Several approaches of adding CV/Gate to the Mk I Odyssey consist of not more than 3 or 4 1/8″ jacks with switch contact that interrupt and hook into the internal CV, gate and trigger lines. While the CV uses standard 1V/octave, the gate and trigger signals require a rather high amplitude of about 10 volts, with the additional impact that the gate level has a direct influence of the achievable output level of the AR envelope. Furthermore, both gate and trigger signals are needed for most modifications.
By adding a simple transistor circuit that resembles the levels generated by the Odyssey’s A board this ARP now works perfectly even with 5V gate and trigger levels and does not require the separate trigger signal anymore, although the trigger input jack is still provided for special purposes.
The prototype for the improved CV / Gate modification in action
After restoring the volume potentiometer from contact cleaner abuse and replacing a bad SSM2044, the Trident still showed some intermittent problems.
One time, the attack on one voice has gone, the next time another voice was lacking release. After some measuring and finally swapping the double transistors, the fault eventually disappeared, but new problems came up.
It turned out that the solder joints of the rivets used as vias on the paper laminate board showed microsopic cracks. Resoldering brought all ADSR features back, needless to say that all rivets of this board need to be reworked.
An example of the affected vias – resolder all of them, there are probably 50+ on this board good for all kind of intermittent trouble.
A side note: this synth uses quite interesting ADSR generators, a 1µF electrolytic is charged and discharged by controlled current sources built around double transistors which are selected by the MOSFETs in a CD4007 package.
The generated envelope controls a VCA built from two selected standard transistors – not always easy to troubleshoot and repair, but the parts are much easier and cheaper to obtain than the typical CEM3310/3360 combo.
After all those plastic synths and heavy metal profile construction with no chance to operate in disassembled state this Yamaha CS50 was a real pleasure to work on. Nice condition and just a minor fault: the external modulation input did not work anymore. There is not much in between the input jack and the modulation source switch – just an OP amp and a capacitor, and unfortunately no protection resistor. This means the OP amp is blown when a high level signal is applied to the modulation input with its level pot turned fully clockwise. After replacing the OP amp and adding a series resistor to protect its input the external modulation was back again.
Some trouble evolved within this Evolver – the barrel jacks permanently failing in laptop computers don’t do better in a synth. Once used on stage, a broken jack, plug, cable… is almost certain. A quick repair on the kitchen table brought it back to live, but a serious improvement would require another connector.