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.
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.