CD4047

The CD4047 is a complex timer chip with quite a few bells and whistles. For the purposes of just being a clock to drive the BBD circuits, we only need to:

1) Set the clock rate with a simple RC circuit at pins 1, 2, and 3. If modulation of the clock is required for chorus/vibrato effects, we can attach a low frequency oscillator here. Lower the "C" value at pin 1 to correct for delay times that are too long and result in an audible high pitched whine (lowering the R or C causes the 4047 to oscillate faster).

2) Connect the 180 deg out-of-phase dual square wave outputs to our BBD clock inputs. These outputs are at pins 10 and 11. See the info on BBD chips to see why they require two out-of-phase clock signals.

All the unused bells and whistles are deactivated by tying them to a power supply rail.

Inside, the RC network controls the rate of a square wave oscillator circuit. These pulses drive the main multivibrator circuit at the heart of the IC. The raw square wave output is buffered and made available pin 13, but we have no use for it on the Memory Man (it is left floating). There are controls for the multivibrator at pins 4, 5, 6, and 8. None of these controls are useful to our BBD circuits, so these controls are tied to either V_DD (4, 5, 6) or V_SS (8).

A frequency divider (another multivibrator) takes the main multivibrator's output as input and produces two 180 deg out-of-phase square waves at half the frequency of the raw oscillator. These are our BBD clock signals at pins 10 and 11. Pins 9 and 12 are more control options that we don't require, so they are tied to V_SS.

Adding Chorus/Vibrato via 4047 Clock Modulation

EH 7810 ("Economy" Chorus) shows a relatively simple dc coupled interface from the LFO to the RC circuit.

EH 7811B (Stereo Memory Man) and EH 7850 (Deluxe Memory Man 5 knob) employ a bit more complicated capacitively coupled interface.

The Chorus/Vibrato control on the 5 knob is simply an attenuator for the LFO signal before heading to the RC circuit.

The Chorus/Vibrato switch selects between two sets of capacitance values to set the LFO's speed.

An obvious and useful mod is adding manual control over the LFO's speed.