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Texas Instruments Speaking Toy Repair

Hello all...

I've got a few Texas Instruments speaking learning toys...the old Speak & Spell, Read and Math. I recently picked up two more, mainly for the add-on ROM word cartridges they came with. One is a Speak & Read from around 1987, the other a Speak & Spell from 1984.

These two units don't work properly. They both fail in pretty much the same way. When powered on, they work normally for a moment or two and then the speech output starts failing. Symptoms range from starting to say a word normally and being unable to finish, to random noise and "hash" sounds coming from the speaker. Every now and then the speech will "correct" itself and try to continue, only to fall down again.

The Speak & Spell is much worse. It will actually start corrupting the vacuum fluorescent display and power off (presumably it's crashing) after some random length of time.

I decided to look at what I could, and found a schematic online. It's of rather poor quality and doesn't cover the "passive" components in the circuit.

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There is a power supply board in the units that outputs multiple voltages from a single battery supply. I did checks on this compared to my working units. It appears to be OK.

So I turned to the only other idea I had. I checked the clock oscillator circuitry to see what it was doing, using the frequency counter range on my multimeter (the only method I have). What I saw was strange...the clock frequency seemed to be wandering all over the place. Doing a cross check with my working unit produced another difference. While probing it, the voice shot up dramatically in pitch any time I touched either of the clock pins on the speech synthesizer IC. That never happened in either of the broken units.

I think this is trying to tell me something, but I'm not familiar enough with the type of oscillator used or the circuit in general to know what. I asked a friend of mine, who said "that looks simple" but he's very busy and hasn't gotten back to me. I'd greatly appreciate any thoughts.

William

Reply to
William R. Walsh
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Well, if the clock is wandering, isn't that a good place to start troubleshooting?

Is the oscillator crystal-controlled, or a simple RC circuit?

Reply to
William Sommerwerck

It's an RC oscillator circuit. The schematic shows an adjustable resistor. None of my units have that--there is a fixed resistor. I cannot tell what the value of the capacitor should be...guessing 60pF?

I forgot to mention--in my working toys, the clock still appears to wander, but much less severely. It seems to fluctuate based on whatever the microcontroller is doing.

Lacking in-depth experience with these circuits, I also want to be sure whether I'm on the right track or not.

William

Reply to
William R. Walsh

I'm guessing...

The circuits are probably not asynchronous. That is, the clock has to be within a certain tolerance range for the chips to "work well with each other". If the clock wanders, the chips may lose synchronization.

Reply to
William Sommerwerck

I've fixed a few of those:

  1. Rotten battery contacts.
  2. Dead batteries.
  3. Replace every electrolytic you can find, even it if looks ok. If you have an ESR tester, use it.
  4. Some units have a tantalum capacitor on the power supply board. Replace it.
  5. Dirt and filth UNDER the big chips. Alcohol and a brush.

That's the symptom of a dead tantalum on the phenolic power supply board. What's weird is that I tested the original tantalum for leakage and ESR. It was fine, but I replaced it anyway. That was the problem.

Argh... gotta run....

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

That dithered clock is a technique we use to make it appear that our designs are 'quieter' than they really are, RF noise - wise.

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Since all parts of the circuit see the same clock, everything remains synchronized.

Relativity in action! :)

--Winston

Reply to
Winston

BTW, if you get a chance, power "Say 'l[owai er=09 e0rj'"

You will say "l[owai er=09 e0rj".

Then it will say:

"No. Say 'ro4w nwubyrmvto u'"

Indicating of course that this is what it expected to hear.

It is quite wonderful. :)

--Winston

Reply to
Winston

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