Following my other DIY (Do It Yourself) hardware project (keyboard beeper circuit) I thought I'd post info about another board (and very useful IMHO) I made a while (some decades!) back: a 40/80 column autoswitch for use with a Videx 80-column card (it probably works with other 80-column cards as well). I don't know its source or have any more info than what I'm posting below, but can say it works great.
I've used it with an Apple II+ clone along with a Videx Videoterm clone. It plugs into the Apple II F14 position IC socket and in turn takes the IC from there in its own "through" socket.
What is it?
The Videx 80-column card has its own video output, and so does the Apple II (40 columns). After selecting the desired output in software (i.e. PR#3) you also need to change the cabling to the monitor. This is traditionally done with a mechanical switch, but the autoswitch eliminates this completely so any chosen video output just appears on the screen by itself. Two LEDs indicate which column-mode the switch is in.
- 3904 transistor (2 pcs.)
- LED (2 pcs.)
- 16-pin wire wrap IC socket (2 pcs.)
- D4011 IC
- 14-pin IC socket (optional -for the 4011)
- 220 Ohm resistor trimmer
- 220 Ohm resistor (2 pcs.)
- 3.3K resistor (2 pcs.)
- 6-way socket
- 6-way header
- 2-way headers (2 pcs. -for inserting into Videx card and video/GND pins on Apple II board)
- phono socket (for insertion of monitor cable)
- connecting wire (suitable lengths for connecting between board and Videx card, monitor etc.)
F14 (Apple II) IC-socket replacement
As with my Apple II Keyboard beeper board I would suggest you start off by replacing the Apple II position F14 IC-socket with a wire-wrap socket. F14 should be a 74LS259 or a 9334) and is a 16-pin socket. Failing to use a wire-wrap socket (or similar -you can use two 8-pin Arduino stackable headers instead) means that the board will fit very loosely and possibly even fall over, short circuiting your computer!
Follow the instructions/photos for the Keyboard beeper (scroll down to "B10 socket replacement (Apple II main board)" on how to do this (only remember to use a 16-pin socket (or two 8-pin Arduino headers) here instead of 14-pins!
How to build/set up
The project is pretty straight forward. Back in the day I hand drew/used rub-on circuit decals and painstakingly etched my own PCB, but if I were to do it today I'd redraw it in some CAD software such as Eagle or KiCAD, then have it professionaly made (if someone were to do that here I suggest sharing the design files by uploading them in this thread).
Be sure to place the two ICs the right way. As for the LEDs -you're free to use different colours for each mode (40 and 80-columns) or the same. It makes no practical difference but might be handy for troubleshooting/indication purposes or visually appealing.
The 6-pin connector has video signals from (a) the Videx 80-colum card, (b) video signals from the Apple II main board and (c) the video output which goes to a (preferrably green/monochrome) display monitor. Make sure you get the polarities right!
The usual video out jack on the back of the Apple II isn't used at all with the auto-switch, but in my case I have a separate colour monitor attached to that output and use it for games, and a green monitor which is connected to the auto-switch output and is used for "serious" software (80-column word processing, CP/M, programming and so on). The best of both worlds without the hassle.
The two trimpot resistors on the soft-switch allow you to adjust the video levels so you can match both modes instead of having to reach for the monitor's "brightness" control each time you change a mode. The video output trimmer on the Apple II (close to the game port socket) makes no difference here -it only adjusts the video level for monitors attached to the rear Apple II video output jack.