Replace Those Old Capacitors
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|
- 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
|7805 Regulator and Replacement Recom R-78B5|
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|
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.|
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
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.
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