SL-1200 MK2 mod possibilities
I downloaded the service manual and schematics for the Technics SL-1200 MK2 and have been staring at the motor control logic diagrams for the past couple of days. This TT uses a Panasonic AN6680 (IC201) chip to handle the phased locked loop for control of the rotational speed of TT. The speed is selected by grounding one of two pins on this chip. That in turn sets how fast the table spins based on the generated frequency pulse rate and the output of the position detection coil. The actual physical control of the speed of the TT is handled by the AN6675 (IC101) chip, but the IC201 chip tell that chip how fast to go. Unless the IC201 chip has some un-described pins that can be used set the 78 RPM speed, I do not see any way to modify the logic with the IC201 without a chip change. There apparently is no magic changing of a resistor that can effect a primary speed change to anything other than 33 or 45 RPMs as existed on the Technics SL-D2. It would be nice to have more information about the chips used in this TT.
That being said, there may be a way to do this with the pitch control logic associated with the AN6682 (IC301) chip and the various related resistors. The question is whether or not the pitch control login has enough range to made the TT spin at or near 78 RPM.
We shall continue out investigations...
Bill