Super 80 – V1 Reproduction

Introduction

The Super 80 is a Z80-based single board computer that had 48K RAM, a full-size keyboard and video interface on board. It was sold as a Dick Smith kit and was published in the Electronics Australia magazine in 1982.

Very few Super 80s have survived the passage of time. They only rarely come up for sale on eBay and other places.

A small group of enthusiasts formed the Yahoo Super 80 Group in 2006. I am a relative newcomer to that group – I joined in 2012.

Development of the V1.0 Reproduction PCB

Prior to my joining the group, there had been talk and some progress towards creating a replica PCB.

Thanks to Gary’s efforts, the group has access to some JPG scans of a de-populated original Super 80 PCB. This allows us to accurately replicate the size and placement of all relevant elements (components, silk-screen legend, tracks, vias, connectors, etc).

Craig (another group member) used those scans to commence drawing a KiCAD reproduction. It seems those efforts occurred in about 2012. He took the very clever approach of modifying KiCAD so that the PCB scans could appear in the background on KiCAD, allowing the new traces to be laid down over them. Some progress was made, but by around September 2012 this approach had stalled. I understand the KiCAD development team refused to include this tracing functionality into KiCAD, with the result that it could not be used in later versions of KiCAD.

In December 2014 I made a new start on drawing a Super 80 replica in KiCAD. I first focussed on drawing new library components for virtually every Super 80 component. I did this so that both the schematic and the PCB would closely match the original Super 80. Once the library components were drawn, I drew the schematic and then positioned all components on a blank PCB. By that point we could generate a “net list” (from the re-drawn schematic) and a parts list, and we had a realistic (but imperfect) silk-screen overlay.

Unfortunately it then took until December 2017 for me to complete the V1.0 design. Overall I estimate I have sunk over 300 hours into drawing the schematic and PCB in KiCAD, and there were long periods during 2014-2017 when I simply couldn’t find time to contribute much effort to this project. I have attempted to make the replica PCB appear very similar to the real thing – each via, component and trace is within about 0.010″ of its true position. The replica is not perfect however, as the original Super 80 was laid out with tape (rather than CAD), and KiCAD cannot mimic the slightly curved traces that the old tape-and-film technique used to result in.

The V1 PCB is intended to be a pure replica. No design enhancements were added, and no design faults were corrected. As at February 2018, two V1.0 Reproduction Super 80’s have been built. We are aware of only one significant design error, and that is easily corrected (see the V1 Notes and Errata section below).

A significant number of people have contributed directly to making the V1.0 Reproduction happen. The contributors that immediately come to mind are:

  • Craig: For demonstrating that a Super 80 PCB reproduction is indeed possible using KiCAD, and for assisting group members with sourcing hard-to-get parts from Anchor Electronics in California
  • Gary: For depopulating an original S80 PCB, and providing JPG scans for the group to use
  • Leon: For providing a high-quality scan of the record-cover sleeve that the original Super 80 PCB was packaged in
  • Peter: For helping with the laborious task of checking the near-final KiCAD design against an original Super 80, building the first V1.0 Reproduction, and providing me with a ROM set (3 x 2532s) to use in my V1 Reproduction
  • Jared: For recreating the Super 80 logo and hatching pattern, for the PCB silkscreen layer
  • Ewan: For helping with the planning of a (yet to be placed) order for a keyboard kit to suit the Super 80 reproduction

Construction

The V1.0 PCBs arrived in January 2018.

Peter Cassar was the first to finish building one of these new V1.0 Reproduction boards. He completed his in late February 2018 (see photo on the right).

During construction Peter discovered one significant error in the V1.0 PCB layout: Pin 15 of U44 is erroneously connected to GND. This must be corrected, by cutting a trace on the top layer of the PCB. It is described in more detail in the “Notes and Errata” section below.

I completed the assembly of my V1.0 Reproduction in early March 2018. As at 09.03.2018 mine still needs to have a couple of things added (RF Modulator and Keyboard parts), but it is working fine with a keyboard temporarily borrowed from my original Super 80. Photos of my V1.0 Super 80 are shown below.

Notes and Errata

Below is a list of the problems and issues encountered when assembling the V1.0 PCBs:

  • U44 Pin 15 is erroneously wired to GND. You must cut the top layer PCB trace between Pin 15 and the via that it connects to, so as to isolate it from GND. This is best done before the I.C. socket is installed, as the socket will otherwise cover where you need to cut. Check with a multimeter to ensure you have isolated this pin.
  • The blank V1.0 PCB has no mounting holes (and neither did the original S80 PCB). If you want to have mounting holes, it is best to drill these before you start construction. I drilled holes in each corner, and an extra 2 holes 6mm above the dotted line above the keyboard. All holes were 7mm from the edge (though I’d probably make this 6mm next time). So 6 holes in total, all 3.5mm in diameter.
  • The holes for the “Transformer In” header are too small to accommodate a standard 0.156″ header. The fix here is to file the corners off the square pins. Thanks to Peter for this suggestion. The pins are 1.13mm x 1.13mm (and 1.50mm on the diagonal).
  • The mounting holes for the RF Modulator are too small. Peter advises that the pins are 3/32″ (2.3mm) in diameter.
  • C17 is shown on the schematic (both in the Assembly Manual and in my KiCad schematic) as being a 33uF tantalum. However, the parts list in the Assembly Manual specifies a 22uF tantalum for this part, and that is what is installed on the real S80 that I have. So I used a 22uF tantalum during assembly of the V1 Reproduction. I suppose a 22uF is the better choice here, given that is what was actually installed on the original S80. The KiCAD schematic should be updated to show a 22uF tantalum.
  • C14 is specified as 25uF/25V in my KiCAD schematic (and as 25uF in both the EA schematic published in October 1981, and the schematic included in the Assembly Manual). But the parts list in the Assembly Manual specifies a 22uF tantalum, and that is what is installed on the real S80 that I have. So I used a 22uF tantalum during assembly of the V1 Reproduction. Again, I suppose a 22uF tantalum is the better choice here, given that is what was actually installed on the original S80. The KiCAD schematic should be updated to show a 22uF tantalum.
  • The schematic and parts list in the Assembly Manual specify 6 x 1.5uF tantalum capacitors and 6 x 15uF tantalum capacitors on the +12V and -5V rails respectively. These are located adjacent to the RAM chips. These values are no longer easy to obtain from some stores (such as Futurlec – but they are available from Digikey), so I replaced them with 2.2uF and 22uF tantalums respectively. The original values (1.5uF and 15uF) are preferred, if you have access to them.
  • R33 (pullup resistor at the base of the video driver transistor) is shown in the EA schematic as being 10K, and that is what is shown in my KiCAD schematic. But this was changed to 15K in the schematic and parts list in the Assembly Manual. My original S80 has a 15K resistor fitted here. So I used a 15K resistor during assembly of the V1 Reproduction. The KiCAD schematic should be updated to show a 15K resistor.
  • The holes for CR1 to CR4 (“4A Rectifier Diodes”) are marginally too small for the diode I selected (MUR420G). The KiCAD PCB specifies 1.27mm holes, and the MUR420G datasheet says the lead diameter is 1.20mm to 1.30mm. But there was no way they would fit. So I ended up drilling the holes out by about 0.1mm, and soldering the diodes to both the top and bottom pads (which was easy to do). Note the original Super 80 used 1N5404 diodes. The datasheet for the 1N5404 shows a lead diameter of 1.22mm to 1.32mm.
  • The holes for the LED are a little on the small side. The diode leads are 0.58mm (on the diagonal) and only just squeeze in. The KiCAD PCB specifies that these holes are 0.6096mm (0.024″) but that must be before plating is applied.
  • The 2513 Character Generator is physically too close to the adjacent U28 and U30. I had to file down the body of U28 (74LS00) and U30 (74LS86) to fit U27 (2513) in its socket. I filed them down until I could just start to see metal leads encapsulated in the IC body. If using dual-wipe IC sockets, you may not need to do this. But I used machined-pin IC sockets, and it was not possible to fit the 2513 without doing this.
  • I did not have access to the basic Z8400 (2.5 MHz Z80CPU) and Z8420 (2.5 MHz Z80PIO). So instead I used the Z8400A (4.0 MHz Z80ACPU) and Z8400B (6 MHz Z80BPIO). The V1 Reproduction runs just fine with these parts.
  • The orientation of the two tansistors (Q1 and Q2) is not obvious from the PCB silk-screen legend. Q1 (BC548 near the RF modulator) faces towards the keyboard. Q2 (BC558 near the cassette relay) faces towards the cassette I/O pins.