Spectrum Issue 7
Start Up Introduction Projects Original Machines Building Tips Links Contact

 

Issue 7 Schematic
Issue 7 Placement
Issue 7 Partslist

ZX Spectrum Issue 7

This is the main page for the ZX Spectrum Issue 7. It's my version of Mike Wynne's 'Speccybob', an excellent project which is basically a ZX Spectrum computer without any custom chips, made with conventional logic ICs which means that anyone with reasonable electronics skill, precision and patience can build their own ZX Spectrum now.

Issue 7

Why did I choose the name Issue 7? It's simple. Sinclair made a lot of revisions for the original ZX Spectrum computer. It ranged from Issue one until Issue 6a. My design will be called Issue 7 because it is designed to fit inside the original ZX Spectrum case. For some people this might be good news because I am sure there are quite some ZX Spectrums around which are for some reason(onboard power supply failure, Ferranti ULA custom chip failure, memory faults, etc) defective. A keen hobbyist can now build a high quality replacement board for their Spectrum ( or 'Speccy':).

Some details about the design

I tried to keep the measurements and details of the Spectrum Issue 7 PCB as closely to the original spectrum PCB as I could. However there are a few things to observe:

- scale of printout on your transparent sheet: make sure your printed layout is the correct scale. Some laser printers are not very precise so you might need to experiment with a few printouts. (use the % scale setting of your printer driver options to get it right) measure the exact center distance between your spectrum's PCB support holes on the lower shell of your spectrum housing, then compare it to the distance between the Spectrum Issue 7 PCB printout sheet's PCB holes. That way you can know if the PCB will fit reasonably. Of course there can be some tolerance in these matters, it does not need to be terribly exact in order to fit. A millimeter or two deviation can still be made to fit with the help of a small round file, I just want to make you aware of the fact that the printout might vary on different printers, something that otherwise might be overlooked.

- the connectors: some changes were made compared to previous spectrum PCBs. The video output connector no longer carries a modulated TV frequency output, instead I have chosen for a DB9 female PCB connector which is placed near to the original Cinch connector where the modulator used to be. This connector gives us seperate RGB signals, as well as composite, horizontal and vertical sync signals. Picture on the monitor will be much clearer with these outputs. The difference in connector shape requires you to enlarge the original connector hole to accommodate this connector. If you don't want to do this, you could optionally choose not to implement the DB9 connector on your PCB and solder a cable directly into the PCB. In my opinion it is the best solution to use this DB9 connector on the PCB. My advice is, be exact, do a lot of in-between-fitting while gradually cutting out the hole. Best way is to use a file to enlarge the last few millimeters so it is a good and neat fit. First the lower shell, until the PCB can fit to it with the connector soldered onboard. Take your time, so it will be a satisfactory result afterwards.

As for the expansion connector, you should be aware of a few things. It is not a 'wobble' ;) edge connector as in the original speccy, I chose for a flatcable boxheader. The advantage is a tight fitting, reliable and flexible connection using a flatcable. The connector does not match the original Spectrum contact order, so if anyone wants to add an original Spectrum expansion, they will need to convert the pin order accordingly. This could be done with a small conversion PCB which carries the original edge contacts, or perhaps by soldering flatcable wires directly on the spectrum expansion. The expansion connector of the Spectrum Issue 7 exactly matches the Z80 CPU's pins, with the extra signals added at the end, same as the Micro-ZX81 PCB. I want to create a 'kind of' standard since the Micro-ZX81. When I made the layout, this connector made it as easy as putting it next to the Z80 and adding all the parallel traces. This saves considerable board space. Have a look at the euro-ZX81 CPU Board to compare. Another advantage in the future might be to have some 'cross platform' expansions. I feel it would be great to have some general Sinclair ZX expansions which can be used by ZX81 users as well as by Spectrum users. This might save me some design work as well in the future, doubling the usefullness of any future expansions. If this indeed will be the case, we will have to wait and see.

As for the other outside connectors: I chose to keep the same shapes as the original connectors used by Sinclair. The idea would be to desolder them from the original PCB and solder them into the Issue 7 PCB. An alternative would be finding some original connectors which fit correctly.

The internal keyboard connectors: these are not in the same locations as the original layouts, because this made the PCB design easier. This will not be a problem because the contacts from the upper shell's keyswitch need to be properly secured anyway. It will be best to use the original keyswitch connectors, solder a flatcable of sufficient length to it, shorten the keyswitch in such a way that you can insert it into the soldered keyswitch connector and carefully superglue this assembly onto a suitable spot on the upper shell. I plan to make some photos of my own modification inside my original rubber keyed Spectrum. As soon as I have them I will add them to this page. Anyway, after modifying the keyswitch into flexible and stable flatcables and using some precision IC-socket connectors as I use with all my designs, it will be easy to connect the keyboard.

Also see the Partslist and Schematic pages for some more details about the connectors and their functions.

A screenshot of the CAD drawing which can give an impression of the density of this project

A few facts about this board:

Total number of pins:             1045

Total pins with connections:     925

Total number of via's:              373

Total number of connections:    717

Number of ICs:                         47

That means that the board has 1418 holes in it.... :)

Current Project Status:

The PCB layout is fully finished, ground and VCC areas are hand-edited in with a bitmap editor. I did a final checkup to compare the PCB layout in CAD pin-wise to the power areas of the bitmaps to make sure everything matches up. I use a technique where I color code the power traces so no accidental inclusion of non-power or wrong power pins can occur. Only after all power areas are done, I finally edited the palette back to monochrome and reduced to one bit colour image again.

The finished layout is etched now, all the holes are already drilled. Next job is to solder in all via wires and use a magnifying glass to carefully study the etched PCB for any errors with a TL light source below and above the PCB. If there are any errors there can be corrected by soldering. It's always easier to study carefully before testing than to find the connection fault after all the parts are soldered in. I can recommend taking some time for this.

Here are some pics of the PCB, here not yet drilled:

ZX Spectrum Issue 7 - top layer (just etched and stripped)

ZX Spectrum Issue 7 - bottom layer (just etched and stripped)

As you can see, the PCB is slightly over etched in the power planes, but the copper is still a solid surface so it's fine. I tried to match the case holes with the original Spectrum PCB. Later on we will see if they can match the case well. Some filing might become necessary for a good fit. Also note that I have manually edited in all the power planes and enlarged the pads for the connectors because they need large holes. For this I use a photo editing program with has good features for drawing straight/diagonal lines and filling in surfaces.

Next phase: soldering in the via's, carefully checking the PCB traces. Sometimes a little local flux cleaning is needed to make sure there are no short circuits.

Last update: 25-9-2006