Valhalla 2701C (early models) level-shifter upgrade

[Le_Bassiste]

early VS 2701C had their optocoupler secondary sides supplied with 15 VDC, which was way beyond their datasheet spec of just 5 VDC, and led to quite excessive heating of the optocouplers. hence, when i acquired my early 2701C, i was looking for a simple way of replacing the optocouplers with "fresh" ones, and also adding the extra level-shifter crcuitry that can be found in later models.
so here you have it: remove the old optocouplers, put in some milled sockets and piggy-back this three-pcb contraption onto the sockets. it will have slightly improved slew rates over the original installment, so it may be necessary to calibrate and re-adjust the unit (on mine, it was).
feel free to use it non-commercially for upgrading your precious 2701C and let me know how it is going.
note that this upgrade is only applicable to those early 2701C that don't have implemented the level-shifter circuitry.