YAMAHA HS 200 REVISITED

In fall of 2024, the owner of the modified Yamaha HS200 I modified some 10 years ago contacted me to have the keyboard fixed. It was still mostly working, but the pitch range was not able to go up to standard tuning. It was unstable and wouldn't stay in tune. We mutually agreed that it would be best to send it back to me for a repair despite the cost and risk associated with shipping overseas these days. There was really no hurry, and the keyboard eventually made it back around the holidays. It was nice to see the old mod again and think back to where I was "technically" ten years ago. I wasn't able to dive in right away because I had just started doing a deep purge and rebuild of my shop space. Ironically, the layout of my shop had not changed in ten years when I moved in to my new space. Things were getting tight, and there were things that I was hanging on to that I really had no interest in pursuing. The work space was nice and big, but it tended to just lead to more clutter, so I thought it would be good to tighten things up and get rid of excess storage. 

before

after

Anyway, once my shop was 90% put back together, I opened up the HS 200 to see if I could easily adjust the pitch range. Upon opening the keyboard, I was reminded of how well this project came together when I was building it. Usually my more ambitious projects are riddled with setbacks and small disasters. For whatever reason, this project came together pretty well considering how tight everything is in the tiny enclosure. 

Unfortunately the entire piece needed to be disassembled to get to the pitch VCO. I had originally used a 74LS624 VCO chip as the pitch oscillator, but the chip can only take up to 5 volts and draws a lot of current on its own. the circuit that was installed had its own dedicated 5 volt regulator. It was really not ideal for battery operation, and was probably adding noise and stability issues to the rest of the power supply. Also, the pitch range was not very great the way it was configured, so I just decided to replace the whole circuit with a CD4046 VCO. The pitch range is now more stable. The 4046 draws much less current, and the chip operates at the keyboard's voltage. 

While I was in there, I also made a few improvements to the LFO and envelope signals. I noticed that many of the potentiometers were buggy. At the time I guess I thought it would be a good idea to solder the housing ground to the lug rivet on the potentiometer. Every pot that I did this to needed to be replaced... The heat must have damaged the connection somehow and created intermittent connection problems.

The echo circuit in the HS 200 is kind of unique in that the feedback path is sent through a band pass filter with two different cutoffs for high and low. The PT2399 does not have a lot of range, so in order to get the most out of the circuit, there had to be a lot of signal with minimum filtering on the input and output of the chip. This was meant to add more range, but also added the potential for the feedback to run away pretty badly in certain settings. To reduce the problem, I limited the feedback depth as much as I could get away with at the loudest levels. To improve the range I added an auto-leveler circuit to the feedback signal. It basically acts as a soft compressor so that the signal can't run away and blow up the output. The feed back can repeat continuously, but stays at a normal level. It might not be very useful since the signal just ends up turning in to looping noise, but it can add just a little bit more range for those weaker signals. 

The face plates on the HS200 were just OK, and I was never really satisfied with them. At the time, I was still reverse engraving clear acrylic, and painting the back. The result was a clear glossy face with all of the color on the underside. The problem was that the paint was easily scratched or bruised by the hardware attached to it. Also, the text graphics ended up looking more splotchy depending on the quality of clear acrylic used. I have since switched to colored plastic and engraving the graphics on the front, which looks a lot better and holds up better in the end. I offered to replace the faceplates with new ones for free if the owner didn't mind using a color that I already had on hand. He agreed and chose orange. 

All of the repairs and modifications came together as fast and easily as the project had been ten years ago. It's funny how some things just work out like that, and other times a project can seem like it is completely cursed. Now the matter of getting it back overseas...