Oberheim Four Voice (FVS-1) Restoration – Part 7: The Programmer

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.

Fig. 1 – Oberheim programmer, post restoration

 

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….

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