Welcome to Mindburner's DAC71-CSB-I & Prophet 5 Site - TOROIDAL TRANSFORMER PSU SWAP
Lockups and outer potential PSU problems
Failure of the 7805 regulator can cause the machine to lockup. The regulator gets very hot and this overheating may cause lockups/ going into tune mode. Other reasons can be bad connections on the ribbon cables and inter board power connectors. For example the 3 wire power connector from the CPU board to the voice board (see image below) was not making good connection. Moving the connector by hand caused the pitch of note played to change. Another classic lockup is the prophet turns on and all the LED's are lit and 88 shows on the display. The so called 'Christmas tree'. Again connections and PSU rails are the best things to check. The ribbon cable connecting the 2 boards with all the pots can fail leading to this issue. A side effect of these problems can be loss of the internal patches stored in RAM.
I had a simple but major problem in that some of the wires in the above multiplug had broken off, namely the 15v and a ground. I tried in vain to remove the metal inserts so that I could resolder the wire. I could not get them out so I used a very fine soldering iron bit and managed to get a good fix on the connector.
There is also a problem with the keyboard to pcb 16-way ribbon interconnector. One of the small pins had broken off, killing a load of notes on the keyboard. I got some new connectors and used an old floppy disk ribbon cable.
Bye Bye Buzz
The power supply in the Prophet has to be one of the most overloaded of any synth. The poor wee 7805 regulator has to dissipate 20v+ of voltage at 800mA, so it gets pretty hot. I have been thinking of trying to improve the design of the PSU or maybe installing a bigger heat sink. Like many prophet 5 owners, I found the buzz and hum from the prophet 5 transformer annoying. They tend to buzz when the resin dries out in the core. I decided to replace the aging transformer. I checked out the DP-241-8-36 and found that digikey sold it but shipping to Ireland was expensive, so I decided to try and get an alternative. I managed to get a high quality toroidal transformer made by Nuvotem. I's rated at 3.33A with 2x115AC inputs and 2x18V DC outputs. The main advantages of toroidal units are that they are very quiet and more efficient.
I had to make up some new wiring and insulated all the connections with heat sleeving. I drilled a hole in the case to fix the mounting kit that came with the transformer.
Below are some pictures of before and after: