ZX-Key: New Lite Version

Introducing the new ZX-Key Lite: A mechanical keyboard for the ZX81 that's very similar to the classic ZX-Key.

The original ZX-Key has been available for some time now, and won't be going away anytime soon. However various requests have come my way asking about kit versions, versions that don't really need the Arduino for PC connectivity and similar, the Lite version should be the answer to many of these needs.

The ZX-Key Lite, ready for use with a ZX81

Fundamentally there are only very minor differences between the PCB of the Lite and Original; what changes there are afford the ability to provide options for selling differing levels of kits, or provide assembled units designed purely for use with a ZX81.

The most noticeable difference between the Lite and Classic ZX-Keys is the now optional Arduino Pro Micro. (In reality the Pro Micro was an option on the Classic boards as well, however the LEDS in particular then have zero purpose without modification.) The presence of an Arduino allows the keyboard to be used with a regular PC, a function mainly intended for use in emulators. A great feature that isn't required if you're only intending to use the keyboard with a real ZX81.

ZX-Key Lite: Complete Unit fit for a ZX81

The ZX-Key Lite version can always be updated to a Full /  Classic at a latter stage by installing the missing components and switching a jumper wire. The new board is more about providing choices, more than a fundamental redesign.

ZX-Key Lite: Board Upgraded to Classic and attached to a Minstrel ZX81 Clone

Housing the ZX-Key Lite Keyboard in an attractive case is just as simple as previously. There are 3 components available on Shapeways

1. ZX-KEY Keyboard 'Starter' Case
2. ZX-KEY Keyboard Case 'Top Plate' - Lite Version
3. ZX-KEY Keyboard Case 'Bottom Plate'

The starter case and bottom plate are the same parts as used for the Full / Classic ZX-Keys. A 'Top-Plate' - Lite Version has been designed specifically to house a non upgraded ZX-Key Lite, and comes without the redundant Serial Expansion slot and now un-required extra LED indicator holes.

ZX-Keys Lite Case Top Available at Shapeways

Limited stocks of the ZX-Keys Lite are now available at Sell My Retro. It's predicted that kit versions will be available early in the new year, after supply chains and shipping times have returned to some normality.

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ZX-Key, Minstrel 3 Keyboard Adapter

The ZX-Keys Keyboard attached to the Minstrel Issue 3 via the ZX-Key Breakout Board.

Since building the ZX81 Minstrel 3 kit I've been using the initial kludge keyboard adapter I'd made up on protoboard with the ZX-Keys. All perfectly fine, although it was a little unsightly, missing the ability to reset the computer and devoid of the reassuring LED indicators.

It's not all about making things tidy though. I still have further plans to encase the Minstrel and need to ensure everything is working as expected before finalising that design. But more of that in latter posts.

The ZX-Key Minstrel adapter is really just a simple breakout board for the standar ZX-Key keyboard. The breakout board can be soldered directly in place, or clipped into the ZX81 keyboard header or slotted into a regular female header. Provision has been made for mounting it firmly to the Minstrel, held in with a bold

To make simple use of the ZX-Keys Keyboard adapter nothing needs to be done other than plug it in and attach the keyboard. However an additional 3 wires may be soldered to the Minstrels 5v, GND and Reset lines to restoring computer reset and LED functionality to the Keyboard.

In the coming weeks I plan to make the Minstrel 3 ZX-Keys Keyboard kit available on Tindie. This will be in addition to the regular fully assembled units designed for the ZX81 that I currently make available on Sell My Retro. So stay tuned for that.

Minstrel Issue 3 ZX-Key Breakout Board.

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RAMming in Some External Retrofits

With the internal updates out of the way in the last 2 Retrofitting articles (Simple Start to Retrofitting a ZX81 and Advancing along with the Retrofitting of a ZX81), it's time to add an external RAM pack. Nothing unusual there, after all the first expansion (and possibly the only expansion) any ZX81 owner attaches to the back of their humble 1k micro is a 16k RAM pack. But this is 2017 and a simple 16k RAM expansion just doesn't cut it, and it certainly didn't cut it back in 2011 either, and so enters the amazing ZXpand, bringing RAM packs and ZX81s into the new millennium.

The Amazing ZXpand

ZXpand 

The ZXpand created by Charlie Robson is firstly a 32k RAM expansion, nothing odd there, however it's also an SD card reader, provides user configurable memory mapping, hires graphics modes, a reset switch, pads on the circuit board to add a joystick port and optionally (via an extra add on expansion) the ZXpand can accommodate an AY sound interface with a fully built in joystick port. For the moment I'm happy with the base model sans AY.

In a nut shell the ZXpand does for the ZX81 what the DIVIDE (in all it's incarnations) does for the ZX Spectrum. Coming pre-assembled, the ZXpand plugs right into the back of a ZX81 (or TS1000) and is ready to go. The build quality of the board is exceptional, and there are even little amusing gems of ZX81 lore written on the boards Silk screen layers.

If you need some convincing of the ZXpands merits check out VectrexRoli's typically flamboyant video review on the Youtubes.

The ZXpand is intermittently available on Sell My Retro, normally released in batches. You'll need to watch out for these appearing as they sell very quickly. The AY Sound interfaces seem to go at an even quicker rate.

ZXpand Needs a Case

Now for all it's merits the ZXpand comes without an encasing. The good news is that there are options, firstly if you have an existing Memotech RAM expansion, then you can gut that, make a minor modification, then slide the ZXpand into void.

The Elongated Cube ZXpand Board Enclosure

Alternatively, should the massacre of harmless ancient peripherals disturb your delicate sensibilities, or more likely you find yourself bereft of Memotech devices, then as luck would have it mhudson52 on Sell My Retro, has recently started producing 3D printed enclosures in various form factors. Models include a classic Memotech inspired design or the ever popular elongated cube form factor. Both versions have counterparts that allow for the AY interface. I obtained a rectangular version of the ZXpand Board Enclosure for my purposes.

The Enclosure  is a well designed smart little black case. The ZXpand fits very snugly inside, and the
whole unit nestles perfectly behind a grateful ZX81. Being 3D printed, the board enclosures finish is a little rough, so don't expect molded plastic perfection.

The only minor short coming in the cases design is the slightly diminutive hole at the back for accessing the ZXpands momentary reset / NMI switch. In order to press the switch through the case you require something the width of a knitting needle or a thin tablet stylis.

Wire Soldered to the ZXpands Pads

Adding A Joystick Port 

ZXpand Joystick Solder Pads

Adding a joystick port to the base ZXpand requires a little soldering, a DB9 male connector, and the usual assortment of wires. The location of attachment points / pads are outlined in the ZXpands manual and these can easily be hooked up to a male DB9 solder-able connector. Wiring should follow the standard Atari 2600 joysick pinouts.

As I intended to attach the  joystick port to the ZXpand Enclosure, I figured the best way would be to separate the metal casing on the DB9 connector. This would allow me to attach the connector to the inside of the enclosure and at the same time cover up any defects created when making some necessary holes in the enclosure on the outside.

Atari Joystick Port Pinout

DB9, with Metal Surrounds Separated

The two metal halves of the DB9 connector are pressed together, a lip in the bolt / screw holes keeps two halves firmly in place. To separate them you can file away the lip around the bolt holes. Once separated connecting wires from the ZXpand joystick pads can be attached to the back of the DB9 shell.

After some examination and testing, I found the most convenient location on the enclosure to place the DB9 connector was at the bottom right. This placement allows the ZXpand to slide in and out of the case without hindrance from the DB9 connector.

Using the outer metal shell of the connector as a template, the placement of the joystick port can be marked out before cutting into the case. I used a Dremel like tool for cutting into 3D printed case. You need to be careful and take your time, as printed plastic likes to re-melt when heated by friction generating power tools. Regardless of how genteel one is, the process will most likely leave a rough edge that needs to be carefully filed away by hand.

The Back of the ZXpand Enclosure, With Holes for the DB9 Connector and Mounting Bolts Cut Out

Joystick Port Mounted

Mounting the DB9 connector is straight forward, if a little fiddly. The enclosure is a little flexible and the mounting point I chose is rather close to one of the ICs on the ZXpand. For this reason I chose to use plastic bolts and nuts for mounting; I didn't want some accidental short circuiting taking place when inserting a joystick.

As the case is designed to fit the ZXpand snugly, the wires to the joystick port had to be routed via the bottom of the enclosure, not a major issue, as these can still be tucked up inside the case mostly out of sight and definitely out of the way.

Joystick Port as seen from the Rear of the Enclosure.

It should also be noted that there is no standard for Joystick interfaces on the ZX81. So much of the software supported by the ZXpands Joystick port has been adapted by the community. There is a list over at the Sinclair ZX World forum of some of the supported tittles.

Getting to that Reset Button

Fully Sprung Reset Tube

As mentioned earlier, the case design makes is a little challenging to access the reset / NMI switch. With fate on my side and some creative thinking I had the means to bodge up an effective solution to that minor issue.

In my draw of miscellaneous items I found a small aluminium tube with a handy flange on one end. The rim fitted over the momentary switch perfectly and the barrel was the perfect length and width to fit through the reset hole on the ZXpand enclosure.

To complete the switching mechanism, a spring extracted from a donor retractable biro was cut down to size, fitted over the tube, and a perfect rest switch was in place.

Another option for the reset switch length problem would be to de-solder the existing SPST momentary push button, replacing it with a longer stalked equivalent. This one from Altronics would do the job quite nicely, any components vendor should have similar items.

So that's it, a ZX81 fit for use, super charged and in better shape than it was 36 years when rolled off the Sinclair (ok Timex) production line.

ZXpand / Enclosure attached to the ZX81. The Reset Switch is the Perfect Length

For full details of the Retrofit, see See  Part 1 and Part 2 of this guide.

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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

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Simple Start to Retrofitting a ZX81

My Sourced ZX81 was Modified for the American Market

Having played around with emulating the ZX81 as part of building a Raspberry PI / Arduino platform in the AZ15, I thought it might be fun to revisit the real device.

To start I'd be needing a ZX81, a process that proved trickier than anticipated. There should be no shortage of ZX81s in Australia, yet they seem sparse enough to command high-ish prices, high enough to send me searching further afield. European and UK prices again seemed rather expensive, particularly if including postage. So to the USA then, a Timex 1000 would be just as acceptable, in the end I found a reasonably priced American fitted out ZX81, obviously sent to the new world before the official deal between Timex and Sinclair.

So what exactly can be done to update and bring a ZX81 into the now? Well, there is a great deal of information out there concerning the upgrading, modifying and repairing of ZX81s. Should you decide to do everything from scratch the information and resources are available. However the really good news is that many of the parts required for a refurbishment and upgrade can be bought right off the shelf.

New or Replacement Power Supply

Before even turning on my ZX81 for the first time (in my possession at least) I needed a power supply, my unit came sans power brick. While I had no option but to source a new supply, it's recommended that even if you still have an original Sinclair unregulated power brick that you should still replace it with a switch mode equivalent.

An original Sinclair unit is rated at 9V, however as the supply is unregulated that can get as high as 12V, this leaves the ZX81s internal regulator doing all the heavy lifting. Governing the required 5v (down from 12V) leads to an excessive amount of heat being generated inside the case.

You can use a regulated power supply of anywhere between 7.5V and 9V with a minimum 1000mA (1 Amp ) current draw. The easiest or most available substitution seems to be regulated 9V supply. The power supply should also have 3.5mm phono plug jack plug at the business end. Power input of +7.5V to +9V should be at the tip of the plug, with the lower / outer ring being ground.

I picked up an appropriate Switchmode Plugpack 9VDC 1.66A from Jaycar (Australian Electronics Chain), the unit provided a variety of jacks that can be substituted as or if required. Similar Plugpacks should be available just about everywhere. Ebay often seems to have packs marketed as being specifically for the ZX81.

Optionally a power switching cable can be added between the supply and the ZX81. This will save some wear and tear on the ZX81s power socket These can be made easily enough, or purchased from specialist suppliers such as the Retro Sparse Shop, that's if you don't want the hassle of tracking down appropriate parts yourself and in all actuality might prove more cost effective.

Composite Video Output

The second thing in need of attention is the video signal. as the combination of modern (not the best quality admittedly) LED TV and RF signal emitting from the ZX81 is terrible to say the least.

There are various video mod available that will provide composite video out, ranging from ones that you can build yourself to others ready to fit into the case with minimal fuss. I've gone with an off the shelf fix in the form of the ZX8-CCB, a video mod created by the venerable Pokemon. The ZX8-CCB is available from The Sell My Retro website. Before going into the fitting of the ZX8-CCB, lets quickly look at other composite video mods.

If you choose to build your own then it would be hard to do better than the mod by Joulesper Coulomb detailed in the YouTube video, 'ZX81 Video Conditioning'. A lot of effort has gone into the mod, and spectacular results have been achieved, which can clearly be seen in the video. Minimal parts are required to undertake the modification, it is easily assembled on strip board and once built, the entire project can be housed in the existing ZX81s modulator case.

There are slightly easier mods to build yourself, most of these use a single transistor and a some resistors, such as the simple mod by the ZX-TEAM. You can see the results of that video modification on Terry Stewart's (retro computer collector extraordinaire) web page. However, these simple mods may not work so well with earlier issues of the ZX81 due to a missing back porch signal, and possibly leaving the resulting picture a little on the dark side.

For off the shelf options, there is the Atari (yes you read that correctly) composite video mod for the ZX81, sold by The Future was 8 bit. Dave Curran on his Tynemouth Software Blog, details the fitting of this interface to great effect. Lastly (of course there are bound to be more), there is the Sinclair ZX81 ZXVid - Composite Video/ULA Fix, available from Sell My Retro. You can see the results of this mod on the YouTube video ZX81 - ZXVid board, Composite mod.

Fitting the ZX8-CCB

The process of fitting the ZX8-CCB is very well documented in the instruction sheet supplied with the kit. There is no assembly of the kit required, and it's down to a little soldering to the ZX81.

To start, it's simply a matter of attaching the input wires to the back of the ZX81 circuit board: red wire +5V to ULA pin 40, the black wire 0V to ULA pin 34 and the yellow wire to video In at ULA pin 16.

For the output wires you have a choice to make, either integrating a small video jack (3.5mm) between the two cover parts (as suggested in the instructions sheet), or re-purpose the modulator casing. The second option is what I decided to go with.

In order to fit the ZX8-CCB into the modulator case, the case needs to be removed from the ZX81 circuit board. There are 2 or 3 wires going into the side of modulator, these can be trimmed of at the base, or de-soldered. Also there are two large pins directly underneath the modulator, these hold the modulator unit in place. These also need to be de-soldered to remove the modulator from the ZX81s circuit board.

Once the modulator casing is free from the ZX81 you can then set about removing the existing contents (though a combination of cutting and de-soldering). What's inside the box will vary depending on the modulator, on PAL models there is a cross bar and the main circuit board to remove, as I have a USA model there was no crossbar.

ZX81-CCB Input Wires Soldered at the Back to the ZX81 Circuit Board

After the modulator has been gutted it can then be re-attached to the main circuit board, with the ZX8-CCB placed inside the casing affixed with a square of double sided tape. The remaining wires are then attached: brown wire for video out to the center of the phono socket, and the the black wire 0V to to the side of the modulator casing.

ZX81-CCB housed insdie the Modulator Case. (Wires latter moved to run through vaccant holes in the casing)

In the initial fitting I had the three wires which are soldered to the ZX81s circuit board running over the side of the modulator casing. With the modulator lid placed back over the top, this was perfectly serviceable but looked a little untidy. I latter decided to move these wires and have them running through the now vacant holes (of which there are three) on the side of the modulator.

Composite Results

So what of the results? In a word 'amazing'. It is unfortunate that I don't have a CRT TV laying around to give the proper picture, though even upscaled through a composite to VGA converter box the clarity is almost as clear as on many an emulators, it's certainly the best video I've even seen emanating from a ZX81.

After Composite Video Mod (with a little HRG graphic teaser)

Of course I'm not the only one to have retrofitted a ZX8-CCB to a ZX81, these things sell like hotcakes. And as is the way with these thing, half way through typing up this post I found another blog post detailing a similar install of a ZX8-CCB. I'd suggest checking out Kevin Phillip's entry 'Rejuvenating My Geriatric Childhood Friend' for extra information. The better you're informed before making any medications there better.

Next Time

There are a couple more modifications I'm wanting to make, along with a couple of interface addons to try out. (Hint one of the is in the final picture). I'll get around to detailing those and more soon.

See  Part 2 and Part 3

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