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Thread: Adding Linear RGB to a Philco CM8764 Monitor

  1. #1

    Default Adding Linear RGB to a Philco CM8764 Monitor

    I have a CM8764 074G monitor, which has a TTL RGB circuit (and composite in), and a cutout on the case and a depopulated circuit for Linear RGB, which I would like to add to the monitor.

    I haven't really been able to find out too much information about this specific monitor, but it's very similar to the CM8762 in design, which also seems to be the same main PCB of a number of Commodore monitors, including 1081 and 1084.

    I've found a diagram for the circuit in the 1084S-P manual (for the CM8705, same board with lower horiz res tube), and an almost components list in the German 1081 manual (again same board lower res tube).

    The only thing that doesn't match up in all this is the tube resolution, the 8764 is a 640 dot horiz resolution CRT screen, and the CM8705 is 390 dot CRT, but I don't think this should matter since they both scan at the same rate.

    Can anyone advise me if this is a crazy idea? Has anyone tried something like this before? It seems like its simple enough to add, 20-something resistors, a few caps and 10 transistors. Just wanted to see if someone has been down this path and has any thoughts or advice.

    JH

  2. #2

    Default

    Give it a try sounds like a cool project. Horizontal resolution of a CRT is totally irrelevant to the chassis

  3. #3

    Default

    You would need to find the point in the gun amplifier chain where the signal processing changes from amplitude limited (square waves) to analog. Normally in TTL monitors the input signals are initially buffered by gates, then pass to a few transistor stages before arriving at the gun output amplifiers which are normally class B (sometimes class A) linear amplifiers to drive the guns. So if you wanted linear control over the R,G &B guns the signal would have to be introduced in the correct place to make that possible. It sounds as though your set was configured for this as an option, so if you can get the information, schematics and parts list it would be relatively easy.

  4. #4

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    I think some of the 1084s auto-sense between digital and analog RGB on the same pins, too. That may make things a little more complicated to suss out!
    -- Lee
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