SPO256 Speech Board Construction Details


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SPO Circuit:

Note: It is not uncommon for the SPO PCB circuit to produce static on the first power-up. If this happens, just remove power and try again. This has been reported by a couple of people - and experienced by myself on occasion - but seems to be only a temporary condition. Probably due to stray charge when getting the board ready.

Small Holes:

NOTE: This only applies to version 2.1. Fixed in all later versions!
There are three minor problems with the boards. All of the traces are correct, but some of the holes are too small. Specifically:

  • All of the header block holes are just a little small - they should be 0.035" but are only 0.029". These can be easily fixed by lightly reaming out the holes and pressing the header block pins in. (There should still be plenty of copper in the holes for good solder joints.) Alternatively, you can just scrape or file the corners of the pins down a little with an Xacto knife and apply a little force to insert them.

  • The holes for the crystal are intended for HC-49U style crystals. My 3.120MHz crystals are HC-50U plug-in type with 1mm diameter "wires". By the time you ream out enough to fit the wires in, there is virtually no copper and hardly any solder pad left. I recommend drilling new [#18?] holes close to the originals with tight tolerance [to provide a better mechanical bonding], bending the wires in [think of a staple], and then soldering small wires from the original holes to the wire pins. [If you try to thin the crystal wires and ream the holes, you might be able to get the crystal in, but I'd be afraid that the joints would fail over time due to vibration and mechanical shock.] Also, the holes are 0.08" too far apart so the plug-in wire-pins may prevent the crystal case from sitting flush on the board.

  • The holes for the timmer cap by the crytsal are way too small for any trimmer I've since found. If you're going to use a trim cap there, you'll have to be creative about how to do it. (If you figure it out, let me(Jim) know - I'll post it here.) On my first board I did NOT put in a trim cap. [Second board, neither.]

    General info:

    Solder all components to the PCB, clean [use rubbing alcohopl (isopropyl)] , and inspect your work. Many faults can be cleared by using thinners and a stiff brush to remove all flux and solder splashes from the PCB.

    Datasheet for the SPO256-AL2 in case you don't alread have one. (And I parked a copy here in case Tandy moves it. Or a PDF of a lousy scan if you prefer.)


    Speech Board Component-side View
    (Enlarged to show detail. Actual size is 2.5" x 1.5")
    (Same view without annotation.)
    Please use this page for version 2.3 and older.




    Audio Circuit:

    Construction Notes:

  • 10K pot: This one can be hard to place if you don't use the Bourns 3386H part. Please make sure to plan ahead as this can be difficult place since there are other components so close. Also make sure you can adjust the pot - my second board has disc caps for the 0.022uF and they partially block access to the pot adjust screw. On my first board I used axial caps which made more room.


    Optional Circuit:

    Construction Notes:

  • If you plan to NEVER use the 12C509 PIC, you can omit the socket for it. You can also just use a TWO pin Berg header pin for H2 - you won't need a connection for R and I at that location. This means that header will only have power and ground.

    With versions later than 2.3, you can omit H2 completely by using a 6x1 Berg header for H1.


    SPO Circuit:

    NOTE: The following circuit schematics originally came from Jon Williams' article "Using the SPO256-AL2 Speech Allophone Processor" in the June 1998 issue of Nuts and Volts [Volume 40]. Go to the Paralax site, Downloads, Nuts and Volts columns for more useful stuff!
    Note: I originally found the gif of this circuit at TheOneSpot - Robotics - Text 2 Speech but that link appears to be broken. :-(

    SPO Circuit


    Note that the output pins QA-QH from the shift register [74HC164] are swapped MSB for LSB going to the SPO256 chip. This greatly reduced the difficulty of the trace routing for the nested shift register [which was nested to conserve board space].

    Audio Amp circuit:

    Audio Amp


    Original audio circuit gif.



    This page made of 100% recycled bits.
    Edited (as my 12-year son brags of his web pages) with Notepad.