[Coco] Strange video issue

[Barry Nelson]

I got it working properly! I made the following modifications. First, instead of using pin 12 of U12 (MC1372) for the input I now use pin 1 of the RF modulator. I also added a 10k pot and connected the input to one side, grounded the other side, and connected the center to the first transistor's base lead. I also added a 100 ohm resistor from the output end of the 22 ohm resistor to ground. It now works great! I was able to fine tune the video using the pot, and compare it to the TV output which also still works. I have reassembled the computer with the new video and output jacks permanently installed in the back of the case. When I have time I will post updated schematics to https://hackaday.io/. I have one hot CoCo1 one now! 64K of ram, floppy controller, double sided floppy drive and composite video mod working! Woo woo!


On Sep 7, 2015, at 11:44 PM, coco-request at maltedmedia.com wrote:

> Date: Mon, 7 Sep 2015 20:29:59 -0500
> From: Zippster <zippster278 at gmail.com>
> To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
> Subject: Re: [Coco] Strange video issue
> Message-ID: <3C0E0231-0771-4A40-8485-8F1DCA3E86A1 at gmail.com>
> Content-Type: text/plain; charset=utf-8
> 
> Signal level/proportions are off.  That circuit (Darlington pair with a decoupling cap) doesn’t allow for any adjustment.
> It also may not help that the CoCo 1 doesn’t have a color burst generation circuit.
> 
> Redesign the circuit to allow for amplitude adjustment and proportion of sync to active video?
> 
> Hard to get more specific without putting a scope on it.  The way it’s coupled through a 22Ω resistor kind
> of makes me wonder what kind of receiving circuitry they were expecting on the other end...
> 
> Can you compensate with the tint controls on your TV?
> 
> - Ed