Bygonebytes

Introduction

I recently came accoss the contruction manual for my Maplin 3800 synthesiser and flicking through it I wondered would it be possible to build the 5600 systhesiser today, nearly four decades on.

With the passage of so much time there will be a few challenges to overcome namely component obsolescence. Integrated circuits, spring line, patch board, keyboard but to name a few.

On the positive side the build process should be simpler. For me the circuit boards will be easier to produce, no more ferric chloride, bubble etch tanks, spraying copper clad board with photo resist varnish and so on. Just produce the gerber files and send them off to JLCPCB.

I'm still considering what to do about the patch bay, try to source one or simply bring the inputs/outputs to limited number small jack sockets so they can be 'jumpered' - no room for 900 sockets!

I am also considering usng the Raspberry Pi instead of a keyboard either as a sequencer or as a 'soft' keyboard. My thoughts are to re-hash my sequencer program for use with the 7" touch screen and fit it where the patch bay would have been.

My first step towards the built in Pi was purchasing the new RPi 3A+, latter I'll get the official Pi 7" touchscreen display which I hope will make it simple to use.



Designing the boards

There are a few integrated circuits that are now obsolete such as the SCL4416 DPDT bilateral switch and the LH0042 op amp. They can be sourced but at a high cost. The question is, do I design the boards with current components or do I leave it as is? Do I move components on the boards to simply the layouts? These and many other questions will be answered as I move through the project.

Boards in order of build

Power Supply

There are two transistors on the power supply board that are obsolete, the MPS3638 and the PN3643, these were replaced with 2N4403 and 2N4401. The Power Supply is now ready to use in the testing of all the other boards.




Oscillator x 4

It is the same issue of transistor obsolesence on the oscillator boards, the MPS3638A and the PN3643 were changed to 2N4403 and 2N4401.

One other change I am made and it is purely down to cost and that is to use 6 way switches for the range selection reducing the range by two settings. The wafercon switches are still available from RS Components but at about 30 each. Maybe towards the end of the project I mught change them.

As pcb space is saved using a double sided board I added the preset board on to the end. It doesn't need to be used as the connections are still available to use the separate board.

The oscillator uses a SCL4416 bilateral switch which luckily I have one left over from by 3800 build 40 years ago! For the other three oscillators I'll need to come up with another solution.

The oscillators are taking shape, all four boards built, tested and set up.





Mixer

Photo:

(1) The Mixer board is complete with only two changes, the BC108C is now a BC107 and the BC178 a BCY70.

(2) and (3) The board and potentiometers are mounted on the aluminium plate and wired, I'll tree all the wiring once the synth is built - just in case I need access to the wiring or board before then.

(4) Testing is now finished and it works well.






Pseudo SCL4416

The SCL4416 Quad Bilateral Switch is quite tricky to get hold of and at a reasonable price. I therefore made a small board that is functionally equivalent from a CD40106 and a CD4016 and at a fraction on the cost.. and works a treat.





Keyboard Controller

The LH0042C is obsolete so I made a small change to the board for IC 16 and 17. The pinout is changed from a TO99 to an 8 pin DIL which will allow a greater choice of replacement including using a TO99. I will be using a TL061.





Keyboard Controller LED Strip

To simplfy the wiring of the keyboard controller LED's I've added this small board.
I'll be using white LED's throughout.




Sample and Noise

The issue of transistor obsolesence is as above, the MPS3638A will be changed to 2N4403.





Transient used in Envelope

The issue of transistor obsolesence is as above, the PS3638A will be changed to 2N4403 and the PN3643 to 2N4401. I have also found it difficult to source 2M ohm double pole switched potentiometers so I have made a conversion board - see below.





Converting 1-pole switch to 2-pole switch

The Envelope board requires a 2M ohm potentiometer with a 2-pole switch which I have found imposible to get. I did manage to source 2M ohm single pole potentiometers which are also used with the Transient boards so I designed a simple board to convert the potentiometer switch from 1-pole to 2-pole.





Voltage Controlled Amplifier (VCA)

No changes were made. Both amplifers have been tested OK.





Transient A/B.

Board layout complete and ready to build. No changes were made to the components.






Retrigger.

Board layout complete and built ready for testing. No changes were made to the components.





Reverb and Phase.

Board layout complete and ready to build. Q1 will be changed to a 2N5320 and Q2 to a 2N5322. The original reverb tank is no longer available so I will use the Accutronics Blue Reverb AMC2BF3.





Voltage Controlled Pan and Ancillary.

Board layout complete and ready to build. Q1 will be changed to a 2N4401 and Q2 to a 2N4403.





Voltage Controlled Filter.

Board layout complete and in production.




External Inputs.

Board layout complete and in production.




Joystick.

Board layout complete and in production.




Headphone Amplifier.

I am thinking a 5 to 8 Watt amplifer is a bit of overkill for headphones so I may use a LM380 2 Watt amplifier instead.

Binary Encoder.

Board layout complete and in production.





Raspberry Pi Interface.

The design for the Raspberry Pi interface is complete, it will allow control of the synthesiser in the same way as my Maplin 3800 sequencer with the addition of 8 input data bits so at some point I will be able to add recording functionality. The board is just a simple Low-High/High-Low voltage level shifter for the data bits to convert the signals to/from the Raspbery Pi 3V3 and the CMOS 14V logic.

Board layout complete and in production.




RPi to Maplin 5600 Synthesiser Interface