Speak and Maths Audio Repair and Clean

This article describes the steps to investigate, clean and repair a broken Texas Instruments Speak & Maths (Speak & Math in the US). This educational toy first sold in 1980 was a successor to the Speak & Spell. Both use an innovative speech system based on linear predictive coding (LPC).

The damage was probably caused by some sort of water ingress, it was very grubby and rusted in places - this inanimate object might be appropriately described as ugly, both inside and out.

The repair involved desoldering and replacing the speaker and a thorough clean.

Some of the photographs feature an enhancement which was made during this work. This circuit bending is described in Instructables: Speak and Maths Circuit Bend - Pitch Control Ribbon.

1. One vintage Speak & Math, dirty and broken.
2. Protection
3. Gloves.
4. Face mask.
5. An old newspaper or similar to cover surfaces while working on dirty parts.
6. Cleaning
7. Citric acid powder is useful for de-rusting contacts and helping to remove some rust marks.
8. Detergent, foam cleanser or similar.
9. Methylated spirits (denatured alcohol) and/or isopropyl alcohol (IPA) for cleaning circuit boards and final cleanse of the exterior.
10. A brush for PCB cleaning, ideally anti-static.
11. A brush for battery contact and case cleaning, e.g. an old unused toothbrush.
12. Cotton buds for tight spaces in the case.
13. Kitchen towel.
14. A dishwasher may be useful for a deeper cleanse of plastic-only parts.
15. Replacement parts
16. Same Sky (formerly CUI) 57 mm, Round Frame, 0.25 W, 8 Ohm, Ferrite Magnet, Paper Cone, Speaker GF0571 (2.25 inch): Digi-Key | Mouser
17. Craft knife for trimming.
18. Dremel-like drill with very thin grinding disc for some minor plastic trimming.
19. Keyboard workaround
20. Thin self-adhesive label.
21. Sharp scissors and craft knife or scalpel.
22. Tweezers.
23. A variable voltage power supply is useful for testing.

The Speak & Maths was powered by an external power supply at 6.0V to see if it worked. The display worked, the keyboard worked but the audio did not work. The speaker was completely silent.

Before any cleaning two simple checks were performed on the audio.

1. Speaker (DC) resistance with multimeter measured in circuit at the speaker terminals. This was 1.1ohms ** which is the same as a previous Speak & Spell.
2. Continuity/resistance of blue wires from PCB to speaker. Both were good.

** This in-circuit measurement is later shown to be misleading in terms of representing the impedance of the speaker.

The inside was surprisingly dirty. This could have been inorganic dirt from one or more batteries leaking and disintegrating or possibly vermin-related organic material or some other sort of debris. The battery compartment is fairly segregated from the rest of the device making the widespread debris rather mysterious. It's possible the debris entered via the holes for the plastic clips perhaps by being flooded with water which would explain the wide dispersal. The speaker grill is another potential ingress point for water. The particles which are very fine black ones could be from a thin foam sheet covering the speaker which had completely disintegrated.

Gloves and a face mask were used in the initial cleaning process. The use of an air duster was avoided to limit further dispersal of the possibly hazardous debris.

All of the electronics can be easily removed from the back part of the case and the dark plastic display but the front part includes the membrane keyboard. It seems very unwise to immerse the front part of the case in water due to the presence of the keyboard.

The battery contacts were desoldered and together with the larger spring contact were soaked in warm water with a heaped tablespoon of citric acid powder** in a small glass jar for 20 minutes. They were then scrubbed with an old toothbrush and left in for a further two hours before being rinsed with water and patted dry.

The back part of the case was washed with a shower head and then scrubbed with toothbrush and detergent and rinsed with a shower.

The front was cleaned with methylated spirits with attention to removing the debris from the nooks and crannies.

The power supply daughter board was far cleaner than the main PCB for unclear reasons. Both sides of the PCBs were wiped with kitchen towel soaked in methylated spirits and then brushed with an anti-static brush and a few cotton buds were used to get into tricky places with a final wash with (more expensive) 99.9% isopropyl alcohol. Fortunately, the PCB tracks looked okay as the dirty marks were cleaned off.

The outside needs to be very clean as it's the part that's handled by users. The inside only really needs to be free from particles and loose dirt and clean enough to see any PCB damage and free of anything corrosive that might cause further PCB damage.

** Beyond Ballistics on YouTube prefers a mix of citric acid with sodium carbonate or sodium bicarbonate.

A Speak & Maths makes sound when it powers up. The keyboard had been removed for cleaning but there's a simple trick** for turning it on without an ON button, the first and second pins on the TMC0270 microcontroller can be momentarily shorted. An oscilloscope showed there was an audio signal with the loudspeaker attached. The speaker was desoldered and the test repeated with identical results.

The paper cone on the loudspeaker felt stiff and required some gentle manipulation to free it. The cone still didn't move smoothly and didn't produce any sound when reattached to the blue wires. The resistance of the loudspeaker coil when unattached was infinite indicating the coil was broken in some way. The previous in-circuit measurement would have been skewed significantly by the coil in the small transformer that drives the loudspeaker. The speaker's wires from its two terminals looked okay suggesting it wasn't a trivial fault.

A quick test with a small 8 ohm loudspeaker showed the Speak & Maths did work fine and produce sound. The sound was quiet which raised some uncertainty of the impedance of the original loudspeaker. A second test with a speaker from that era indicated 8 ohms was correct. It measured 57mm (2 1/4 inch) across its diameter. This is a common size from that era and happens to be the same size as the original IBM PC loudspeaker.

** fastmatt says in the comments of a YouTube video that this doesn't apply to the first version of the Speak & Spell with push-button round keys.

The front part of the case had some thin brown marks on the sides near where it joins with the other part of the case. These didn't appear to lighten with vigorous and repeated rubbing with detergent or alcohol.

The ribbon cable from the membrane keyboard was put inside a plastic bag and taped up to reduce the chance of water and moisture entering it. The sides of the Speak & Maths were brushed with detergent diluted with water and it was left overnight with a mixture of citric acid and washing soda crystals (sodium carbonate) on the left side and stain-remover washing powder (contains sodium percarbonate) on the right side. The case was placed so the membrane keyboard was lifted above the surface to avoid pools of water (with chemicals) seeping in.

The powder was carefully removed the next day and the case was wiped down with damp kitchen towels.

This wasn't successful and didn't change the appearance of the brown marks. This is also too risky in terms of exposing the membrane keyboard to moisture.

Speaker replacement is trivial on the Speak & Maths. The Same Sky GF0571 (on the right in first photograph) is almost a perfect like-for-like replacement with the frustrating exception of some extra depth.

The speaker was soldered first and tested to ensure it worked. The polarity of the connections doesn't matter for typical uses of a single (i.e. mono) speaker.

The new speaker was made to physically fit by the two modifications listed below as the first one wasn't sufficient.

1. Very carefully trimming away about 1mm (40 thou) of the dense cardboard-like material (MDF?) with a craft knife.
2. Trimming another 1mm (40 thou) from the speaker's plastic mounts with a Dremel-like drill.

The initial reluctance to remove the membrane keyboard was overcome by watching an inspiring video on the Retro Tech Repair channel on YouTube.

1. The membrane keyboard was lifted by gently pushing a thin blunt object through the two small rectangular holes in the case. These are next to the plastic clips which hold the two parts of the case together.
2. A thin card (like a credit card) was used to break the glue seal between the underside of the membrane keyboard and the case. This works best with many runs across the keyboard with gentle pressure both downwards onto the case and sideways against the glue breaking about 5-10mm (1/5-2/5 inch) each time. Some gentle heat might help here too.
3. The glue which was still a little tacky was cleaned off with gentle circular rubbing with kitchen towel and alcohol. There are two vent holes in the underside of the membrane keyboard - it's probably best to avoid rubbing glue or alcohol into these.

The glue is best removed immediately and completely. The glue can still be sticky and if the exposed part of the ribbon cable touches it then the conductive carbon tracks may be partly or fully lifted off.

The two parts of the Speak & Maths case without any electronics were placed alone in the dishwasher and washed on the intensive cycle. It was then left in the dishwasher for four more cleaning cycles.

This may have had some effect in but certainly didn't completely remove the stubborn brown marks on the side of the case.

This set of tests were used to test the Speak & Maths before reassembly.

1. Power on - wait for the announcement.
2. Maths test - does it speak and score correctly
3. Test all buttons including power off.
4. Test at 6.0V, 5.0V and 4.0V to see how it behaves at voltages for fresh and partially discharged batteries.
5. Test with an external power adapter, 6V DC centre-positive.
6. Test external audio using the 3.5mm socket.
7. Play the SOLVE IT game.

Testing at different voltages covers a range for batteries discharging. If the external power jack doesn't work at first then it's worth removing it and re-inserting it a few times to try to clean the (old) contacts a little.

The Speak & Maths appeared to function well based on this set of tests...

The components shown above are listed below.

1. Back of case.
2. Front of case.
3. Battery spring and two battery connectors.
4. Membrane keyboard.
5. Main PCB with PSU daughterboard and attached (new) speaker.
6. Dark black plastic display cover.
7. Battery cover.

There would normally be a square of thin foam across the front of the speaker to prevent dust ingress. This has probably disintegrated and wasn't replaced.

Display Cover

The display cover was reattached by angling it and clipping it in on the on left side and then firmly pressing on right side near the speaker to click that side into place.

PCB

The PCB sits on some stands which are part of the front of the case. It was placed there and checked to ensure it was flat. The speaker wires need to clear the plastic stands. The speaker was clipped in by sliding one side under the two plastic mounts and then pushing the other side to force it under the third clip.

Battery Compartment

The battery connectors were soldered to the two wires and then pushed back onto the left side of the battery compartment. The top one is positive (red) and the bottom one is negative (black). The double spring was pushed back into right side.

Keyboard

The old glue needs to be completely removed to ensure the underside of the membrane keyboard is free of lumps and can lie perfectly flat. This also needs to be true for the part of the case it will be attached to.

UHU all purpose adhesive was used for this, partly because it smells very similar to the original glue which still had a mild odour after over forty years! This glue is described as "Not suitable for Styrofoam®, polyethylene (PE) and polypropylene (PP)" but this is likely to be related to poor adhesion. The Speak & Maths case design includes a rough surface for the keyboard to sit on which is probably there to improve the adhesion.

The glue was spread in a thin even layer across the grey case in the keyboard well. This needs to be done rapidly as the glue will start to set (perhaps 90-150 seconds at 22 degrees Celsius) so ensure everything is ready before starting to spread the glue. After checking the glue layer was flat the keyboard was placed on top of the glue and the centre was pressed gently moving outwards in each direction to the edge to adhere it. The keyboard was then left on five sheets of kitchen towel cut to keyboard side and one large sheet of paper to absorb any glue seepage on a flat surface with a little bit of weight on the case for four hours.

Double-sided tape is a less fiddly alternative.

Case

After checking the Speak & Maths still worked and the PCB was in the correct place the back of the case was aligned with the six clips and pushed back on.

Some more extensive testing of the SOLVE IT maths game revealed the "key" for 6 would sometimes produce a 3. This wasn't rare, more than half the time 6 was pressed a 3 would appear. If it was pressed towards the top of the circular 6 key it would produce a 6 most of the time. Some more experimentation showed pressing - and * keys would sometimes produce a 3 too. Pressing the areas between the 3 key and all adjacent keys (marked with red crosses in the photograph above) would produce a 3 - this was not true for any other key.

This was a disappointing discovery but a crucial one in revealing the Speak & Maths would be confusing to use in this state of partial repair.

The Speak & Maths would occasionally behave strangely or crash. On these devices this is often an indication of multiple key presses confusing the program on the microcontroller.

The keyboard is a membrane keyboard similar to a common 4x4 numerical keyboard used by hobbyists. These have an upper and lower layer with conductive tracks on and a spacer layer with holes in between them. The isolated nature of the problem suggested a mechanical/physical issue rather than an electrical one.

After removing the membrane keyboard again and cleaning off the new glue on it the keyboard was inspected and it looked okay. On closer inspection and with the light catching it from a certain angle some minor indentations could be seen near the 3 key. These are shown above with red circles highlighting them.

This seems like an unlikely cause but this type of membrane keyboard is constructed from very thin layers meaning a small change in height could cause premature contact resulting in the false keypresses.

A list of possible solutions was made.

1. Heat the back of the membrane keyboard up with a hairdryer and hope it flattens itself.
2. Open up the membrane keyboard and then attempt to (permanently) re-flatten that area of the plastic.
3. Add a spacer around the 3 key to prevent the indentations from causing these false key presses.
4. Use something abrasive to remove some or all of the carbon conductive track on the small raised parts of the indentations.

The first idea was tried and didn't work. The third idea was tried and worked well rendering trying the second and fourth ideas unnecessary.

The spacer was made from a self-adhesive laser printer label cut to the right shape with scissors and a scalpel. The scalpel was used to carefully separate the layers of the membrane keyboard and slid along to cut a 5cm (2in) hole. This must be done at the top of the keyboard as the bottom appears to have critical electrical connections from the top and bottom plastic layers. The adhesive backing was removed from the label and tweezers were used to put it into position and then gentle pressure with fingers was applied to stick it to the "holes" layer.

This was tested thoroughly before the keyboard was re-glued. The spacer worked well and there were no false presses for 3. The amount of pressure required to produce a 3 increased but only slightly.

The keyboard was glued back on for the second and hopefully final time.

The Speak & Maths was tested again thoroughly - this time it worked well.

After a final clean this became a Christmas gift and was entrusted to the postal service for its long trip to the young recipient.

Hopefully, this article has inspired you or helped you to repair a vintage Speak & Spell or Speak & Math(s).

Repairs and technical information:

1. Retro Tech Repair: TI Speak and Maths Repair (YouTube) - track repair on the ribbon cable of the membrane keyboard, also shows opening up the membrane keyboard.
2. fastmatt (YouTube)
3. Troubleshooting Speak & Spell - won’t turn on
4. Repair a glitchy Texas Instruments Speak & Spell Speak & Read Speak & Math - capacitor issue
5. Furrtek: Speak & Spell (french version) - reverse engineering and creation of a new ROM.
6. Ingmar's Retroblog: Speech output in the 80s – Speak and Spell (german version)
7. Datamath Calculator Museum:
8. Texas Instruments Speak & Math (US Type 1)
9. Texas Instruments Speak & Math (US Type 2)
10. Texas Instruments Speak & Maths (UK Type 1)
11. Texas Instruments Speak & Maths (UK Type 2)
12. Instructables: Speak and Spell Power Board Repair - a repair to the power supply daughterboard and a workaround for the ON button not working properly.

Electronics repairs:

1. Mend It Mark (YouTube) - no Speak & Spell repairs, but many other inspirational repairs by the talented Mark Maher of Perton Electronics.
2. Electronics Repair School (YouTube) - complex repairs on laptops, tablets, smartphone and more.

Small speakers:

1. EE Journal: Speakers in Audio Design -- Same Sky and Mouser Electronics (YouTube) - "Nick Grillone from Same Sky and Amelia Dalton explore the characteristics of speakers in audio design and the parameters you should consider when choosing a speaker".