[Coco] Cassette Program Loading Help Needed

[Gene Heskett]

On Sunday 19 March 2017 01:43:45 RETRO Innovations wrote:

> On 3/18/2017 11:12 PM, Dave Philipsen wrote:
> > In theory, a square wave should work.  The only reason for the sine
> > wave was to be more accommodating to the analog circuitry of a
> > cassette recorder.  You might try a square wave with some sort of
> > attenuator on it to control the amplitude of the signal going into
> > the CoCo.  As I recall, the CoCo was somewhat sensitive to level and
> > you had to tweak the volume control on the cassette player to get it
> > to work reliably.  You can always put a 'scope probe on the output
> > of the comparator to see what it's passing (pin 13 of U14 or pin 2
> > of U4).
>
> I continue to try things.  As the DAC did not work at all (no amperage
> to send to the Coco), I went back to the PWM.  I moved to a 2.2uF cap,
> and I am using 1 uF DC bias blocking cap to center the output without
> DC bias.
>
> Here's what it looks like going into the Coco:
>
> https://s6.postimg.org/cz7evq2ip/IMG_7728.jpg
>
> I'll see if I can do a square wave, but the dc blocking cap and such
> are messing it up really bad right now.
>
> I am getting a "IO error" now, so things are "better", but I wish I
> had an app on the Coco I could use to see where the read is failing. 
> Anyone handy with ASM and looking for a small prj? :-)
>
> Jim
>
Jim, that looks a lot like what I have always called ground bounce. And 
one of those bounces is awfully close to hitting the zero crossing.  
Where is the scope probes ground lead attached? It looks as if its 
bouncing around by the current flow in the zero crossing detector. 
Getting the scope probes ground on a good solid ground, and some 
additional rail to rail bypassing can work wonders, particularly in our 
30+ yo hardware. Many of those low voltage electrolytics marked as 10 uf 
at 6.3 volts or even 16 volts have a huge ESR at this late date, and if 
carefully measured might not have .01 uf left for actual capacitance. 
You should be seeing the typical t=rc curved ramping from one voltage to 
the other.  Those relatively narrow kickbacks near the peaks are 
artificially being injected from something else.
>
> --
> RETRO Innovations, Contemporary Gear for Classic Systems
> www.go4retro.com
> store.go4retro.com


Cheers, Gene Heskett
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