[Coco] CoCo 3 Power Supply Question

[Mike Rowen]

Thanks, Gene. This is good information. I can only assume that this configuration provided external power with the minimum modification to the mother board. It also makes more sense to simply connect 5V to the board than to utilize the regulator - as you indicate, just excess heat. Thanks for the insight.

Cheers,
-Mike

On Feb 27, 2013, at 3:28 AM, Gene Heskett <gheskett at wdtv.com> wrote:

> On Wednesday 27 February 2013 03:14:55 Mike Rowen did opine:
> 
>> Hello all. I have a little reclamation project going on. At last year's
>> CoCoFest, I purchased an old AT chassis that was used for some sort of
>> kiosk application that contained a logic board and an entire CoCo3
>> motherboard. I have removed the CoCo3 board and it seems to be
>> unmodified except for one component and this is what my question is
>> about.
>> 
>> There is a 100 ohm resistor soldered between the cathode of D2 of the
>> bridge rectifier and the cathode of D13. Power was connected from the PC
>> switching power supply at the normal transformer connector, but the
>> input power was -12VDC and +12VDC, rather than 12VAC from a
>> transformer. I have not encountered this method of powering the CoCo
>> before.
>> 
>> Can someone with circuit analysis skills tell me if this would work, why
>> the 100 ohm resistor would be necessary, and whether this configuration
>> could damage or shorten the life of the power supply components? Your
>> help is appreciated! Thanks!
>> 
>> Regards,
>> -Mike Rowen
>> 
> It should work Mike, the question is how long.  The problem is the pass 
> transistor with a heat sink sticking out of the left edge of the 
> motherboard.  Its going to be making about half again as much heat as it 
> would on the regular transformer because its getting a full 12 volts that 
> it has to knock back to 5 volts for the logic on the coco's board.  I, when 
> I did that to mine, pulled that whole cookie baker out, and fed 5 volts 
> from the AT power supply right into the emitter pin hole in the motherboard 
> where that transistor formerly lived.  The whole coco now runs at about 2F 
> over room temps, and has been for a good 15 years.  And its got a disto 2 
> meg kit, that and the gime are its major sources of heat now since the cpu 
> has been an HD63C09EP since a few weeks after it was discovered.
> 
> My psu is remote, lives in the bottom of the drive stack tower case, and I 
> have replaced 2 of the drive power plugs on the AT psu with 4 pin polarized 
> trailer lighting connectors.  2 because I've also stripped the power 
> components out of my mpi.
> 
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> 
> Cheers, Gene
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