Oxford Synthesizer Co OSCar
Someone contacted me recently asking if I would be interested in possibly making some additions to the OSCar firmware. I knew nothing about the OSCar, but decided it would be interesting to learn about it.
First, some new schematics:
Having easy to read schematics which accurately reflect the production units is a great asset, both for learning how things work, and for doing any repairs. Since the only schematics that seemed to exist for the OSCar (except the filter section) were scans of hand-drawn ones, I decided to enter them into a schematic capture program and make them available to others. I checked all of them except the filter and vca sheet against an OSCar with V1.3 boards that I was able to borrow for this project. Here is an archive containing them. Use them at your own risk. Please let me know if you find any errors.
Replacing a faulty rotary switch:
Replacing switches and potentiometers in synths from the 1980's can sometimes be tough, as the parts are no longer made and very hard to find. But in the case of the OSCar rotary switches, luckily, this is not the case. These switches were made by Lorlin UK, and are still available. A single-pole twelve position switch was used. This switch has an adjustable stop to set the number of available positions. For the OSCar, this would be set to give six positions. A single pole switch should be used, as pcb traces are routed through the unused pins and these should not be connected to anything. The mfr part number is CK1044, and the Mouser part number is 10WA144. The plastic shaft is longer than the exact part that was used in the OSCar, so you will need to cut off the excess. I used a hack saw for this, after covering the switch itself with tape to prevent any loose plastic bits from getting inside.
Fixing another issue:
The S&H timing around the DAC in the OSCar is pretty tight. The DAC output needs to settle quickly after new data is applied to it, so that the voltage can be sampled and held properly. To accomplish this, a wide bandwidth LF357 op amp with a fast slew rate was used to convert the DAC's current output to a voltage. I was trying to repair an OSCar recently which had noise in both oscillator waveforms all of the time. When I looked at the DAC output voltage waveform with my oscilloscope, I could see that it was changing a lot more slowly than it should have. As a result, the DAC had not settled to the new value by the time its voltage was sampled for the oscillators. I could see that the LF357 op amp had been replaced at some point. It seemed likely that the replacement chip, also marked "LF357" was counterfeit, and was in fact an inferior (slow) garden-variety op amp. When I replaced this op amp with a different LF357, the DAC changes were much faster, and the noise was gone.
New OSCar MIDI board:
The M2 OSCar MIDI firmware EPROM is completely full. There is no room for any new code. Simply plugging a larger EPROM into the same socket would not increase the available space, since the CPU address decoding would also need to be changed to allocate more space to the EPROM. To accomplish this, I designed a new version of the OSCar MIDI daughter board. It mounts a little differently from the older ones, but is compatible with the OSCar processor board. Here is a photo of the new board:
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