Jupiter 4 Voice Card Recap

Overview As part of my Jupiter One project I needed to recap and upgrade the Jupiter 4 Voice Card, ready for fitting to the Controller and Pot PCB’s  that I have designed. Hopefully these notes will help others recapping their own voice cards.

Why Recap? My voice card was made sometime in 1979 and 40 years later the capacitors have deteriorated, losing the original value. This is especially important in the timing capacitors in the envelope generators and the VCO, as well as the power supply bypass. I have used modern capacitors which are higher quality, and specifically upgraded the HPF filter capacitor and the audio capacitor between the VCO and VCF.

Further Changes To fit the Voice Card to my Controller PCB I need to carefully remove the 9 pin connectors as these will be replaced with SIP connectors. I also want to replace all the trimmer pots with cermet types to ensure the tuning of the card is accurate and stable. This includes replacing VR10 which had been swapped out for a potentiometer.

Power Capacitors The 100nF power supply bypass capacitors are C31 and C32 are are replaced with MLCC ceramics. C20 and C29 are 100nF Mylar capacitors (power bypass) and they have been replaced with modern versions that are smaller (this helps fit the Voice Card into 11 mm of clearance). C33 and C34 are 33uF 16V electrolytics mounted on the back of the PCB, and are very important for a stable power supply. I have uprated the voltage to 25V (to provide a safety margin on the 15V power rails) and used 39uF Panasonic FC power capacitors.

Timing Capacitors The envelope generator timing caps C12 and C17 (220pf), C18, C19, C23, C24 (100pf) were all upgraded to MLCC ceramics, along with C2 in the VCO.

VCO Capacitors I have left the two polystyrene timing capacitors alone as they look in good condition. I have replaced C36 and C37 which are kludges by Roland onto the rear of the PCB, they are old ceramic plate capacitors, which I have replaced with modern MLCC type.

Filter Capacitors C10 is the 10uF coupling capacitor between the VCO and HPF which I have upgraded to a nice audio quality capacitor, and at a higher voltage rating. The ceramic disk HPF 470pF capacitor at C11 was upgraded to a precision polypropylene capacitor, which improves the HPF sound. I have chosen to leave the 150pF VCF capacitors as the original disk ceramics until I conduct audio testing. In the AM8104 I use Polypropylene 1% types.

The coupling capacitors going into the VCF and VCA are quite low value 22nF Mylar film, I use much higher 10uF electrolytics in the 8104 JP04 Filter to improve the bass response. I have swapped them out with Vishay MKP 1% types at 68nF. This opens up the bass response of the filter a little, and Roland started using 56nF on the E revision of the PCB.  The capacitor on the CV summing Op Amp at C28 (22pF) has been replaced with MLCC type.

New Trimmer

Trimmers An ideal replacement for the single turn is the Piher PTC10LH10 which has the same pin spacing, however they are difficult to locate at 100k value. I also need a trimmer which can fit within the 11 mm clearance of my Controller PCB, so a smaller trimmer is needed. I have used the Copal CT6 which need Pin 2 bending out to fit the PCB. They are similar to the type used in the Jupiter 8 and are just 7 mm in height. I will also use this trimmer on my Controller PCB.

The VCO multi turn trimmers are easy to replace with Bourns 3006 types which just drop in. I have used real versions rather than cheap copies.

Project Status By early May 2019 the new power supply PCB for the Moog 60HP case has been prototyped and proven. The Controller and Pot PCB’s have been designed, checked and ordered. The panel has also been designed and you can see a mock up picture at the start of this post. More updates to follow!