Re: [stella] The Atari 2600 Puma, where did you get those skills?

Subject: Re: [stella] The Atari 2600 Puma, where did you get those skills?
From: Kevin Horton <khorton@xxxxxxxxxx>
Date: Wed, 10 Jan 2001 19:10:04 -0500
At 11:55 1/10/01 -0600, you wrote:
> Just thought everyone would like to see my latest hardware Atari 2600
> developments. :-)

This is very impressive.  How did you learn how to do this?  Do you have a
background in Electrical Engineering?

Thanks. Actually, no. I've been doing this stuff for 21 years on my own and just sorta picked it up in that time.

I would LOVE to be able to do something like this, but my main problem is:
How do you know when to use a capacitor, and how do you know what size to
use?  How do you estimate how much voltage and current you will need?

"A woman calls a plumber since her water pipe was leaking. The plumber comes out, hits the pipe with a hammer and charges her $100.

 '$100?  but you just hit the pipe with a hammer!'
'Yes.  I charged you $5 to hit the pipe, and $95 to know where to hit it.'"

After many years, one gets a "sense" of what to use and where to use it. I can't describe it; I just "know" what to use.

Doesn't cutting out the excess 2600 parts in your "Puma" design alter the
voltage, line noise, and timing requirements, and how do you take that into

I had to totally re-design the video circuitry to interface with my chroma decoder. Also, I "cheated" on the crystal oscillator and used the oscillator on the PIC microcontroller to run the TIA :-).

For example...on your web page, you had a great schematic of the audio
output from a NES.  It seems they are reducing the output voltage, inverting
it, and then amplifying it again...with occasional grounding connections and
a seemingly random sprinkling of capacitors.  Why go through all this...why
not just sprinkle a couple of caps to filter out line noise and then tie it
directly to the audio jack?

The inverter gate is in its linear region, and is no longer acting like a logic gate. The resistor across the gate biases it into the linear region. The capacitors are used for AC coupling (Between stages) and the caps to ground are making a pole of a lowpass filter. (Incidentally, the cap across the logic gate is also making another pole of a filter). This filter/amp thingy is used to attenuate (reduce) the high frequency components that would otherwise cause problems with the RF modulator, resulting in interference in the picture among other things.

How do you learn all of the "analog glue" that holds the digital design

Many years of working with it. I didn't totally design the chroma decoder for instance though. The chip maker's datasheet has a design test circuit to get
you going. It has a "typical" application circuit on it which can be used to get the chip up and running. Of course, most final designs use this circuit with minor tweaks.

Another interesting tidbit is that on your Bankzilla page, you mention that
it would cost $100,000 in order to have a custom-made plastic case designed
and built.  Where did you get that info from?  I have been kicking around
the idea of trying to melt some old plastic and re-forming it...but I hear
that melted plastic gives off hazardous fumes...or is it burning plastic?

You would have to do that in an inert environment, to prevent burning of the plastic. As for the cost, I'm just guessing. :-) At work, we had a custom designed aluminum injection mould made and it cost $5000. This type of mould is only good for 10K pieces or so. Most "professional" moulds (like the ones Atari would've used) are made out of hardened steel and can withstand many more mouldings. The mould we had made was a VERY simple 1 piece job. To make a device enclosure would require two or more pieces (remember, each plastic part needs its own mould). Typical "Atari" style moulds would've been multi-cavity (i.e. more than 1 part made at a time) and would be made out of steel. So, $50K-$100K for a complete mould set for say, a 2600 or 7800 enclosure is probably not too far from the mark.

Incidentally, this is why I used the Nomad case for my Puma. It was already made professionally, and it is nearly perfect as-is. I don't have access to a $300K computer-controlled milling machine or anything so it is all I could afford :-). IMO it looks "honest" as it stands now, and it works good so I'm happy with the final result.

BTW, if anyone has any more tech questions, please direct them to me 'cause it is probably going to be off-topic for this list.

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