[Coco] CoCo floppy disk subsystem

[Salvador Garcia]

 Thanks Gene. Yes, I saw the mention of the 7551 in your post from yesterday. Alas, my electronics knowledge is limited and I did not fully understand how this fit into the circuit. I understand the notion of a charge pump, but searching for 7751 charge pump did not yield any results. Salvador
    On Friday, January 15, 2021, 04:58:45 PM CST, Gene Heskett <gheskett at shentel.net> wrote:  
 
 On Friday 15 January 2021 16:58:04 Salvador Garcia via Coco wrote:

>  Hi all, I think I found the 26-3022 controller's schematic. Can
> anyone verify if this is it, or whether there is a better option?
> Thanks! 
>
> https://colorcomputerarchive.com/repo/Documents/Manuals/Hardware/Weste
>rn%20Digital%20Floppy%20Controller%20-%20WD1793%2012V%20Schematic%20Dia
>gram.gif
>
>
> If that is the one, other than going to caps (top of diagram), the 12v
> line only goes to R17 (750 Ohms). Salvador

Yes, that is it. The total load is 1750 ohms +- the tolerance of the 750 
and the 1k pot, plus whatever few microamps the chip might draw from the 
pot arm. That would be nominally a 10 milliamp maximum draw.  A charge 
pump as I wrote in yesterdays msg would handle that nicely, drawing 
about 15 ma from the 5 volt line. A7551 running at 100 to 250 kilohertz 
would use very small caps, 2 of them, and a couple 1n914's to stack 
about 4.5 volts on top of the 5, the 2nd cap being a filter to reduce 
the 7551's pulse noise on the output.  Even that one could be quite 
small if the 7551 is doing north of 100khz. That raises the parts count 
to 3 small caps and a resistor to make the oscillator work plus the 2 
diodes.

Cheers, Gene Heskett
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