Yamaha CS-60 part 5: Power Supply Finish and Calibration

I received a big box of capacitors from Mouser and quickly set to work finishing up the power supply and PRA boards. Of course I failed to take any pictures of the finished power supply board before I reassembled the power supply, but you have seen new capacitors before. I had one trace get damaged in the rebuild so I bent the lead I would normally cut off over and soldered it to another solder pad/connection on the same trace.

GOLT! goes through the power supply theory of operation, safety precautions and adjustment better than I ever could in this video. Power Supply section begins at 17:22. Thanks to Emanuel for the link.

I realized in all my work I have not discussed safety at all. Safety for myself and safety for the custom ICs in this synth.

Protect yourself: You need to protect yourself from chemicals and shocks. To protect yourself from chemicals you want to solder with a fan blowing across your work and ideally out an open window. I’ve never seen a breakdown of the contents of solder smoke, but it can’t be good, even if it is just run of the mill smoke. Other chemical exposures come with cleaning fluids, fader lubes, adhesives and any other stuff you end up bringing to bear on your project. Read the warnings and follow the recommendations. You only get one set of lungs / liver etc. Protecting from shocks is as simple as making sure your synth is unplugged before you do any work.

Protect your synth: The Yamaha CS lines are in a class of their own with respect to hard to get ICs. There are no spares other than the very odd examples what show up on eBay or Reverb. A machine has to die for more to be sourced. I would be willing to bet CS-50 and CS-60 survivorship will turn out to be fairly low due to their being broken for parts. So how do you protect your precious ICs? Wear an antistatic wrist strap and connect properly to ground so you don’t zap your CS-60 when you touch it. This is especially important because a CS-60 has a wood chassis and the only dissipation path for static is into the circuit boards via the wiring.

Okay, now we and the synth are safe.

Power supply reinstalled. If you are not good at remembering where the screws go you should make a diagram. 6 same size screws hold the power supply chassis to the wood chassis, 1 nut and bolt secures the green ground to this chassis, 2 screws secure the power cord strain relief to the wood chassis and 1 holds the white power cord clip.
Set up for calibration. Power connector to the rest of the synth (the one on the left) gets disconnected. Since it is disconnected before power on you have to jumper the +15V to the +15V sense and -15V to -15V sense or you will kill your power supply. Jumpers stay in place for entire calibration. Replacements can be had, but are very expensive. Black from your meter goes to the ground between the + and – 15V wires, red goes to the + or – 15V. The jumpers stay in place for the reading. First white pot seen here (closest to the red jumper) is 15V adjustment, after that is the -15V. A little goes a long way. -15V adjust goes the same.
-6.5V is lower down in the row of connections, doesn’t have a sense connection and has a different ground point all the way at the bottom of the row of connections you are dealing with.
And the 8.5V.

After I got the voltages set I reconnected the power connector to the rest of the synth, made sure I didn’t have any tools or other stuff that could short out and I tested the synth. Sounds better than it used to. Sounds are more distinct and the controls seem to have more of an impact on the sound.

After I confirmed functionality following power supply calibration I recapped the PRA board. Take your time and confirm all caps are in correctly at the end. I have replaced many many capacitors over the years and even so I managed to get one cap in backward. I had this on a towel when I was doing the work. No sense scratching up the paint on the end panel or key tops.

Power supply is rebuilt and calibrated. All but the voice boards are recapped and we still have a fully functional synth. What is left to do you ask?

  • Order all ICs I will replace along with suitable sockets
  • Order LED lights and resistors to make voice on indicators for the voice cards
  • Replace capacitors on the voice boards & add LEDs
  • Install new ICs in sockets
  • Calibrate the various tone sections

I admit, it is tempting to stop now since I have replaced a lot of capacitors and it still works, but I need to see this through.

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