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.

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 PS3638A 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 with one tested so far.





Mixer

The Mixer board is complete with only two changes, the BC108C is now a BC107 and the BC178 a BCY70. The board and potentiometers are mounted on the aluminium plate ready for wiring.





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

In production.




Sample and Noise

In production.




Transient used in Envelope

In production.




Voltage Controlled Amplifier (VCA)

In production.




Transient A/B.

Board layout complete.




Retrigger.

Board layout complete.




Reverb and Phase.

Board layout complete.




Voltage Controlled Pan and Ancillary.

Board layout complete.




Voltage Controlled Filter.

Board layout complete.




External Inputs.

Board layout complete.




Joystick.

Board layout complete.




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.

Raspberry Pi Interface.