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Tim Stinchcombe's Synth Stuff

 

A-155 16-step Modification

Figure 6 in the A-155 Owner's Manual section shows a simple patch, using an A-160 Clock Divider and an A-150 Voltage Controlled Switch, to turn the 155 into a simple 16-step sequencer. Someone in the group suggested it might be a nice idea to make up a permanent 'adapter' to do this, and when I looked into it, it was so simple I made one up straight away! The 155 itself contains a '4024 counter which already provides the necessary clocking (which the 160 would do externally), and so the only other components required are an analogue switch chip and a couple of LEDs, plus resistors.

The mod is (relatively) straightforward, and allows for permanent Trig and CV outputs to turn the 155 into a simple 16-step sequencer. I deliberately don't want to be too 'prescriptive', as what I have done is quite a lot more than really needs to be (for several reasons: I move my modules around a lot, so a separate 4hp plate with 10-way IDC + ribbon fits the bill; and I was too lazy to consider pulling various boards off the 155 to drill extra holes etc.). I used a piece of stripboard because I had some, and I more-or-less just added the wires as I needed to, cutting tracks where needed - the mod could be done by sticking the IC on its back and soldering directly onto its legs.

Here is the schematic for the extra circuitry:

The chip is a CD4053 or something equivalent, such as an HEF/HCF4053 etc. Resistors could be 2k2 for use with 5mA LEDs (I used 5k1 as the LEDs I had were low-current 2mA types, left over from some kit I built).

The following pictures show where the signals are picked off the 155:


A general view of the back, showing the ribbon cable I used. Note that the blue wire is a factory-fitted mod, and not part of this one!


Location of the row 'post outs', these are the legs of 100 ohm resistors, which can be traced directly to the 'post out' sockets.


This shows the other outputs: +12V taken from the top pair of pins on the back of the power connector on the 'controller board'; GND taken from any of the middle 3 pairs (I used the pair second from bottom).


This shows how little there is, most of the complication arises because of my use of the 10-way IDC header. It should be easy enough to wire point-to-point straight from the schematic.


My new panel on the right of the 155: the top socket is the combined trigger rows, the bottom one is the CV; the LEDs are in line with the pots on the row that is active.

Pin 6 on the '4024 is just that. There is no other connection to this pin on the board (the 4024 is right next to the other connector on the controller board). Trig 1 is the second pin down on the back of the connector on the 'trigger/gate adapter board'; Trig 3 is the second pin up on this connector (you can easily trace tracks to the sockets to be sure).

There's no need for any sort of switch: when not in use there are no loading effects on the normal 8-step circuitry - when you want to go 16-step, just plug in!

The only thing I haven't catered for is if negative voltages are input to the bottom row external inputs. Since I have taken Vee, pin 7 to ground, the 4053 will not be happy with such voltages (it's highly unlikely I will ever want to do this). If bothered about this, there are two 79L05s (one on each potentiometer board), and the -5V from either could be taken to pin 7 (the 4053s already in the 155 are wired that way).

[Page last updated: 14 Dec 2009]