Advancing along with the Retrofitting of a ZX81

At the end of the last article, 'Simple Start To Retrofitting A ZX81', a new power supply was sourced and a composite video mod was fitted. This entry sees the near completion of the internal retrofitting project; where some capacitors are replaced, a new voltage regulator is fitted, a lower Amperage CPU inserted and some heatsinks affixed.

Replace Those Old Capacitors

Over a number of years, particularly if exposed to continual high temperatures, electrolytic capacitors tend towards failure. While there is no obvious evidence that the capacitors in my ZX81 have bitten the dust, they are 35 old and probably due for replacement even if purely as a precautionary measure.

Depending on the Issue number of a ZX81, there will be either 2 or 3 electrolytic capacitors on the main board. I have an Issue 1 USA board, as such there are 2 capacitors I'd be replacing. The caps are numbered C3 and C5, are located up the back of the ZX81 PCB, to the right of the modulator and left of the expansion edge.

New Capacitors in Place

Visually the 35 year old capacitors seemed okay, there was no tell tale bulging, or leaking of electrolytic fluids. What wasn't so brilliant was the illegibility of the markings on C5, as the the outer plastic / paper casing of the Caps had shriveled over time (and heating). Handily the ZX81 Service and Assembly Manual is available from numerous online services, confirming the values required:

C3 22uF Electrolytic 16V minimum
C5 1uF Electrolytic 5V minimum

Both the existing capacitors had voltage levels specked well over the minimums at 50V. Higher voltage ratings are a good thing for capacitors, as a higher voltage rating will prolong their life expectancy. I had some 63V Electrolytic 22uF and 1uF caps at hand and so simply swapped those over for the existing ones.

Switch Mode Regulator

Heat is possibly one of greatest enemy of the aged microcomputer, and lurking inside a typical ZX81 are two major heat manufacturing components, the ULA and a linear voltage regulator. The ULA we can't do to much about, however the regulator can be replaced with a modern switch mode equivalent.

7805 Regulator and Replacement Recom R-78B5

Linear Regulators can get quite hot as all excess energy is converted to heat, the higher the voltage drop required the hotter the regulator becomes. As the case of the ZX81 has minimal ventilation this heat has very few places to go. Modern Switch mode regulators on the other hand produce very little in the way of heat, and no heat equals no ventilation problem.

There are a number of manufactures producing switching regulators, I'm using a Recom R-78B5.0-1.0L, which I sourced from Element14. If you're conducting a similar mod, the main things to be sure of are that the switching regulator is a drop in replacement for 7805, is 5 volts and rated at 1A or 1.5A. (2A is overkill, unless your ZX81s controlling a nuclear power plant)

The voltage regulator is not hard to spot, it's attached to what looks like a tractor part, or aluminium heatsink if you prefer. If you ever wondered why your left hand got so toasty on a cold winters night programming (OK gaming) on your ZX81, this thing is the most likely culprit.

Recom R-78B5 In Situ

I started the process by removing the bolt which secures the heatsink to the 7805 and circuit board, once unscrewed the heatsink slid out easily. The heatsink is not required after fitting the new regulator.

Removing the 7805 regulator itself proved slightly trickier due to the large amount of solder holing it in place, plus there being a channel selection switch (being a USA ZX81 variant) and some rubber like trim isolating the switch from the 7805 obscuring the pins at the base of the regulator. In the end I found it easiest to remove the channel selection switch before finally removing the 7805. Note that the process of converting the ZX81 to composite video out, had already made the channel selection switch redundant so decided not to reattach it latter.

The Recom R-78B5 dropped in easily and after soldering it to the board I powered the ZX81 on with no issues or incidents. The only thing to notice is a comparative drop in heat production, the keyboard certainly won't be roasting slowly over an aluminium hot plate anymore.

Cooling the ULA

Self-adhesive Heatsinks mounted onto the ULA.

With the heat now taken out off voltage regulation, this leaves the ULA as the only other major heat source. While there is no easy way to extract radiant heat from the ZX81 case, the overall temperature level inside has vastly subsided.The heat generated by the ULA is now able to permeate the cool void sans 7805. Hopefully attaching some self adhesive heatsinks to the ULA will help keep this vital part a little cooler in general, prolonging its lifespan.

I used two self adhesive heatsinks sourced from Jaycar, laid end to end on the ULA, these are a little overhang and don't quite cover the whole chip. If you're a little more OCD, then it is possible to track down exactly fitting 40 Pin DIP IC heatsinks at specialists stores like Retroleum, or more general and extensive component suppliers such as Element14.

CPU for a CPU, a Fair Swap

Swapping out the existing CPU is more about power consumption than anything else. There are no gains to be made in speed or reliability (unless the old CPU is on the blink of course). The original CPU in my ZX81 was Z80A made by NEC an NEC - D780C NMOS chip. NMOS chips use quite a lot more power than the more modern CMOS equivalents.

An NMOS Z80 CPU requires 200mA, where as a CMOS Z80 CPU requires only 20mA when running at 4MHz. Considering I dropped the theoretical output of the regulator by 500mA when replacing the the linear regulator for the switching regulator that's a saving worth making.

You can drop in any 40pin DIP 8bit Z80 processor, no matter what the MHz rating. The CPU will only work as fast as permitted by the ZX81s oscillator circuit set at 3.35MHz. My drop in replacement is a 10MHz Zilog Z80.

Z80 processors are still readily available, they come up on Ebay (though proceed there with caution), specialists stores such as Retroleum, and once again though I procured mine via Element14.

Next Time

I still haven't covered everything, I'll attempt a summing up of the (semi) final fixes and extras and addons (which I failed to get to this time), next entry.

See Part 1 and Part 3