Oberheim Four Voice (FVS-1) Restoration – Part 8: The SEMs

In the spirit of saving the easy stuff for last (…or so I thought), I tackled the restorations of the four Synthesizer Expander Module (SEM) units.  These were more work than I had anticipated, but then again I opted to completely disassemble them, clean, re-cap, and repair any issues along the way.

Fig. 1 – SEM #1 prior to restoration

 

I have to say, the SEMs are brilliantly designed – everything from the board layout to the physical construction and user interface… It says a lot that they have been reissued after all these years with very little change to their form and function.  One of these alone would be a blast to play, and to hear four of them in a polyphonic configuration is truly mind-blowing.

My initial assessment of the SEMs in this particular four voice had revealed the following:

SEM #1

  • There was usually no signal present from VCO 2; however, it did seem to come on intermittently at times

SEM #2

  • VCO 1 was dead

SEM #3

  • This SEM had no issues

SEM #4

  • This SEM had no issues
  • The machine screw that was lodged under the PSU distribution board (back in the second installment) turned out to be one of the PCB mounting screws from SEM #4

My service regemine on all SEMs involved removing the knobs, dropping the PCBs, cleaning everything thoroughly, replacing any old electrolytic and tantalum decoupling caps, and cleaning all switches and pots.  In the process of doing this work, I also swapped out all of the red LEDs with non-diffused amber LEDs and upped the value of their current limiting resistors to suit.  They give off a soft, subtle glow reminiscent of an incandescent lamp, and the color change just served to personalize things a bit.

 

Fig. 2 – Preparing for replacement of old electrolytic and tantalum capacitors

 

Fig. 3 – Caps replaced and amber LED installed

 

Fig. 4 – Pot board after cleaning

 

Fig. 5 – SEM amidst reassembly

 

Fig. 6 – Testing restored SEM

 

As mentioned before, both SEM #1 and #2 had some issues to sort out.  The silent VCO on SEM #2 traced back to a dead CA3080.  I carefully extracted the chip and temperature compensating resistors, and replaced with an NOS chip.

Fig. 7 – SEM #2 dead CA3080

 

Fig. 8 – SEM #2 dead CA3080 removed

 

Fig. 9 – SEM #2 CA3080 replaced with tempcos reinstalled

 

SEM #1 posed the biggest challenge by far.  To my surprise, the intermittent output from VCO 2 traced back to its frequency potentiometer.  I had no idea what I was in store for me here initially, but the nature of the failure became obvious upon disassembly of the pot…

Fig. 10 – SEM #1 VCO 2 frequency pot

 

Fig. 11 – Pot removed for inspection

 

Fig. 12 – Potentiometer disassembly

 

Fig. 13 – Potentiometer disassembly

 

Fig. 14 – Houston, we have a problem!

 

I think I laughed and cried a little as this point.  The resistive element had been completely worn away in the middle 2/3 of the pot’s travel!  This was a new one for me, and I got a kick out of it since I recall seeing comparisons (on the guitar sites) between the construction of CTS and Alpha pots citing that the CTS units had a superior conductive surface due to the perceived thickness.  As it turns out, the thickness is all just substrate with a thin resistive coating applied over the top.  Apparently, this pot had seen its share of action!

I realized that finding a replacement pot with the fine-tuning apparatus would be nearly impossible, and so I decieded that my best chance of success would stem from replacing just the resistive element.  I test fit one from a CTS pot of different value, and it seemed to work fine.  After prowling the Internet, I found someone on Ebay selling a bunch of NOS 50K CTS pots, and bought enough to service the entire synth if needed.

The donor pot worked out great!  I tested the resistance throughout the travel of the pot, and it appears to function as new.  The only challenge lied in the the physical connection.  The PCB mounting pins on the donor pots were positioned differently than the original Oberheim pots.

My solution was to drill out the rivits and scavenge the connection prongs from the original pot, carefully lift the crimped tabs on the donor pot, clamp the original prongs in place, and then tack solder them down for rigidity.  With a little finesse, this worked out well and I was able to install the modified pot into the original PCB mounting holes.

Fig. 15 – Donor CTS pots

 

Fig. 16 – Donor pot on left, worn out original on right

 

Fig. 17 – Test fitting the donor resistive element

 

Fig. 18 – Original PCB prongs transplanted onto donor substrate

 

Fig. 19 – Testing functionality of repaired pot

 

Fig. 20 – Original substrate after removal of prongs

 

Fig. 21 – Test fitting the repaired pot in the SEM’s PCB

 

As demonstrated in the photos, this potentiometer fix took a lot of doing… but the end result worked.  Mercifully, the remainder of the pots seem to be in good shape.  SEM #1 is now restored to its former glory, and with that I moved onto the next project – restoration of the MS-1A sidecar sequencer and its integration with the FVS-1 via a custom 12-pin Molex cable.

I’ve long since finished this whole restoration effort, and I’m slowly catching up on the blog entries as time permits.  Please check back for more installments to come.

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