PAiA 4780 Sequencer Module

The 4780 is a 12 stage sequencer.  This module is built from the inside-out with a lot of wiring so is quite difficult to work on.  I had to completely disassemble the module to restore the PCBs and then completely rewire it which took 9 hours.  All front panel controls except the three position slide switch were replaced.  I upgraded the pin jacks to banana jacks to be compatible with the rest of the 4700 series modules.  This kit was $59.00 in the 1976 catalog.



There are three separate manuals for the 4780 sequencer.  The schematic for the sequencer is in the Using The Sequencer manual and not the Assembly Instructions.  I extracted the schematics and block diagram and combined them on one page and used them as the figures in the Design Analysis document.

PAiA 4780 Sequencer Assembly Instructions

PAiA 4780 Sequencer Assembly Illustrations Supplement

PAiA 4780 Using The Sequencer

PAiA 4780 Design Analysis

PAiA 4780 Sequencer Schematic


The module consists of four PCBs.  PCB A is 6 stages of the sequencer.  This image shows PCB A with the LED PCB attached after cleaning and restoration.  I am just starting to add wires to the PCB for assembly.  The 20 turn potentiometers are shown along the top of the PCB.


This image shows the rear of PCB A.


PCB B contains the control logic and 6 stages of the sequencer.  This image shows PCB B with the LED PCB attached after cleaning and restoration.


This image shows the rear of PCB B


I added a standoff between the PCBs for additional stability.  The top board is mounted first, and then wired to the panel.  The second board is wired and there are a number of interconnects between them that are a bit difficult to reach.  This was definitely not a beginner's kit.


The bottom has relatively little wiring.  You can see the 100K resistor I added to load the Trigger output on the right.




This scope image shows a 12 step sequence CV, Gate, and Trigger.  The Trig Out is loaded only by the scope input impedance so has a very slow decay.


I added a 100K load to Trig Out which generates a nice 2 mS pulse waveform.  All the other scope images are with the Trig Out unloaded.


The minimum rate is 2 Hz.


The maximum rate is 31.65 Hz.  This scope image shows a bit of glide on the CV.


The Gate output has a variable width.  The minimum Gate width is 23 mS.


The maximum Gate width is 346 mS.


This scope image is showing the sequencer clocked by an external square wave on Sync (green trace).




I was sent a 4780 Sequencer that had multiple broken potentiometers.  These potentiometers were made by CTS and are no longer available.  I swapped good potentiometers from PCB A to PCB B so all the replacement potentiometers would be on PCB A.  I found Bourns 3059 Trimpots, Mouser 652-3059Y-1-503LF, which were large and had a full 22 turns and used these to replace the broken potentiometers.  Their size was appropriate for the PCB so I drilled new holes to mount the trimmers.  The trimmers need to be mounted about 0.062" above the PCB so I made spacers out of raw PCB material.  The two end terminals were close enough to the pads that I could simply bend the leads over to solder them and they hold the trimmer firmly to the PCB.  I used a wire to jumper the wiper to the pad.

I used brass tubing which was the same OD as the trimmer brass screw.  I flared one end of the tubing, tinned it, tinned the trimmer brass screw, and soldered them together quickly so as to not damage the trimmer with heat.  I then cut off the old shaft and expoxied it into the other end of the tubing.  As the epoxy starts to set you can rotate the tubing to center the shaft correctly.  I then trimmed the end of the knob which narrowed down to fit inside of the original potentiometer so it would fit flush against the brass tubing.  They work quite nicely.


One of the stage LEDs needed to be replaced as well.  I found Mouser 606-4037T1 SMD LED to be a perfect physical match.  The new LED is more efficient so brighter.  However, you cannot reduce the brightness as the forward voltage drop of the LED determines the maximum voltage output of the stage, and matching the brightness dropped the stage output level by over 1V.  The gain of all the stages is fixed by a non-inverting op-amp and it would have required a number of modifications to compensate.