The Junior Computer hardware and software is described in detail in the books and articles, see there for details.

Note that the Junior has the Elektor bus and coul sue a lot of the cards developed for the EC65/Octobus, but memory layout differences may require adaptations.

Base Junior computer, quite a small system!

Circuit diagram

PCB front

PCB back

PCB layout front (from March 1980 Elektuur article)
Note that this layout is not as shipped, the Junior PCB’s were front/back swapped. The book is correct.

PCB layout front (from Junior Book 1, as shipped)

PCB layout back (from March 1980 Elektuur article)

PCB layout back (from Junior Book 1, as shipped)

LED display PCB and layout

Address sticker

Photo of (stripped clean) PCB front

Photo of (stripped clean) PCB back

Interface card, adds more memory, cassette interface, TTY interface

Circuit diagram part 1, ROM, RAM, VIA

Circuit diagram part 2, TTY interface, audio cassette interface

Interface card  PCB front (note not on scale!)

Interface card PCB back (note not on scale!)

Interface card layout

Backplane PCB, part of it could be used to connect Junior PCB to Interface card.

PSU

Backplane layout

Memory layout

Elektor bus

Musicprint kit and cassette interface

The Junior was sold as a kit by the firm Musicprint, read here the extra instructions in the kit.

It took Elektuur some time to add a cassette interface tot the Junior design, a year until the Interface PCB appeared. Many users therefore bought the socalled musicprint cassette interface. See here the description and circuit design.

A 2732 EPROM can be used instead of the 2708 EPROM when you perform the following:
– cut the traces +12 and -5V
– K6 to the VIA,
– K7 to the EPROM.but leave K7 to the 7401 intact for the STEP function.
– /CS from VIA to K3 to move the VIA to page 0E (jumps in the EPROM are adapted for that)
– /OE from the 2732 to ground
– A10 and A11 of the 2732 to the addressbus
– /CS from the 2732 via an inverter to A12 , note that there are two unused ports in the 7402 with grounded inputs,: cut those! and add a pull up resistor