Now that we have a working Decoder Board from 'Easy Add-on Projects for Spectrum, ZX81 & Ace', it's time to get down to some projects. There are a couple in the book I've really been wanting to try out, Project 2: Picture Digitser and Project 7: Light Pen. Despite our guide books numeric ordering, the Light Pen is by far the simpler build, so lets start there.
The Light Pen Project
As everybody from the 80s knows, the Light Pen is the control mechanism of the future, mice touch screens and keyboards all redundant when you have a light pen by your side, it's truly the wonder input device for the ages.
Typically light pens work with only on CRT monitors; as lines are drawn on a CRT screen a light pen detects the CRTs scan line once it reaches the pen. The time it takes for the light pen to detect the scan line allows the computer to pinpoint exactly where on the screen the pen is placed.
The light pen featured in Easy add-on Projects for Spectrum ZX81 & ACE does not work in the typical way. Essentially the projects light pen functions as a light detector that doesn't detect CRT scan line progression. Working out where the light pen is on the screen is left entirely up to our BASIC programs. As it turns out (thirty plus years on from the books publications) this simplicity is exceptionally beneficial for the projects light pen, marking it just as usable on LCD monitor as it ever was with a CRT.
Original Light Pen Circuit from Easy Add-on Projects for Spectrum, ZX81 & Ace
Building the Light Pen and Circuit is not a difficult. I constructed the majority of the circuit on a breadboard, which sits quite nicely just behind the ZX81's keyboard. The LDR is mounted in the tip of an old pen, with a cable running down through the centre of the x-writing implement. I hot glued the cable and resistor legs to removable nib holder (is that what it's called?) and the cable again where it leaves the top of the pen. This hold everything together quite firmly.
ZX81 and Light Pen attached to the Decoder Board
I was forced to make some minor changes to the circuit as some components can be harder to come by than others. The local (read Australia) walk in Electronics supplier Jaycar don't stock the projects prescribed ICL7611 CMOS Op Amp. In order to get the Light Pen up an running with minimal fuss I substituted the ICL7611 for a general purpose LM741 Op Amp.
Modified Light Pen Circuit
The IC change also made the addition of a couple of resistors necessary. The Output of the Substitute LM741 Op Amp doesn't lower the voltage close enough to TTL logic levels when detecting variances between the light-dependent resistor and reference potential. Two resistors, a 10k and a 4.7k are added to the IC Output that's feed back to the Decoder Board.
ZX81 Light Pen Test Menu Application
Based on the books listings and general project information I wrote up a simple BASIC application to test out the new pen with my trusty ZX81. The light pens status is read by input 3 of the Decoder Board, lines 10 and 20 of the BASIC listing reflect this. Line 10 also stores the machine code routines required for a ZX81 to read the Decoder Board: Refer to Appendix A of 'Easy Add-on Projects for Spectrum, ZX81 & Ace' for how all that works.
Finally, I've added a short YouTube Video of the Light Pen and the BASIC program in action. As can be seen it's really quite effective for simple input tasks.
My NEC PC-8401s' LCD Module Faces Capacitor Issues
I'd noticed, well couldn't help noticing really that the PC-8401s' LCD screen would flicker, and every now again turn off unexpectedly after extended use. Over the last week the occurrences became more frequent. Interestingly or comfortingly the screen flickering has no effect on the Main Computer, everything stays resident, you can even continue typing while the screen is off. So what ever the problem? It is appeared localised to the LCD screen. Time to pry open the computer and see what's happening.
A quick look over the main board revealed nothing of immediate concern, though at some point I'll want to replace the NICAD backup battery and the 30 year old capacitors, thankfully though none of these components have been leaking. Semi confident the main board is okay it was time to crack open the LCD bezel, not the most convenient thing to do.
NEC PC-8401 Main Board
Unlike the main case the bezel is mostly held together with molded plastic clips, taking it apart took some considerable carefully spent time. Once off however it was immediately clear where the problem (hopefully the only problem) lay.
NEC PC-8401s' LCD Panel
We've got 2 exploded leaky electrolytic capacitors that need to be replacing, and while we're at it the other 10 should be swapped out for good measure. I guess considering the age of the computer this is not unexpected, all this is really part of the excitement of playing around with old hardware.
Some Picturesque Leaky Electrolytic Capacitors
Well then, before continuing any further it's of the the electronics supplier to procure some of these tiny caps, of which I have exactly none of in my parts draw.
As a quick aside the panel was manufactured by Epson.
A while ago now I fitted the very cool and reasonably priced ZXCCB composite video mod into a ZX81, yielding a much improved video experience, yet this is only half the story in the quest for retro ZX81 video perfection.
There is many a web page or YouTube video devoted to video outputs on retro computers and gaming consoles. Mostly these deal with the somewhat later computing or console incarnations such as the Sega Mega Drive, Nintendo NES or Commodore 64s. There are some excellent write ups on the subject, and you can't go past RetroRGB.com for some solid information well beyond the scope of this article. The ZX81s video demands though are rather less unassuming than most, requiring only the display of a (hopefully) clear black and white image.
What kind of perfection are we after? I say video perfection, this is a perhaps a little misleading, as what I'm really after is a video experience, an experience that captures the heart of ZX81 usage. This is of course a rather esoteric requirement, if perhaps partially at the heart of using the real deal instead of a emulator to start with.
Below are some of the variations on a theme I've tried, this is not an extensive list of every possible configuration available. So this is not a guide, more of a primer before launching your own quest down the rabbit hole of ZX81 monitor selection.
To CRT or LCD?
Back in the 80s the best monitor for a stock ZX81 was a black and white television. The perfect monitor for a ZX81 with a composite mod in 2017 is probably a black and white monitor with composite inputs. Simply put, the ZX81 does not output a chomiance (colour) signal, only a luminance (black and white) signal, any colour induced into the picture is by pure accident. There are not a great deal of black and white monitors available these days, so what easily available monitor options are available in 2017 and beyond?
There are a whole host of options available these days, from old CRTs to video converter boxes and new LCD televisions. The choice is only limited user preferences and in some cases by the constraints of the ZX81s video signal and it's interaction with modern devices. My personal preference varies, on the whole though I'm rather taken by Sony Broadcast monitors as these induce that all important retro feeling.
Option 1: An LCD TV
ZX81 with LCD television as monitor.
Most (I'm not going to say all as there are weird regional variances) LCD TVs still have Composite video ports. The ZXCCB video mod, the ZXVid and others, are designed to take full advantage of the Composite inputs, outputting a stable and clear image to modern LCD TVs. Frankly the clarity of the ZX81s output on a decent TV is astounding, even on low rent LCD TVs the image quality is far sharper anything you've ever experienced on an unmodified ZX81.
At this point, If you're happy with LCD displays you can stop right here, your never going to get a clearer display out of a ZX81, still there are minor issues with the display, and your personal preferences play a part in the ultimate monitor selection.
It's difficult to find an LCD TV with a 4:3 aspect ratio, meaning that text and therefor the underlying pixels are always going to look elongated. This may or many not be a worry, certainly the enhanced video quality more than makes up for the distortion. Then again anyone after retro video purity may well take some offence at lack of near squareness in the aspect ratio. Of course an LCD monitor may already fall into the abyss of impure horrors for those with high retro tastes.
I have found that some LCD televisions play up a little when loading tape, loading from the ZXpand or going into fast mode. Anytime the Sync signal is dropped by the ZX81 an LCD televisions may revert to a blue or black screen momentarily, where most CRT monitors will retain the greyish background or display the loading bars normally associated with a ZX81.
Option 2: The CRT Colour TV
Late model TV with the ZX81 using RF out.
The best part about these is that they are (at the time of writing) cheap, though a slowly disappearing resource. I was lucky enough to find a late model 32cm (14inch) xxxx on the side of the road a couple of months ago, I was initially quite happy about that. I say initially as while the general image clarity falls in the okay-ish zone, the refresh rate on the monitor is headache inducing.
Refresh rate is probably misleading here as technically speaking all PAL TVs and monitors are refreshed at 50htz. This may really be a phosphor persistence rating problem. The rating of particular coating thi inside particular model seems to be just high enough to mostly maintain an image between refresh cycles. Doubtlessly this is not the only reason for this units headache producing flaws.
On the positive side, the TV has AV inputs for a very clear image, though more interestingly it also has no issues in picking up a ZX81s RF signal, producing one of the clearest images I think I've ever seen RF wise. So if you're not keen on modifying a ZX81 for composite out a late model CRT TV might be the best video output option.
Regardless of the positives, using a standard TV as a computer monitor in 2017 is just plain awful, even if you're after that retro CRT experience. Sitting up close to a standard flickering television is underwhelming, did we really do this 30 years ago?
Option 3: A Professional / Broadcast CRT Video Monitors
Sony 9inch PVMs
Broadcast and Professional monitors are without doubt one of the better go ways to go if you're after an old school CRT feel without compromising high legibility. These monitors are designed for people who's job it is to make, view and breath Television 24 x 7, so the clarity gained, particularly from the high end editions of these monitors is breath taking for CRT.
Presently being discarded by the TV industry, PVMs and BVMs are highly coveted by many a serious retro gamer. Sought after PVMs are typically 20 inches plus, with extensive features including Composite, S-Video, RGB and YCbCr inputs. All this providing an expansive, high quality retro gaming experience suitable for most retro consoles. Of course a ZX81 in no way needs all the higher end features, we're just after a clear picture.
A ZX81 isn't going to be used from the couch or armchair, most likely it'll be desk based, so clarity is probably the foremost factor in choosing the monitor. Sizes over 14 inches aren't going to add much to experience, and RGB and YCbCr are nice to haves but not important unless using you're also planning on using the monitor with more advanced micros.
Nicely I have 2 Sony PVM monitors, a PVM-9040ME a PAL only monitor that has S-Video and Composite ports and a PVM-9042QM monitor with a full spectrum of video inputs available. Both monitors have screen sizes of 9inchs, a small size by today's standards but not so tiny by the standards of the day. These monitors provide a beautiful viewing experience.
Option 4: Video Upscalers and Converter Boxes
Given the vast supply of LCD VGA and HDMI monitors in circulation, video converter boxes seem like an obvious usage case. But, and this is a big but, not all converters are created equal. Finding a converter box that worked with my ZX81 has proven problematic at best, though individual mileage may vary considerably.
The ZX81s video output, even with a ZXCCB installed is dependent on the version of ULA inside. While the signal can be corrected for missing back porch, the underlay signal generation may not be exactly to PAL standards. I'm not am expert on the subject, but given the lack lustre results using various converter boxes which for all intensive purposes work fine other conversion duties I'd guess this is the cause of the problems I've encountered.
The best converter box I've found is a rather ancient one, an Aver Media TV Box 5. It's unfortunate that the maximum resolution this will pump out is 1024x800, somewhat below the capabilities current generation LCD VGA monitors. For this the TV Box suffers a little, as monitors perform best at their own native resolutions and the resulting image is just a touch blurry. Of course that does feel a little more retro.
I've put up a little video that highlights the problems or successes of various converter boxes at my disposal. The best I can say is buy one at your own risk, and at the end of the day a cheap LCD television seems by far a better option.
What is Monitor Perfection?
The choice of monitor really comes down to personal preferences. If after a clean and sharp emulator like quality, I've found even the least expensive LCD television is capable of delivering a stunning image (for a ZX81). For a that fuzzy feel good retro experience my preference falls defiantly in favour of PVMs and other broadcast monitors. Just remember that most of these video options are only available once the ZX81 is fitted with a composite modification such as the ZXCCB or ZXVid, both of which are regularly obtainable from the Sell My Retro web site.
