COMX-35: Part 2 (Modernising)
14Summary: I've modernised my COMX-35 with the following:
[1] Replaced the 7805 linear regulator with a modern equivalent,
[2] Converted the RF modulator video output to composite,
[3] Replaced the boring red power LED with amber to match the keyboard,
[4] Made the speaker wires detachable
And it works! Just some keys not working.
I explore a bit more of what's on the board:
[1] The CDP1802 CPU
[2] The CDP1871 keyboard encoder
[3] The CDP1869 & 1870 for the Visual Interface System
[4] 32KB of RAM in 8 x M3732 (32Kb x 1b)
[5] 16KB of BASIC ROM in the HN613128 (same pinout as 27C128 EPROM)
[6] 3KB of video RAM in the HM6116 (2KB) and two MSM5114 (1KB x 4b)
Part 3 should be getting the keyboard working and then loading some software.
Replacing the 7805 linear regulator dropped the current consumption from 200mA @ 9V to about 130mA.
I used the CDP1870 data sheet and the COMIX-35 schematic to convert the video output to composite. This involved gutting the RF modulator can, removing C33 (a 47µF capacitor), replacing R33 (33Ω) with a 10Ω and implementing an emitter follower as per the COMIX-35 video output stage:
[a] a 2N3904 NPN transistor,
[b] 10Ω resistor from collector to +5V (R33 replacement),
[c] base connected to video signal,
[d] emitter connected to ground via 220Ω resistor and RCA socket via 75Ω resistor.
Remember to put the insulating cardboard underneath the modulator can before you re-solder to the board! There's some vias under there that could short to ground otherwise.
The CDP1870 actually outputs separate chroma & luma so perhaps S-video output is also possible? There is also an RGB version of the chip.
The 7805 replacement is a PSU3 from EzSBC.com (as always):
https://www.ezsbc.com/
Emma 02 emulator for CDP1802 systems (BASIC ROMs & software):
https://emma02.hobby-site.com/
COMX-35 software archive:
https://archive.org/details/comx-35roms
COMIX-35 homebrew clone:
https://github.com/schlae/comix-35
And various home brew projects using the RCA 1802:
http://www.sunrise-ev.com/1802.htm
Discussion forum on the 1802 processor (and COMX-35):
https://groups.io/g/cosmacelf/topics
Assembly Programming on the 1802:
https://tomswan.com/pub/collections/
Data sheets for the CDP1870, etc.:
http://www.cosmacelf.com/publications/data-sheets/
#comx35 #comix35