[Coco] Converting CoCo 3 to USB power

[Bruce W. Calkins]

What I did in 1997 was plug +12V to one side and 12V ground to the 
center to run my CoCo 3 off the cigarette lighter plug in a '94 
Peterbuilt.  Pretty much the same thing with the disk drive too.  I 
never bothered adapting a printer or any other accessories.

Bruce W.
~KV4OE~


On 1/28/24 15:02, Rocky Hill via Coco wrote:
>   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
>>
>