[Coco] Converting CoCo 3 to USB power

[Rocky Hill]

 Hi Lee,
The transformer converts your 120VAC to about 16VAC and that gets rectified by 4 onboard diodes to produce about +12VDC and -12VDC.Those voltages are then fed to the SALT chip. The SALT chip in conjunction with Q1 produces +5VDC from the +12VDC to power the CoCo.The SALT chip uses the -12VDC to produce the -5VDC needed for RS232 communications(bit banger). 

 You can unplug the transformer and directly power the CoCo with +12VDC(or close) and the SALT chip + Q1 will still produce+5VDC and the CoCo will power up but since the SALT chip no longer has the -12VDC available the bit banger will not work. 
If you don't need the bitbanger, this works. If you want to have the same functionality and run the CoCo off of DC then you'll need a solution like the pepper board. The RS232 level converter used in the pepper board internally produces +-5VDC for RS232 communications from a single DC source.
If you are modding a CoCo 3 to work from a DC source, then you must short a capacitor(C62). This capacitor feeds about 12VAC to a rectifier circuit that is then brought down to 8VDC to power the transistor audio amplifier.
Once you have the CoCo working how you want it to work from a DC source then you can go ahead and add the USB-C PD board. 
I hope this helps.
Pedro
    On Sunday, January 28, 2024 at 01:06:16 PM EST, Lee via Coco <coco at maltedmedia.com> wrote:  
 
 Patrick,
So you think he's done something more than just cut the 3-pin connector
cable from the power supply he removed and connect the 3 wires where he cut
them to the +9v (he demoed it in the video with a 9v battery) and ground of
the tiny USB PD board?
-----
Lee

On Sun, Jan 28, 2024 at 8:33 AM Patrick Ulland via Coco <
coco at maltedmedia.com> wrote:

> The CoCo supply is more complex than you’d think. Starts easy, a
> transformer rated 8 volts either side of center tap. Runs about 10 volts
> under light load. Those 3 to connector.
>
> The two (+/-) are rectified and sent to the SALT (or pepper
> replacement). The salt internally regulates these to become the +/-
> power rail for analog and zero crossing work – cassette, bitbanger,
> joysticks. Low current supply done.
>
>  The raw +8VDC also goes to pass transistor Q1 – what many call a
> ‘voltage regulator’. SALT has the 5v reference, and switches Q1 at the
> right speed to keep the + side of C15 a perfect 5v, which feeds the main
> 5v rail.
>
> I don’t see from this video what Bob’s done with Q1, maybe hidden under
> tipped over c29? Else there are more secrets;-)
>
>  -ricku
>

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