There’s life in the old dog yet

After a recent trip to my parents house and being gently persuaded to take all my old “junk” with me I found my trusty old Amiga tucked away in the loft complete with old boxes of disks. The case had severely yellowed which seems very common with these old computers.

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Back in the day I spent my formative teenage years virtually glued to the keyboard of this thing. After the initial infatuation with playing games had worn off I taught myself 68000 assembler and started writing utilities and demos which I guess with the exception of writing a bit of BASIC on my old Amstrad CPC 464 was my first real venture into programming.

So fast forward 20 plus years would it still work ? The first thing I did was take it apart. Just to check everything was still in one piece and check nothing had been damaged. Now being designed when it was all components are through hole. No surface mount in here. Img_3426The majority of the chips are all socketed with the exception of the memory. Everything looks good a bit of dust but that’s about all. Even the battery on the memory upgrade seems to be good a tiny bit of leakage but not enough to have damaged anything.

Initial power up was fine. The red power LED illuminates and the internal floppy starts clicking. So far so good. After connecting the AV-scart lead to my 40″ Samsung LCD TV no picture. Just a white screen. Not good. In the past I had always used an old 15″ Sony CRT TV/monitor which has long gone now. However after rummaging through the box of peripherals I found the original TV modulator. Feeding the composite video from the modulator to a scart-phono adaptor worked just fine. Few so there doesn’t appear to be anything wrong with the Amigas video output stage.

So how am I going to get a decent (ish) picture on a modern LCD TV ? I guess there are a number of options. A lot of people use scan doubler/flicker fixers. Something like the Indivision ECS scandoubler from Amigakit.

Img_3430This simply slots into the video chip (Denise) socket and runs a 15 pin SUB plug to provide VGA output up to 1024×768. There are also a number of scandoublers on eBay that appear to be based on the CGA/EGA/YUV to VGA converters I used in my Weecade arcade machine. Both these options retail for around £80 which is more than I really wanted to spend just to get an old computer up and running.

After a bit more googling I found an article talking about the signal voltages on the video port. It seems these voltages may be too high for most modern TVs. The SYNC signal from the Amiga is apparently around 4.8V peak-peak. Which makes sense if the Amiga is running 5V TTL throughout. Modern TVs expect a synch signal on the composite video or synch on green with a peak voltage of around 1V peak-peak. Since they would probably be operating at 3.3V/1.8V levels. The composite synch input has a 75R terminator which when coupled with a 330R resistor along with the 47R inside the Amiga gives a SYNC voltage of around 1V peak-peak.

So I connected a 330R resistor between pin 10 of the Amiga video port (TTL synch) and pin 20 of the scart lead (composite video). For each of the colour signals Red, Green and Blue I added a 220uF electrolytic just to block any DC bias on those signals. The AV mode signal (pin 8) on the scart was fed from the 12V signal on the video port via 1K resistor. The RGB mode signal (pin 16) was fed from the 5V signal on the video port via a 75R resistor. The logic grounds (pin 13 video port, pin 18 scart) were connected together. The remaining video grounds on each connector were tied together. The audio signals from the scart to two phono plugs was left untouched. Again a couple of small electrolytic capacitors could have been added but I choose not to bother.

After hooking everything backup and firing up the Amiga the iconic start-up screen appeared in all its glory. First hurdle overcome.

With everything up and running I tried booting from the original workbench disk. After inserting the disk the drive starting making some rather strange noises the AmigaDOS screen appeared but then the disk error dialogue appeared soon after. After trying a hand full of other disks it became clear the drive has had its day.

Time to find a replacement internal floppy drive.

 

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