Speech & Sound Pack
Modification for High Speed

Automatic switch by Robert Emery (theother_bob@yahoo.com)

March 1, 2003

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modifications in place

A little background info:

This modification is based on two existing articles detailing the modification of the Tandy Speech and Sound Cartridge so that it works with the high speed poke. I will not cover the details of these articles, the links are included.

The first article is located on CoCo3.com, and suggests replacing a transistor to fix the negative voltage circuit, which fails at high speed on a stock SSC. This article also provides instructions for adding a mechanical switch so the user can manually set the cartridge to work at the desired speed.

The second article I found is on Robert Gault's homepage and also features a manual switch, but instead of replacing the transistor, Mr. Gault points out that the same effect can be achieved by simply adding a capacitor across R16 (or C26 as in my photo). Beautifully simple, and it works.

The switches are located differently in the two articles, but provide the same function... they provide the sound effects chip with a 2MHz clock, which is normally provided by doubling the CoCo's Q clock. If the CoCo is running at high speed, we no longer need to double this clock.

Someone asked if the switch could be automated...

Looking at the CoCo3 article, I figured out that I could replace the mechanical switch with a 74LS08 AND gate. This gate could then act as an automatic switch if it is fed a signal which is controlled by the CoCo's clock. That signal is provided by the Turbo Light circuit located on my Projects page. Any Turbo Light circuit for the CoCo could be used to control the switching.

If you don't have the Turbo Light circuit, you can simply install a jumper on the plug for high speed or remove it for low speed operation.

The Hack...

Since the AND gate switch uses the same connections as the CoCo3.com article, it requires cutting the same trace on the circuit board. The trace goes from card-edge pin 7 (Q Clock) to pin 9 of IC3 (74LS86). I lifted C8 and cut the component-side trace as indicated in the image. The eyelet here also makes a good place to get the Q clock to feed pin 9 of the AND gate.

The AND gate (74LS08) is piggybacked on top of IC3. Make sure to align pin 1 of both chips. Only pins 7 (Vcc) and 14 (Gnd) should be soldered to IC3... all other pins of the 74LS08 should be bent out, preferably removed with the exception of pins 8,9 and 10, a single AND gate. I actually cut my 74LS08 from an old 8088 motherboard with some small diagonal cutters, desoldering only pins 7 and 14.

Connect pin 8 of the 74LS08 to pin 9 of IC3. Connect pin 9 of the 74LS08 to the Q clock at the eyelet under C8 (the blue wire in the photo). Connect pin 10 of the 74LS08 to one pin of a 2 pin jumper plug, the other pin goes to ground.

Finally, take a twisted pair of wires with a 2 pin socket (such as an LED or Reset switch connector from a PC) and connect the wires to pins 6(/Q) and 7(Gnd) of the 74LS74 of the Turbo Light circuit. Make sure you don't plug the connector into the SSC plug backwards or you'll be shorting you're CoCo's Q clock to ground... probably not a good thing.

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