calibration and then the cosmetic stuff.  Oh, and
actually playing it!

 David


--- In yamahacs80@yahoogroups.com, "David Rogoff" <david@t...> wrote:
> 
> Latest update (I'd been waiting for some more parts):
> 
> Power Supply (pics under Photos > CS80 renovation > Power Supply):
> I replaced all the bypass caps on the power supply circuit board.  As
> some of you know (especially if you've read Crow's site), the big
> electrolytic caps dry out after many years and stop doing their job,
> which is to smooth out the voltage.
> 
> I used parts with at least the same capacitance value (in general,
> more is better) and voltage value.  The catch is that the size of caps
> have changed over the years.  It's not just micro-chips that have
> gotten smaller (see pic "caps are smaller now!" to see new/old caps
> next to each other).  In fact, the old 5600uF caps had a third lead,
> which wasn't electrically connected, just to support it on the board.
>  This made mounting the biggest one a little tricky.  
> 
> My wife didn't appreciate that the old, lavender-colored caps have
> been replaced by ugly black, grey, and brown ones. Oh well.
> 
> 
> TKC (pics under Photos > CS80 renovation > TKC):
> This board was 100% old CMOS digital chips with no bypass caps!  I
> definitely needed the desoldering station for this one.  It's also
> tricky, since there are about 12 types of chips on this board, which
> all look virtually the same.  In addition, they all face one way,
> except about four of them!  If you look at "Back - after with caps",
> you'll see why Crow was smart to use the sockets with the built-in
> bypass caps.
> 
> I think the only board I have left is the KAS.  I should have my
> de-soldering station for sale in a couple of weeks!
> 
>  David