Modify your ZX81 to use a composite monitor.

If youíre like me, you probably donít have any UHF compatible TVís lying around anymore. So to use a Sinclair ZX81 or a Timex Sinclair T1000 you must either find an obsolete TV set or modify your Sinclair to use another means of displaying the video output. I personally have Apple Color Monitor IIís and Green Phosphor monitors readily available. However you canít connect a composite monitor to the UHF output jack. The jack will physically connect but it will not display the video output. To be able to connect and display properly, you must install a RCA jack and connect it to the circuit board in the right place. You also have to be careful as to where you locate the RCA jack. If you locate it too far to the right it will obstruct the 16k ram pack.


WRONG

RIGHT

In the picture to the left is the RCA jack needed for the conversion. Note it has two solder lugs on it. You should use a piece of shielded cable to connect the jack to the circuit board. The center conductor goes to the center lug on the jack, and the braided shield goes to the other lug for ground. When you look at your circuit board, you will see a silver box. This box is the modulator for UHF output. Coming out of the modulator are three wires going to the circuit board near the "ear" jack. The center conductor of the cable should be connected to the modulator connection closest to the edge of the board. The ground shield should be connected to the closer of the two larger solder connections that hold the modulator on the board for ground.

All of the above connections were done to U.S.A. specifications. For other countries you may have to solder the center conductor of the cable to one of the other two modulator connections. You can figure this out through trial and error. Since this is an output instead of input, no harm can be done to your Sinclair if you use the wrong modulator connection.I purchased the jack and cable new locally for total of $2.59 including sales tax. If you have any questions or concerns, email me using the link below.


Do this modification at your own risk. I will not be held responsible for any damages.
My next project is to add an external PC type keyboard to a Sinclair ZX81. If anyone has any information or experience with this modification, please email me using the link below.