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Prophet 5 DAC Issues

Prophet 5

Prophet 5

Prophet 5

Prophet 5

The Sequential Circuits Prophet-5 is a 5-voice analog synthesizer. What makes it notable is that it is one of the first to use a CPU and digital circuits to save and recall all of the parameters of a patch. It also has a great sound, and lots of modulation options. The early Prophet-5s used Solid State Music (SSM) chips. The later versions (also available with MIDI) used Curtis CEM chips.

This particular Prophet-5 had several issues. On power up, the display and LEDs flashed in a cyclical pattern, and the buttons and keys acted erratically. The tuning was also randomly shifting about.

Digitally controlled analog synths frequently exhibit a large number of problems at the same time, and it can be a bit overwhelming. The best approach is to start with the power supply.

Prophet 5 Power Supply

Prophet 5 Power Supply

Prophet 5 Power Supply

Prophet 5 Power Supply

I've Got The Power

The power supply still had the original capacitors (huge!) from about 1980, and they were due for replacement. I could tell that the power supply was running extremely hot, as there were char marks on the circuit board. The heat-conductive grease between the regulators and the heat-sink (the fins on the back panel), was almost nonexistent.

By the way, the #7 card in the middle of the photo is just my obsessive way to keep track of how things were taken apart (and how they have to go back together).

Prophet 5 Factory Changes

Prophet 5 Factory Changes

Prophet 5 Factory Changes

Prophet 5 Factory Changes

While I was poking about, I noticed many factory changes. These are repairs and updates done during the manufacturing process, after the design has been finalized, and synths have started to roll off the production line.

This Prophet-5 was an early Rev-3, so there were extra resistors, capacitors, and jumpers, tacked in all over the place. Shown here are some resistors hand-soldered to a comparator chip. The pink goo on the trimmer in the upper-left is just so that it won't move during transport.

Prophet 5 Troubleshooting

Prophet 5 Troubleshooting

Prophet 5 Troubleshooting

Prophet 5 Troubleshooting

Troubleshooting

Once the power supply issues were resolved, I started poking around to figure out what was causing the other strange behaviour of the synth.

From experience, I could tell that there was some type of issue with the digital control circuits. Usually on this type of synth, the CPU runs a program in a continuous loop. It reads all of the knobs, switches, and keys, then updates the display and output. It then goes back to the beginning of the loop, and does it over and over again... very quickly. When there is some type of odd cyclical display issue, it often means that something is slowing down the CPU.

As it turned out, the main issue was a malfunctioning buffer chip between the CPU and the DAC. This is also why the tuning was off.

There were another half-dozen or so issues with some banks of switches not working, and some bad hum on the output.

Prophet 5 Repair

Prophet 5 Repair

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