[Coco] WTB Orchestra 90 cartridge

Zippster zippster278 at gmail.com
Wed Aug 3 08:39:10 EDT 2016

Yes, I’m reluctant to open the one Orch 90 cart I have here as well.  :)

I’ve been wondering about the choice in value as well.

Using .1uF does seem like a really low value, until you take into consideration
that the output cap, in conjunction with the input circuit on the receiving end
form a high-pass filter.  To see what the effects of the value chosen will be,
we need to know what that load impedance will be.

What I can gather on the internet indicates it is typically 10,000Ω or more,
at least for typical consumer gear, and walking around the house with a multimeter
here would seem to confirm that.

The lowest I could find were the small speakers shown in the CoCo2 test,
at 12,000Ω.  One TV came in at 50K ohms, and everything else I could find
ran into many, many megohms.

So, in a worst case scenario (10K), you’re looking at a -3dB roll off of ~159Hz 
using a .1uF output cap.  At megohm input impedances, it’s negligible, certainly
beyond the range of human hearing (mine at least), and wouldn’t really matter.

If the schematics are correct at .1uF, I almost suspect Tandy might have intended
the high-pass formed from the connection to aid in filtering out low Hz noise,
but who knows.  The other consideration I suppose would be just what frequencies
is this circuit even capable of producing.

I may bump the value up of the output caps to something like 10uF or more on
the final revision, not sure.  BTW, I was able to find .1uF bi-polar electrolytic caps
at Digikey.

<http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=203424567&uq=636057666275979890 <http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=203424567&uq=636057666275979890>>

- Ed

> On Aug 3, 2016, at 4:51 AM, Jayeson Lee-Steere <cocolistemail at titaniumstudios.com> wrote:
> Very cool Ed.
> On capacitors, I think perhaps the schematic is in error with the 0.1uF
> non-polarized cap values. The smallest NP electrolytic I can find available
> anywhere is 1uF, and if you look at the pic of the Orch 90 board, what
> should be the 0.1uF NP caps are physically larger than what is supposed to
> be a 1uF NP cap. And then there is the question of, if 0.1uF is the
> intended value, why not use polyester caps as was done with the opamp input
> circuit?
> In fact, the caps look suspiciously like the polarized 10uF bypass cap on
> the 5V rail.  This happens to be the same value as the output cap used for
> Tandy's Coco 2 and Coco 3 audio out. Not the best evidence since the output
> impedance of the Orch 90 DAC is indeed around 1/100th of the  Coco 2/3
> circuits mentioned. Still, makes more sense to me than using a 0.1uF NP
> electrolytic where a 0.1uF polyester cap would do.
> Having the screw under the label sure is a curse, eh.
> Jayeson
> On Wed, Aug 3, 2016 at 6:47 AM, Zippster <zippster278 at gmail.com> wrote:
>> Thank you.  :)
>> After playing some tunes in your CoCo, you can just flip the switch and
>> plug it into a Dragon.  Thanks to Pere Serrat’s adaptation of the Orch 90
>> ROM.
>> <https://drive.google.com/open?id=0B1sgl615FlGnNzRhNC1xSjRBSFk <
>> https://drive.google.com/open?id=0B1sgl615FlGnNzRhNC1xSjRBSFk>>
>> Just plugged into the TV speakers, but you can see it works…
>> - Ed
>>> On Aug 2, 2016, at 5:38 PM, Bill Pierce via Coco <coco at maltedmedia.com>
>> wrote:
>>> <Applause> Good job!
>>> Bill Pierce
>>> "Charlie stole the handle, and the train it won't stop going, no way to
>> slow down!" - Ian Anderson - Jethro Tull
>> --
>> Coco mailing list
>> Coco at maltedmedia.com
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