Stop press! Dieter has mentioned a much simpler modification (adding just 1
resistor) which should alleviate the problem of driving an A-117 from an
A-111 - see message no. 4134.

Modification to A117 Digital Noise Generator to reconfigure the circuit 
to allow it run off a much ‘sloppier’ clock input.

The three photos show:
- a top view, identifying the components
- an overall view of the solder side
- a detailed view of the bottom, with the cut tracks highlighted

An astable Schmitt trigger oscillator, followed by 2 Schmitt triggers, 
is normalled to the ‘Ext. Clc’ input to give the internal clock signal 
when nothing is connected. From the input the clock signal is switched 
through a transistor and then on to the shift register producing the 
digital noise. The modification moves one of the Schmitt triggers to be 
between the transistor and the shift register, thus having the effect 
of ‘squaring up’ the incoming signal, before it goes to the shift 
register.

("Top, bottom, left, right": for the component side, when viewed with 
the face-plate held downwards; for the solder side, face-plate held 
upwards.)

On the component side, locate the leftmost hex Schmitt inverter chip, 
40106, and the shift register, 4006, on the left side of the board 
(both close to the potentiometer – note there is a second 40106 on the 
right of the board, this is not affected). Find the 10k resistor 
(brown, black, orange, gold) running vertically immediately to the left 
of the 4006 chip. Locate the 1k resistor (brown, black, red, gold) 
running horizontally below the 4006. Also note the position of 
transistor Q1. Flip the module over.

Cuts to 4 tracks need to be made:
1. There is a short track connecting pins 4 and 5 on the 40106 – cut 
this track.
2. Find the track running between pin 6 of the 40106 and the left end 
of the 1k resistor. Cut this track.
3. Cut the track running from the 10k resistor to pin 3 of the 4006 
(top end of the resistor viewed on the solder side).
4. Pin 3 of the 4006 also connects to the collector of Q1 (the topmost 
pin as seen from solder side). Cut this track (so cuts 3 and 4 
'isolate' pin 3 on the 4006).

Holding the PCB up to a light should help to determine that a track 
_has_ been cut.

Solder a wire from pin 4 of the 40106 to the left end of the 1k 
resistor.
Solder 2 wires to the (topmost) end of the 10k resistor (the end which 
used to be connected to pin 3): one of the wires should be soldered to 
the collector of Q1 (this re-establishes this connection severed by 
cuts 3 & 4); the other wire is soldered to pin 5 of the 40106.
Solder a wire from pin 6 of the 40106 to pin 3 of the 4006.

Double check that the soldering has not caused any 'bridges' between 
neighbouring tracks that should not be connected, and that the ends of 
the wire straps are not touching any nearby pins or tracks (again, 
holding up to a light may help with this).

Done!

Tim