In 1976, the Four Voice began being offered with a programmer module. Oberheim’s patch storage implementation was a bit different from that of the microprocessor-controlled polyphonic synths to follow (Prophet-5, OBX, etc…) in that it was capable of multi-timbral storage/recall of sounds. In other words, each SEM could be setup differently using the programmer’s controls and saved as part of a single patch. Patch recall could also be enabled or disabled for each SEM, leaving select SEM units under manual control.
Outside of the patch storage capability, the programmer also provides a subset of SEM voice controls that can be leveraged to affect quick timbral changes across all SEMs simultaneously. This feature helps immensely when attempting to use the FVS as a “conventional” polysynth. Other nice features include the quantized VCO frequency controls, facilitating quick interval adjustments between oscillators, and a dedicated vibrato LFO.
Physically, the programmer design consists of several circuit boards: a potentiometer board, the overhead logic board, and the channel boards – each controlling two SEMs (the FVS has two of these boards, and the EVS has four). The boards are stacked together using aluminum spacers and interconnects, and molex edge connectors for interfacing with the polyphonic keyboard module and the SEMs.
Fig. 2 – Programmer edge connectors and custom VC envelope generator chips
Each of these boards was in need of restoration. The pots and switches on the pot board needed cleaning, the battery and old tantalum capacitors on the overhead board needed replacement, many of those old striped mylar caps were beginning to crack apart, and I felt that some chip replacements were warranted in order to ensure the longevity of the synth.
Fig. 3 – Old, cracking mylar caps
Fig. 4 – The programmer’s potentiometer board
Fig. 5 – Potentiometer board restoration
Fig. 6 – Replaced old CMOS
The overhead logic board contained several tantalum caps that should be replaced, and the original battery must have leaked and caused some damage to the PCB.
Fig. 7 – Overhead logic board, post restoration
Fig. 8 – Damage from a battery leak
Fig. 9 – Overhead logic board, post restoration
Fig. 10 – Channel boards, with refreshed caps and LM324s
Fig. 11 – Programmer with external battery box
Fig. 12 – The growing castaway parts pile
Fig. 13 – Programmer up-and-running, post restoration
I’ve taken a first-pass at refreshing the obvious components in the programmer module and cleaning the board interconnects; however, the patch storage/recall problems still exist. I will need to trace that problem out with the scope, and I’ll update this post with more info on that as I make progress. In the meantime, the remainder of the programmer works great and I’m onto the SEM repair/restoration….