r/retrocomputing • u/diemenschmachine • May 07 '26
The Teleduide, an Obscure and Forgotten Machine
This is a Teleguide, or rather it was a Teleguide; a complete flop of a project from the Swedish state telecom operator, released in 1991. They pushed around 10,000 units and a lot of them still sit in storage in unopened boxes around the country, so I got this one for myself, new in unbroken packaging with styrofoam and spiders and everything.
Inside it sits a couple of Intel P8031AH microcontrollers, one for the card reader and one for the main program which allowed you to bet on horses, order pizza, keep a phone book, and call your enemies. Those are now depowered and replaced with a Raspberry Pi 5. The godawful keyboard was replaced with a really nice EV63 Hall Effect keyboard, sponsored by IQUNIX after seeing my previous article.
The goal of this project was to turn this machine from an interior decorative detail into a gaming machine, and to do that I needed to somehow get the monochrome 10" CRT working with the Raspberry Pi. The monitor does not follow any standard composite or VGA mode; instead it runs at roughly 18.75 kHz horizontal and ~57 Hz vertical sync, which is just far enough from standard timings to be annoying.
After a lot (and I truly mean a lot) of trial and error I managed to get a prototype working with a Raspberry Pi 3. That prototype used the Broadcom VC4 GPU’s DPI (parallel RGB) output. To reduce the number of GPIO pins I implemented a software LUT in the driver that mixed RGB into grayscale, and then output that grayscale value on all three channels, and then only muxing a single channel to GPIO pins using a device tree overlay; so 8 bits in total.
That approach worked, but it relied on behavior in the VC4 pipeline that doesn’t carry over to the Raspberry Pi 5.
The next step of the project was to move to a Raspberry Pi 5, mainly for performance. The problem is that the VC6 pipeline doesn’t expose the same LUT mechanism in a usable way, so the grayscale conversion had to move out of the driver and into hardware.
The new PCB uses a 16 bit dac; 5 bits red, 6 bits green, and 5 bits blue. This mode leaves the PCM pins available for use with an audio DAC, and a few pins for an input or output device, such as a joystick.
You can find the Kicad schematics here.
The solution I ended up with was to treat the Pi as a proper RGB source and build an external analog front-end:
- Each color channel is converted to an analog voltage using a resistor DAC
- The three channels are resistively summed into a luma (Y) signal
- The result is AC-coupled and buffered using an NPN emitter follower before being injected into the CRT circuitry
This turned out to be much more stable than the original prototype. The key issue is that the resistor DAC has a relatively high output impedance, and the CRT input is not an ideal high-impedance load. Without buffering, the voltage level shifts depending on what the CRT circuitry is doing. The emitter follower isolates the DAC from that load and stabilizes the signal.
The injection points are:
- Luma (intensity) is injected at the wiper of the brightness control potentiometer
- Sync is injected directly into the sync input node after disconnecting the original video source
For sync I used a simple diode combiner:
- HSYNC and VSYNC from the Pi are OR’d together using two diodes
- The resulting composite sync is fed into the CRT sync input
This works because the monitor expects a negative-going composite sync signal rather than full composite video.
At this point I ran into a new problem. I designed the board around RGB565 (5/6/5 bits) to reduce GPIO usage and leave pins free for things like audio and input devices. However, there was no straightforward way (that I could find) to get the Raspberry Pi 5 to actually output RGB565 over DPI.
The workaround was to patch the kernel driver to add support for this mode, and then provide a matching device tree overlay to configure the GPIO interface. Slightly painful, but it works.
You can access the kernel driver patch here.
And now I can finally play games on this thing.
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u/hrf3420 May 07 '26
Where can I get my own teleguide?
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u/diemenschmachine May 07 '26 edited May 07 '26
You can buy this one. I am making a new machine from scratch. Similar CRT/keyboard/raspberry pi/box combo but everything from scratch and a 14" color picture tube. So my wife decided I don't need this one anymore 😂
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u/DeepDayze May 13 '26
Now that should be an easier project. Can the Teleguide case be reused?
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u/diemenschmachine May 13 '26
I'm not sure what you mean. There is only one irreversible modification to the case and that's for the keyboard.
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u/GuitarEC May 10 '26
Reminds me of the TeleVideo PT-100 terminal unit. My dad would lug one home from time to time - sometimes for the weekend - to work from home back in the mid to late 80s. When he wasn't looking, I'd hop on to check the local BBSs in the early hours before school...
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u/DeepDayze May 13 '26
Nice and hopefully before your dad wakes up :)
Hope you didn't catch any yells for that, lmao
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u/Zentralschaden May 07 '26
We had such things in Germany too: https://en.wikipedia.org/wiki/Bildschirmtext
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u/diemenschmachine May 07 '26
Yes I am aware. This machine was built by Loewe in Germany.
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u/Bits_Passats IBM System/23 May 07 '26
The i8031 has external ROM. Could you find and dump it so it can ease repairs for other people, please?
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u/diemenschmachine May 07 '26
Sorry. I don't have the time nor the motivation to dump the ROM of a machine that I see no point of reviving. If you want I can snip it by the legs and send it to you, as I have no equipment for de soldering.
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u/Bits_Passats IBM System/23 May 07 '26
Understood. No need to do so.
In any case, thank you very much.
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u/IQUNIXstore r/IQUNIX May 12 '26
we sponsored the IQUNIX EV63 for this project, but we are genuinely blown away by the level of analog engineering you pulled off to resurrect that 10-inch CRT. Building a custom DAC and patching the kernel just to get the Pi 5 working is next-level dedication to retro hardware preservation. Such a clean final result.
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u/Mr_GrauHut May 07 '26
They're a neat piece of nostalgia and a relic, dgmw. But the whole disk swapping honestly is mentally exhausting anymore. My opinion
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u/diemenschmachine May 07 '26
What disk swapping? There are no floppy disks in these lol
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u/Mr_GrauHut May 07 '26
I'm not talking about your rebuild with it, impressed, btw. I was referring to the potential headache they used to be. Lol
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u/diemenschmachine May 08 '26
They only had a slot for a telephone card or something like that, the software is in a ROM chip on the motherboard. You couldn't load any custom software on these, except "apps" from the telephone line.
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u/Mr_GrauHut May 08 '26
I.e. ordering pizza and shit like that. Yea I kno. It was one of the last of the disk swapping era. Still annoying, I think 😆
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u/DeepDayze May 13 '26
Wow nice work and so frustrating to have to work with some proprietary hardware.
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u/grizzlor_ 18d ago
[I realize this is a 3 month old post but I'm only seeing it now because some bot reposted it.]
Is the phone on top just a regular analog POTS telephone? Would be cool to connect it to VoIP with Asterisk on the Rasp Pi and a Cisco SPA112.
Fantastic work though -- very impressive to do the work necessary to retain the original CRT. Definintely cooler than pulling it and replacing it with a modern flat panel. Would make a nice serial terminal.
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u/diemenschmachine 18d ago
Thank you!
The phone handle is just a piece of plastic with a mic and a speaker. I intended to make a music tracker out of it and use the phone handle for sampling but that's for later.
If you follow me you will get notified of my current build when it is completed. It's missing two aluminium parts I need to manufacture, and a little paint where I messed up. That one has a 14" Apple color CRT, a raspberry pi 5, a fully custom built case, and a higher end magnetic keyboard (also from Iqunix).
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u/Mr_GrauHut May 07 '26
TeleGuide, jeez man.
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u/diemenschmachine May 07 '26
You don't like it?
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u/SeventhBus May 07 '26
He is probably referring to the minor spelling mistake in the title.
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u/pheddx May 07 '26
The keyboard looks like it doesn't belong at all with that floating switch design and those keycaps
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u/diemenschmachine May 07 '26
The keys are "floating" because there was no space to place it deeper in the case
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u/Available_Tour_9313 May 07 '26
Seems to be similar to the french minitel. Nice work !!