Amiga restoration project
As mentioned back in September, I bought an Amiga 1200 in unknown condition from ebay. This page simply documents the steps I took to get it working. I also have an ongoing thread on the fediverse about this process. There’s also a much more nicely written and entertaining Amiga restoration on Console’s blog.
Amiga 1200
I received an Amiga 1200 rev 1D.4, with an installed spinning-disk harddrive, and a MBX-1200z expansion card, which has a floating point processor, realtime clock, and 2MB of RAM fitted. No power supply, peripherals or cables were included.
For power, I ordered a modern replacement from Retrokit.com.au. I’ve seen some reports since that this design of power supply is unreliable, or might even damage the computer in the long term, but many other people seem to be using them without issue.
Video signal problems
Once I had the Amiga and a power supply, I was ready to start testing. Power LED lit up when I supplied power, but I couldn’t get a video signal on the first screen I tried. I only have cables for testing the composite ‘tv’ video output, and there’s not many screens around that still have sockets for such a connection. I managed to temporarily get an old portable flat panel display to show .. something:
First thing I did was replace all of the barrel capacitors on the board. This is a strongly recommended step for Amigas of this era; the capacitors are old enough that they’re basically guaranteed to leak. If not already, then soon.
Sensitive areas of the board near the surgery sites were decorated with flimsy filmy Kapton tape, which didn’t feel like it would offer much in the way of protection, but apparently it works because it did a fine job of stopping surrounding components from melting under the deadly breath of my heat gun. The old capacitors did not surrender easily. I heated them to melt the solder holding them in place, but the same heat also caused these leaky bois to hiss, spit and in a couple of cases loudly pop at me. Eye protection mandatory. Ventilation also recommended, the leaking electrolyte has a horrible fish smell and is probably not recommended for breathing. Replacement capacitors, which I’m assured should never leak, went on fairly easily.
Unfortunately the recap didn’t make any difference to the composite video problem, but I’m actually glad it didn’t work immediately. I wanted a hardware project and expected this kind of problem when I placed the order. This was an opportunity to use my oscilloscope to explore the board, measuring each point of the video system that I could get to with the probes. The excellent amigapcb.org has interactive diagrams of the circuit board, making tracing the routes between pads and components fast and easy.
Eventually I found that the ‘LUMA IN’ signal needed by the composite video encoder chip was not being received. There’s a ‘delay line’ designed to generate the signal based on the the ‘LUMA OUT’ signal from the same chip, and nothing was coming back from the delay line.
I experimented with shorting the LUMA_OUT to LUMA_IN pins on the encoder to bypass the delay line and see if I could get something recognisable on the screen..
The dead delay line components are basically unobtainium, but some research revealed wise masters have devised an upgrade that replaces the current encoder and delay line with a single (available!) encoder that has its own internal delay. I ordered a replacement from utsource.net.
More surgery later, and I now have working composite video output :)
Booting Workbench
When I bought my PSU I also included a Compact Flash to IDE adapter, letting me use a CF Card as an internal harddrive on the Amiga. Plugging the CF card into my desktop PC, I was able to install a copy of the Workbench 3.0 operating system and a few pieces of software, directly from an emulator. Plugging this into the Amiga seemed to work first time.
Kybrd Prblm
But almost no keys on the keyboard were registering. Removing the connector, cleaning it and replacing it didn’t help. The end of the keyboard ribbon connector looked like the conductor had turned black with corrosion. Later I learned that this is actually a layer of conductive carbon that’s supposed to be there. I trimmed back the ribbon and managed to get about 2/3 of the keys working.
Rather than attempt to fix broken traces on a flimsy membrane, I opted to buy a replacement from a local distributor. It arrived quickly and installation was quick and easy.
Now all keys are working, except for the Right ‘A’ button, which acts as a modifier key like alt or shift. Haven’t worked out the problem yet.
Audio
Audio output was functional at this point, but the volume seemed to wander up and down (mostly down), sometimes barely audible. I reflowed the solder of the replacement surface mount capacitor I had added nearest to the audio outputs, and the problem was immediately fixed. Hooray for easy fixes!
File system corruption
About this time I started noticing that large files seemed broken somehow. Any large program seemed to not work. Some would crash, others just error out. It took me a while but I found a more comprehensive guide which recommends limiting the maximum transfer size per write to a partition. An arcane setting easily overlooked. I guess there’s some kind of buffer issue or bug that this works around.
Since setting this parameter and (re)installing workbench 3.1, I’ve had zero of these mysterious corruption issues.
Networking
Next up I wanted to get the Amiga on the network. While it’s decidedly underpowered for browsing modern websites, having networking makes moving files on and off of the machine much easier and quicker.
The A1200 has a PCMIA slot, a cross-platform standard for small expansion cards. The most popular compatible networking cards are fairly expensive due to demand, but during my research I found a newly released driver for the xircom range of PCMIA network devices. These are much less in demand and I was able to buy one for about $10.
One issue I didn’t anticipate is that some PCMIA devices are actually ‘two units tall’. The plastic shell of the device made it too large to physically fit into the slot in the Amiga, despite the actual connection fitting when the case was removed from the Amiga.
I didn’t want to cut a hole in the Amiga to accomodate the card, so instead I trimmed the shell of the network card. I told the driver author about this and I think he was somewhat horrified: ‘I just bought a pcmia extender on ebay…’. Sorry Neil.
Finally I can log into IRC and hassle my friends in #DosGameClub and let them know how much better IRC is on the Amiga (:. File transfer also works.
Future
Currently I’m awaiting an adapter to let me plug a vga cable into the 23 pin Video output port. It might allow me to get a better quality picture than composite, but relies on being connected to a compatible monitor. I don’t have high hopes that my monitors will support the 15Khz signal, but I could get lucky. The alternative is to try and find an older supported monitor, or use some kind of signal processor to transform the signal into something my monitors do support. Those are expensive and I really don’t want More Devices being plugged in if I can help it.
I also have an 8MB stick of RAM in the post to slot into the expansion port. Currently there’s only 2MB of expansion RAM, and I’d like more for games and graphics creation later on.