Silent TR-77

The TR-77 went silent. All, except for the metronome, which sounded normal. There is a single IC in this machine, a Motorola MFC6040 "electronic attenuator" (essentially a VCA) that is switched on or off by a flip flop on the control board, triggered by the touch plate. This action also simultaneously resets the tempo oscillator, so that at a touch, the machine should spring to life starting from the top of the pattern (essentially the pattern is always running, you don't hear it when the VCA is switched off). The metronome sound is added to the audio buss AFTER the VCA...


The CV in measured 1.4 volts despite triggering the flip flop - from my previous investigations I knew that wasn't right. Audio was present at pin 3, but barely audible at pin 5.
I managed to find a replacement part at great cost. Apparently these things were also used in certain pinball machines. Sure enough, replacing the IC fixed the problem. Now the voltages changed as they should, and the drums became audible again.
Pin out for the MFC6040: note the transverse ridge across the body from pin 1 to 3.

This system enables the celebrated "fade out" feature, where the CV is controlled by a capacitor discharge, rather than an abrupt switch, allowing the drums to fade out just like on a record! Priceless.


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