The Microtan 65 used the National Semiconductors DM8678 or equivalent DM86S64 series character generators as part of its on-board video circuitry. The upper case characters are generated by the xxxxBWF device and the lower case by the xxxxCAE device. The CAE device was an option my Microtan never had one fitted, and the device was obsolete many years ago. In TANDOC 2 a design was published by Keith Jones and Tony Kersey to replace these devices with some logic and an EPROM, and this design was adapted for Microtan-R. My Character Generator Replacement board is a development of the original circuit into a plug in replacement that requires no modification to the Microtan.
The first iteration of the design was not perfect. There were some faint part dots visible on the screen in some places. The fix was to change from the 74LS165 asynchronous load shift register used in the original design to a 74LS166 synchronous load one. Full documentation including a circuit description can be downloaded below. The board is designed in KiCad and if anybody would like the designs please get in contact and I will be happy to share them. I have a few spare PCBs if you would like one please get in touch.