retro – modrobert's weblog

Amiga 1200 chip RAM repair

Another adventure with my Amiga 1200, recently when booting I was met with a green solid color boot screen instead of loading AmigaOS 3.1 (workbench) as expected. According to boot error color codes that’s most likely bad chip RAM (the first 2MB of RAM shared with custom chips). I burned Diagrom v1.3 on EPROMs using the MiniPro TL866A programmer and installed those in the motherboard ROM sockets. Diagrom outputs diagnostics via serial port (9600 baud 8n1) on the A1200 which I connected to the COM port on my PC running Linux, the output from Diagrom confirmed that the chip RAM was bad.

The chip RAM consists of the four chips located close to the center (upper) part of the motherboard, the two ROM sockets are below. Here’s a close up photo of the area with the RAM chips.

Amazingly I managed to find the same RAM chips KM416C256BJ-7 sold on eBay, so bought five of them which arrived a week later. These chips are in a SOJ package, directly soldered onto the motherboard, so I used my SMD rework station to desolder the suspected bad chip using hot air, then cleaned up, and added solder.

I soldered in a new chip, but this was apparently the wrong chip, RAM test still failed, but instead of switching back I just replaced one by one until three chips were replaced and I ended up with address line errors in the Diagrom output. This was turning into a nightmare, and I started to doubt the “new” chips were working.

After going over the schematics again I figured out a way to cross check by measuring zero Ohm (connection) on the data lines and address lines between RAM chips using a multimeter, this made me discover several bad solder joints so had to apply more flux and reheat the chips as needed several times before the solder joints were solid. Admittedly I’m a rookie with the SMD rework station (hot air), much prefer the soldering iron, but things started to improve after upping the temp to 380C.

The Diagrom chip RAM test finally went through OK, so I happily assembled the A1200, put the original ROMs back and AmigaOS (workbench) booted fine, but only 1MB of chip RAM was detected. Frustrated, after looking at the schematics again I noticed that the A1200 3.1 ROM will boot (pass tests) even if only 1MB chip RAM is detected (U16 and U17 required, U18 and U19 optional).

I disassembled the A1200 once more and started measuring the data and address lines on the motherboard, all seemed OK. After some head scratching I decided to measure all pins (not just address and data lines), and found out that on U19, one of the RAM chips, the UCAS line (“Upper Column Address Strobe”, pin 28) was not connecting to the board. After soldering pin 28 on U19 manually using my iron the full 2MB chip RAM was detected, and passing all tests. The repair was finally over.

Repairing the Amiga 1200 keyboard

Here’s my Amiga 1200, it’s a lot of fun. I use it mostly to play games, program in C language (sometimes 68k assembly) and to connect to the HP 1660AS logic analyzer / oscilloscope through the serial RS-232 port. The nerd specs are; CPU 68EC020 @ 14 MHz (stock), 68882 @ 40 MHz, 2MB Chip + 8MB Fast RAM, RTC, 3.1 ROMs, IDE-CF+4GB, WiFi WPA2/AES.

In 2024 I replaced my Amiga 1200 keyboard membrane because some of the keys were failing, bought it from retrofuzion.com which is a good store in my experience, highly recommended.

Then a few days ago my shift key on the A1200 keyboard started failing, after taking it apart turns out it was the key plunger (aka key post) for the shift key which was bad, it looks like this.

The black part is some kind of conductive rubber which is what makes contact with the keyboard membrane when a key is pressed, over time (decades) the rubber loses its conductivity which can be checked by measuring resistance with a multimeter.

Bad 1200 key plunger — *Bad A1200 key plunger*

The bad key plunger as shown is 1511 ohm which is way too high. I found working key plungers on eBay for the A1200 keyboard. After measuring a few of these by now the good ones are usually around 100 ohm, the bad ones over 500 ohm in my experience. After replacing the key plunger with a new one which is 88 ohm the shift key works again.

Console Déjà vu

I just got a request about scene releases of SNES (Super Famicom) games, unfortunately my file collection went into a dumpster in the form of quick-80 tapes a while back, though I did manage to find an old capture from one of my visits to the A K I R A board (multinode bbs running AmiExpress /X) which was a Sneakers HQ back then.

Memories of US Robotics courier, blue boxing and failing calling cards comes to mind.

usr_courier_node1 — US Robotics Courier – Best modem ever made