Extreme Hardware Debugging: Amiga 500, Pi1541

Things are moving forward. I was able to repair three critical things:

Amiga 500: Corroded Kickstart Socket

The Amiga almost never started when it was cold. Today I took out the 68000 and the Kickstart and looked at the pins. And lo and behold – one slot on the Kickstart socket was totally corroded green. I cleaned it up as best I could with contact cleaner. Now it works again. I hope it will keep working when it is completely cold.

Amiga 500: Video Cable

You almost can’t buy a 23-pin sub-D connector anywhere anymore. I found one in this shop: store.griederbauteile.ch. It is no so funny that the dealer charges almost 50 EUR for shipping. That is not worth it to me. That is usurious. So I simply grinded off the last row of a 25-pin connector. After that I rebuilt the cable exactly according to the schematic from the last post. Works wonderfully. The crucial thing was the 2.5V at Scart pin 16, otherwise the TV doesn’t recognize that it’s supposed to take the signal from the RGB pins.

Pin 20 is also important. I thought at first that this is just an additional composite signal. But in RGB mode this line is a sync signal (comes from Amiga pin 10). Without it, the picture continuously rolls through.

So it looks like the Amiga is running fine again!

Then I had a very nice experience. I found a disk with my old demos. They are all still ok. The Ghost demo. Also the vector demo with the TV. And other little demo programs. I’ll have to figure out how to get the files onto the PC. The floppy disks won’t last forever.

Pi1541: Short circuit

The Pi1541 just didn’t want to work. I still had the hope that maybe something was wrong with the config. Eventually I started to check all the connections according the schematic from https://cbm-pi1541.firebaseapp.com/. After a while I noticed that there was almost 0 Ohm between the CLK signal and GND. Something made a short circuit there. I started to desolder the components. First the level shifter, which looked a bit cheap. But that was not the cause of the short. Then the 7406. Then a socket. Desoldering those parts was a horror. Finally I got those parts out with the help of desoldering braids and a desoldering vacuum pump. But the short circuit was still there.

The suspicion grew that the problem was directly on the PCB. In order to be able to limit it further, I scratched open a trace at a certain spot on the board. One half with the two sockets was not at fault. It was on the other side with the IC and the level shifter, but I had already removed both of them. Inspecting the pin on the level shifter where the CLK signal comes in, I could finally see that there was no clear separation between the pin and the ground plane on one side. I scratched something with the knife and – eureka – the short was gone. Then I soldered everything back in. With the hope that during desoldering no more damage was done to the board. In the end it actually worked. I tried a couple of games, and quickly wrote a tower calculation program that I was able to save and load to an empty “virtual” disk. Only the OLED doesn’t seem to work anymore. I’ll have to investigate that further. Since it was already working at some point.

Fantastic that this worked. One lesson though. Do not desolder chips if it is not absolutely necessary. I should have checked the traces to the individual components first.

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