Sunday, 18 June 2017

Minstrel ZX80 Clone Issue 2.4 Boards

I am now shipping the V2.4 version of the Minstrel ZX80 Clone PCB.
These are now also available in blue, which goes well with the RC2014 keyboard. There is now a kit or fully built Minstrel with RC2014 keyboard available from my Tindie store.
The Minstrel ZX80 clone kit (also in my Tindie store) now comes with the choice of blue or green PCBs.
The blue wasn't actually planned, it was a mistake by the PCB manufacturers, but it's grown on me.
The v2.4 boards fix the issue identified on earlier boards with the pinout of the edge connector.
This means you can use things like the ZXpand (or here the new ZXpand+) directly with the Minstrel.
Note this is still an issue with the CAT command when you have more than a screen full of files in a directory (see previous article). I am currently working with the creator of the ZXpand to address this.
There are also a couple of minor mods such as larger spacing for the main input capacitor and through hole plating on the bolt hole for the regulator.
Also wider spacing on the pads on the video buffer transistor to make it easier to solder.
I've had a few queries about using larger size crystals on the Minstrel. The board was designed for a low profile crystal with an HC49S style case (e.g. this one from DigiKey), but some people have had difficulty locating those.
You can fit a normal HC49 size crystal, just bend the legs to rest the can on the components to the left or the IC to the right. Maybe add a drop of hotmelt glue to stop it moving around.
Regarding the frequency, I have been told that 6.5536MHz is close enough to work, but I have not verified that. You might need to change some of the values of the associated components to match the rated capacitance of your crystal, either the 47pF capacitor or the 1K resistor.
It is not possible to speed up the Minstrel by increasing the frequency of the crystal as this is tied to the TV signal generation, and a high frequency would not give a valid TV signal.
You can still fit these board inside a ZX81 case.
That build used a switching regulator, but you can still fit the original ZX81 heatsink and 7805 regulator.
The full range of PCBs, kits and fully assembled Minstrel ZX80 clones are available from my Tindie Store.
Finally, a video from Louis Scully building a V2.4 Minstrel clone in a white ZX81 case, like the one I built (full instructions here).