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I want to modify or rebuild the video amplifier circuit of the Macintosh Classic so that it accepts 8 bit grayscale input. Original circuit accepts 1 bit 5V P-P video signal. First idea was just to supply it with 0-5V output from a DAC. I am not well versed in analog circuitry however it seems like video amplifier has clamping diodes and will drive the beam just ON/OFF based on the input voltage.

I will leave the rest of the circuit such as horizontal/vertical deflectors intact. In the picture below is the video amplifier section.

Video amplifier circuit

Farid83
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  • I dare say you’ll have to build a video amp. Generally a couple of transistors in a cascode arrangement for the voltage and speed. The crt vga monitors used an ic that did the three amps, but i think they’re obsolete nowadays. National Semiconductor had their LM24xx series. Shouldn’y be too hard to find example circuits in crt tv and monitor schematics. – Kartman Dec 22 '21 at 01:05
  • I assume you're putting some new machine in the chassis? Because the mac classic can't output anything other than 1-bit video. – Hearth Dec 22 '21 at 01:33
  • Yes, CRT will be driven by Raspberry Pi, FPGA or something similar. – Farid83 Dec 22 '21 at 01:35
  • @Kartman, I've seen AN-861 from TI but it seems like it's an overkill for a presumably simple (maybe not) mod. – Farid83 Dec 22 '21 at 01:37

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I connected a variable resistor between GND and VCC and fed middle terminal to the video amplifier. It indeed senses only low/high level thus it would be impossible to do grayscale with default setup. However I found an interesting circuit from 90s that does exactly what I want - sufficiently simple video amplifier that can accept analog greyscale video input.enter image description here

Farid83
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