Ten Years at the Mill

13 Jun 2024

A big and bitter-sweet change yesterday; Cydar has moved out of Bulbeck Mill.

Bulbeck Mill

A few weeks and ten years ago, I started work at Cydar, and went from working in a perfectly nice but fairly generic Business Park office to something very different.

There’s been a mill on the site since Roman times, but the current building dates from the early 19th century. As well as a mill it’s been a medical instruments factory, and more recently a photo gallery and yoga studio. The ghosts of these latter uses were still very much in evidence when I arrived, in particular in the floor-to-ceiling mirrors covering most of the walls on the first floor.

At first, the furniture was a bit of a hodge-podge, including not only a large desk made from an Ikea wooden kitchen countertop, but sofas, chairs, and a ping-pong table (generally used for meetings, rather than actual ping-pong). For a while, there was a baby grand piano in one corner, though I don’t recall anyone playing it.

Over the years, the furniture and fixtures and fittings became less esoteric, but the ancient building still made itself felt. Alas, as the company has grown and changed, and the kind of work we do has become more varied, it became increasingly hard to fit the two together. The time had come to move on.

One of the things I’ll miss most about working there was not the building itself, but the surroundings, and in particular the nature. We’d regularly see squirrels and foxes, egrets and swans, herons and kestrels. A kingfisher used to hide in the tree directly outside the window opposite my desk, and if you were very lucky you might catch a glimpse of it fishing in the millpond. Occasionally an otter would swim by. This year a pair of red kites are nesting on the other side of the pond, and just a couple of weeks ago, a stoat peered in through the window. You don’t get that on a science park or in the middle of a city.

On Monday, I’ll be turning up not to the mill, but at the new offices near the station in Cambridge. It certainly won’t be the same, but there will be different perks and annoyances. The past ten years have been good; let’s see what the next ten bring.

ZX81 Composite Mod

9 Jun 2024

After my unexpected success with the hacked-together version of the ZX81 composite mod, I was keen to press on and do it properly. To skip to the end, it went pretty well, and I now have a usable ZX81:

ZX81 with composite mod, displaying a sin plot

As mentioned in the previous post, my ZX81 is a later version that generates a sensible video signal with back porch, so the mod is very simple — essentially just a transistor to amplify the ULA’s video output signal to the level needed for the external connection. The only question was how to fit it into the case. I wanted to leave the RF modulator as intact as possible (after all, the machine is an antique, so I feel like I should treat it with respect), so the basic idea is to disconnect the relevant lines but leave the components in place.

As to where the new components go, I’ve seen two options: put everything inside the RF modulator case, or keep it all outside, and route the necessary connections through. The first option looked ambitious: I wasn’t confident that I could work in such a confined area, and more importantly it involved cutting rather just desoldering the old connections, so would be harder to reverse. The second option has neither problem, but looked a bit messy.

Looking at the board, I came up with a compromise I was happy with: putting the transistor outside the modulator, with the collector and base soldered directly to the +5v and video signal connections previously used by the modulator, and the 100Ω resistor on the inside. One lead of the resistor is soldered to the modulator case for ground, and the other end is soldered to the pin of the RCA jack for output, and also routed through the insulated hole to connect to the emitter of the transistor.

Description below Description below Description below

The above images show the changes. From left to right:

  • The unmodified RF modulator. The three connections to be desoldered are marked with red circles.
  • The modification, showing the video signal and +5v connections to the board.
  • Another view of the modification, showing the resistor lead connecting to the RCA pin and out to the transistor.

The old connections are insulated yellow tape to avoid shorts. This is probably the messiest part of the project; otherwise, it’s pretty neat, and doesn’t get in the way once the cover is back on the modulator. Once I’d made the changes, I plugged in the board on its own to check the signal and got a recognisable but madly flickering picture, indicative of a loose connection. I redid the soldering on the RCA pin, tried again, and the picture was good. After reconnecting the keyboard ribbon (I’ve been lucky in that regard; it’s in amazingly good shape considering its age) and reassembling the case, I had a working ZX81 that could be used with a (relatively) modern display.

You may notice from photo at the top of the post that the image looks a bit washed out — the background is the expected light grey, but the “black” is very much not black. This is at least in part due to the cheap RCA-to-HDMI converter I’m using on my monitor; it looks better on a TV with built-in composite input. I tried adding additional resistors on the video output as described here, but they made the whole picture darker rather than increasing the contrast. I’m not sure if there are other things to try, or if that’s as good as the picture will get, but in any case it’s good enough for my purposes. Now I can start to explore the ZX81 itself.

Bread and Crocodiles

1 Jun 2024

As mentioned previously, my recently-acquired ZX81 wasn’t providing a usable picture over the original RF connection, and so I’d decided to try to modify it to produce composite video output. The oldest ZX81s produce a slightly out-of-spec video signal lacking back porch, and so properly fixing the video involves adding circuitry to modify the signal using a 555 timer. However, opening up my unit revealed that it was a later version1 that does produce a proper signal. For these, the required mod is almost comically simple; a single transistor and a resistor, and maybe a second resistor if you need to adjust the brightness. You can get such a mod pre-built, but I decided to give it a go myself.

Being a coward prudent, I assembled the components on a solderless breadboard before trying to permanently modify the original hardware. Once I had this, I decided to try starting up the ZX81 with it temporarily connected, as an experiment. I was pretty confident that I wouldn’t damage the ZX81 itself (apart from maybe the RF modulator, which wasn’t useful anyway), so the worse that could happen would be no picture.

A ZX81 PCB attached to a breadboard circuit via crocodile clips

As you can see, the setup is a little janky. The crocodile clips seemed likely to either not make proper contact or short with something they shouldn’t. Moreover, I’d not disconnected the RF modulator, so that would also be driving a signal on the same RCA socket.

With expectations suitably modest, I connected it to the TV and powered it up, and… it worked! A solid, crisp K-cursor appeared exactly as it should. Now I’ve confirmed that the mod works, the next step is to solder it in properly and put the whole thing back together. Wish me luck.

  1. Specifically, an issue 3 board with a 2C210E ULA. [back]

Long Service

22 May 2024

I’d like to commend this Varta battery on 36 years of faithfully not leaking inside a Mac. Battery, we salute you.

A black half-AA battery labelled "VARTA" and "Made in West Germany"

As a bit of a #MARCHintosh holdover, I finally got around to replacing the PRAM battery in the Mac SE. It’s done fine service since the late 80s — not only had it not leaked, when I booted it up in January after many years on the shelf the clock was still set — but I felt I’d pushed my luck long enough, and so installed a fresh one along with a holder to make the next replacement easier. Rest in peace; you were a good battery.

Retro Frustrations

20 May 2024

On a recent visit to the Centre for Computing History, our eldest noticed a poster for their upcoming “Bring and Byte” sale and was keen to go along. I’ll admit I didn’t argue too much with the idea, and a few weeks later we were back, cash in hand, poring over the tables of old hardware, software and assorted technical knicknacks.

We picked up a couple of classic devices: an Atari VCS1, and a ZX81. Neither are rare (in fact, both are notably for selling like hot cakes and jump-starting their respective markets), but they’re interesting from a technical, historical and aesthetic perspective, and I was keen to set them up and explore them hands-on. However, we soon ran into a problem: RF.

Some context, for those who weren’t around in the early 80s (or were around, but weren’t messing around with home micros). TVs of that era didn’t have inputs for external video sources — no composite or SCART, let alone component, VGA or HDMI. Their single input was a connection to an aerial, in order to receive over-the-air broadcasts (Radio Frequency, or RF). Several channels would be sent using different frequencies, and the TV would include a tuner that picked out a particular frequency and decoded that into video and audio to present.

But what if you wanted to use your TV to view something other than live broadcasts? Other devices — VCRs, games consoles, microcomputers — worked like a TV broadcast tower in miniature, and skipped the over-the-air phase. A modulator inside the device would encode the signal at a particular frequency, allowing the TV to be tuned to that frequency and display the result. It wasn’t the most straightforward approach, and it was a bit prone to interference and fuzziness, but it was a practical way to hook up your shiny new technology to the display you already had.

Back in the present, our TV is fortunately old enough to still have an analogue RF input alongside more modern options. I attached the new-to-us devices, hoping that a usable picture would only be a tuning away. No such luck. With the ZX81 there was definitely something there, but with the Atari it was nothing but static.

Asking around, I gathered that more recent TVs only supported standard broadcast formats, and had trouble with the more unusual formats generated by early consoles and computers. I tried again with an old VCR, and with a dedicated RF-to-HDMI converter box. Both gave slightly better results with the ZX81; the text wasn’t legible, but I could at least see where it was, and the way it changed when I typed in some simple BASIC commands gave me confidence that the machine was working, and the problem was just the display. I need to confirm that this is a problem with the RF signal, but if it is it turns out that there’s a straightforward solution — the system can be modded to output composite video, bypassing the RF modulator, with kits as simple as a few resistors and a single transistor. This seems like an eminently doable project, and not too much of a risk as ZX81s are fairly easy to come by.

What of the Atari? Even with the alternative modulators, I still couldn’t get a whiff of a picture. See nothing seemed unlikely, so I began to suspect that the problem might be elsewhere. I decided to open it up to look for any obvious damage, and as soon as I did I spotted the problem. Whoever had been in there last had suffered an off-by-one error when reconnecting the board with the switches and RF modulator to the main unit, leaving an empty connector at one end and a dangling pin at the other.

I corrected the mistake, but I was painfully aware that I’d been powering it on with essentially random wiring to key components. Something could easily have been damaged beyond repair, rendering the whole thing a non-functional (though admittedly handsome) display piece. With some trepidation I hooked everything up again, and flipped the switch… and it worked! Low resolution Space Invaders started immediately marching across the screen to staccato beeps, as colourful and slightly fuzzy as they were in 1980.

Not only was I chuffed that the hardware was working, but it proved a surprise hit with our youngest. The crude-by-modern-standards graphics didn’t put her off, nor did the lack of the gentle learning curve of today’s games. What the VCS does have is immediacy and responsiveness. It can’t do a lot, but what it can do is focussed on the essentials, and four and a half decades later it still shows. I’m glad I was able to bring it back to life, and we’re both looking forward to playing with it some more.

  1. Specifically, a PAL “Light Sixer”, so manufactured between 1978 and 1980. Not the original (“Heavy Sixer”) design, but close, and still with the iconic wood grain front. [back]

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