This post is about a simple modification that can be done to many CRT TVs to add an aspect ratio switch.
The main uses for this nowadays would be to make the TV more usable on a distribution system set up for 16:9 TVs, or to allow for correction of the aspect ratio for retro gaming consoles which were designed for NTSC then bodged to produce a letterboxed PAL picture for export to PAL regions.
This mod can be carried out on most TVs except those made at the end of the CRT TV era, which typically use the service menu for control of the vertical size.
The main objective is to find the V Size potentiometer, then add a switch and a second potentiometer. In the circuit below, decreasing the resistance between the wiper and top end of the track on VR301 will decrease the height of the picture. There are two main ways of connecting it up - remove the wiper connection on the existing potentiometer and connect it to the other leg of a double throw switch, which has the benefit of the second aspect ratio's height not depending on both potentiometers, or switching in the second potentiometer, which has the benefit of avoiding the problem of what happens during the time that the switch is not making contact with either throw when changing its position - that shouldn't damage the TV but doesn't look as good in my opinion.
If the purpose of the mod is to add a 4:3/16:9 switch, then the original potentiometer will set the height for 4:3, and the new one will set the height for 16:9.
If the purpose of the mod is to add a switch for the aspect ratio required for PAL consoles, then the original potentiometer will set the stretched height for that, and the new one will set the height for 4:3.
If 4:3, 16:9 and the stretched aspect ratio for PAL consoles are all required, then a double throw (SPDT) switch can be used. The second throw will connect a second potentiometer to the lower side of the V Size potentiometer. In this case, the original potentiometer will set the height for 4:3, and the new ones will set the aspect ratios for 16:9 and gaming.
The next step is to select an appropriate switch and find somewhere to mount it. Switches with any exposed metal should be avoided unless you are absolutely sure that the TV is powered through a fully isolated transformer. The new potentiometer or trimmer should be accessible once the new switch is installed so that the picture height adjustments can be carried out reasonably safely once the modification has been assembled.
Modification on a 21" colour CRT
A hole was cut in the dummy speaker grille.
Modification on a 12" black and white TV
This TV is powered from a 12V DC input or a transformer. The method of using both throws of the switch to select between the original and new potentiometers was used on this one, with the consequence of the vertical height jumping if the switch is moved too slowly between the two positions.
Once the mod is assembled, assuming the original potentiometer has already been set accurately, the new potentiometer can be adjusted while looking at the picture until the correct aspect ratio is achieved.
