Equipment in use

Posted on Feb 6, 2014

The Comtech has become the de facto standard now for those entering the Amateur TV scene. Sadly the production may be coming to an end with the lower demand for analog tuners and the ICs used becoming obsolete. With the demise of the G1MFG website there is only a limited amount of information about modifying and upgrading these useful workhorses.

Equipment in use
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I have attempted to collect together the most useful information and links to make it easier for you to source the details you need. This is still `work in progress`, more to come. Remember that the Comtec modules have to have correct pre and de- emphasis added, the on-board C/R hf lift filter next to the preset are non standard and should be by-passed. A regular pre-emphasis to CCIR 405 standard should be used. I recommend removing the original filter components on the receiver board and fitting CCIR405 de-emphasis to the receiver but extra gain will be needed afterwards to raise the video level to 1volt pk to drive monitors etc. The original common base addon board used on the MFG boards suffered from field crushing and generally poor sync handling. A revised version using a simple emitter follower instead is shown below and can be fitted onto the original add-on board. There are a few different versions of the board with the latest ones having a dual audio chip and switching regulator IC, but otherwise the layout is very similar. The NE592 is a high gain video amp that was designed for dvd laser output amplification. The built in non-linearity and sharp drop in bandwidth at higher gains means that it is working at full stretch trying to overcome the losses of the de-emphasis circuit. The audio level from the Comtech is rather low for most uses so a simple line amp like this one will give sufficient boost to feed a...

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