It’s sad but true that the ZX81/TS1000 machines tend to be sensitive to overheating in the summer. With a solderless mod, you can in most cases cure the “summertime blues.” No, there’s no drilling and carving involved either.
Most overheating problems can be attributed to the “workhorse” chip, or ULA (Uncommitted Logic Array) that co-ordinates the CPU and memory with TV monitor and cassette jacks. Less often, the CPU develops oddities (especially the SGS unit used in the later Timex’s), and on occasion the 5V regulator is to blame. Generally, the regulator is quite capable of holding its own, unless the bolt to the heatsink is loose or you have a large peripherals current demand.
Before we go on, let’s look at the cause of overheating and possibly bury some myths about “keeping cool.” Heat is generated primarily in the silicon chips that make up the computer; how much heat depends on the power each chip consumes. Typically, the regulator “burns” 2.5 watts (with 16K RAM), the ULA 1.3 watts, the CPU 1.1 watts and the built-in RAM and ROM together only about .1 watt. How hot each chip gets (temperature) depends on its ability to get rid of heat to the surroundings. Heat developed by the chips has to pass through the IC package, where convection inside the case carries it to the case, and conduction takes it to the outer surface for outside convection to remove. Heat transfer due to radiation (“infra-red”) is negligible (this is why there’s not much point in painting heatsinks black, etc. Actually, most paints actually make matters worse, as they’re not “black” as far as infra-red is concerned.) Note that since there’s not much convection from inside to outside of the case, there is no real advantage to a larger regulator heat sink in terms of overall operating temperature or IC junction (chip) temperature. At best it might help out a borderline regulator and reduce hot-spots slightly. So short of putting big vent holes in the case there’s no easy way to improve heat flow from inside to outside of the case.
We can reduce the thermal resistance between the chip and the inside of the case using a metal heatsink to aid in the internal convection process. This is already done for the regulator, which is why it’s usually the least of our worries.
To significantly reduce the operating temperature of the ULA and CPU chips, all you need is two strips of aluminum, 1″ x 3″, 1/16″ to 1/8″ thick, and a good grade of cyanoacrylate (“superglue”) adhesive. We’ll cement the aluminum heat sink to the top of the large chips and solve perhaps 90% of overheating problems.
When you have the materials, open the computer and remove the board (or carefully flip it over so you don’t have to remove the keyboard tails.) After thoroughly cleaning the sinks and scoring with emery paper to ensure a good bond, lay each heat sink next to its respective chip, and mark where the chip should be so that the heatsink doesn’t stick over either edge of the board. DON’T try to glue the sinks straight to the installed chips, if any superglue runs over the edge you’ll be in trouble. Instead, gently remove the chips. Lay down the heatsink and apply a thin line of glue between your end marks, and cement the chip to the metal upside down, Let the bond cure overnight, preferably with mild clamping. Be sure you cement the right heatsink to the right chip, as the positions of the ICs on the strip are not the same. Also be careful not to glue them on backwards!
When cured, carefully re-install the chips with attached heat sinks, re-assemble the machine in the case, and test it. If you followed the instructions and used fresh, good-grade cement, the bond will hold even as the chips heat up. If you prefer, you can use a good epoxy instead, just don’t make the bond too thick. Silicone cement probably works but I haven’t tried it. –A good bet for this would a high conductivity adhesive, RTV silicone or epoxy. TB
Chances are, you just cured your machine of strange crashes no matter how hot it gets where you live. The following section gives an additional aid if you’re still having trouble.
(RE-EDIT NOTES: Since this was written, I’ve found out that the replacement ULA chips supplied by Timex come with a small ribbed heat sink glued on top, ie., they come “stock” with “custom coolers” now. It might be a good idea to get one of these as a back-up while they’re still available. TS1500 owners – your ULA chip is in the smaller and virtually nonremovable “flat-pack” package. The smaller size could aggravate heating problems, and the new package makes replacement a real night-mare. Not only are they soldered in, but the solder joints are UNDER the chip! DEFINITELY consider installing a “custom cooler” on these! this is especially true if you live where the weather gets unbearably hot. Another alternative is to do like I did and move to Canada.)