Commodore Amiga 1200

Introduction

The Amiga 1200, made available in 1992 (the same year as the Amiga 600), was seen as an improvement on the Amiga 600 in a number of ways. Firstly, the A1200 was fully 32-bit, featuring the 68EC020 at 14MHz, as well as 2MB RAM, a proper expansion connector (under the trapdoor) and the keyboard had a numeric keypad (the Amiga 1200 is slightly wider than the Amiga 500 of which has a similar keyboard). Examples of uses of the expansion slot were to add more RAM, a real time clock or an FPU (Floating-Point Unit) to speed up calculations (interestingly, it seems there were plans to have an FPU on the A1200's motherboard). The Kickstart ROM that came with the A1200 was V3 but can be upgraded to version 3.1.

Introduction

Overview

Repair

Floppy disk not detected

You can email me at james.boshikoopa@gmail.com

Overview

In the photo that follows you can seen an Amiga 1200:

As with other Amiga computers the A1200 has a built-in keyboard and includes a numeric keypad that was absent on the Amiga 600, but similar to the A600 the A1200 has two blank keys either side of the keyboard that were intended for international characters. At the top right of the computer are three status LED's for power, floppy disk access and hard drive access (on the Amiga 600 the hard drive LED is simply called 'Disk'). At the top of the photo is the Amiga's power supply which is smaller than the power units of previous Amiga computers. On the Amiga itself is the mouse which has a switch on the bottom to switch between Amiga and Atari ST compatibility modes (the Atari ST was a rival to to the Amiga computers).

At the left of the computer is the PCMCIA type II slot for expansion and all other ports are at the back (not counting the expansion port hidden by the trapdoor). The ports are mouse and joystick, external disk drive, serial, parallel, audio L and R, video RGB, colour composite, RF for TV, power and a blanked off port (there are adapters to make use of the available space).

The main form of storage was either floppy disks, making use of the internal floppy drive, or hard drive (or compact flash) by way of the internal IDE connection (just like with the Amiga 600). By holding the two mouse buttons as the A1200 turns on boot options are available, as well as an expansion board diagnostic screen.

The Amiga 1200 (above) was bought used and had been fitted with a 500MB 3.5 inch IDE hard drive of which contained a copy of Workbench and some games. It is not recommended to use a 3.5 inch hard drive in an Amiga 1200 (or 600) because it may put a strain on the power supply, however, I had yet to see any problems with my Amiga. Whoever installed the hard drive had to daisy chain the internal floppy drive and hard drive power supply connections. I have since installed a compact flash card and adapter which makes the system run very fast compared to using floppy disks.

When I bought my A1200 it came with a Power computing LTD PC528 (issue 1) expansion board with the markings '4M/CLK/FPU'. It adds 4MB RAM, a RTC that is battery backed up and has a socket for an FPU (possibly a 68882). An example of a piece of software that can take advantage of an FPU is Imagine, which is for 3D modelling. I did have problems with the expansion board when the Amiga kept freezing up shortly after booting. Although I don't know for sure that it was the expansion board at fault I found its battery was dead which I replaced and since putting the expansion board back into the Amiga there has been no more freezing. If you do change a soldered on battery such as the type used in an expansion board it's a good idea to install a battery holder to make it much easier to change the battery again when you need to.

The Amiga 1200 has support for reading CD's using an external CD-ROM drive much like you'd find it a PC but for the Amiga the CD units typically connected using PCMCIA. One popular Amiga CD drive was the Zappo 'Overdrive' by Archos but the one that came with my Amiga I wouldn't even connect to the computer as it was in such as bad condition. It had no power supply so I opened up the unit to work out what voltage and current would be needed. I found inside a Mitsumi CRMC-FX001DE PC CD-ROM drive which is a 2x IDE drive and this was connected to a small board. Upon finding a 7812 voltage regulator connected to the input socket I deduced that the input voltage was about 15V (a 7812 needs about 3V above the regulated voltage). However, it soon came to my notice that the voltage regulator's input had been connected to its output, which I can only assume was a user modification as apart from the obvious a small bit of wire had been soldered to cause the short. I can only guess that either the original user blew the regulator and thus needed to bypass it or just wanted to feed in 12V. I was curious where the 5V came from to power the CD-ROM's logic circuit and found it to come from the PCMCIA port. This is not the best design as it can load down the Amiga; it would be better that the CD-ROM drew no power from the Amiga.

Repair

While this section covers the fixing of the Amiga 1200 it may also be useful for repairing other Amiga computers.

Floppy disk not detected

If you find that putting a floppy disk into the drive is not detected or only at start up and putting in another disk isn't recognised then it's possible that the floppy drive's disk detect switch has failed. Inside the drive towards where the disk is inserted and opposite side of the floppy eject button is a switch that contains two moving parts, one of which detects that a disk has been inserted and the other lets the computer know the status of the floppy write protect tab.

I tested the switch with my multimeter after taking apart the drive and found that both parts of the switch were defective (not that the switch is of the normally closed type). The drive was very dusty and I did clean it up as best as I could but there wasn't an easy way to clean switch. So I had a look in an external floppy drive that I happened to have and was pleased to see it had the exact same type of switch. I tested the switch to make sure it was working, which it was, and then replaced the bad switch with the working one.

It was quite tricky to put the floppy drive back together so if you do try the repair yourself make sure to take photos or record a video. The repair was a success and so far my A1200 detects disks no problem. You may be wondering why I didn't just put the external drive into my A1200: the original one fits better. If you do want to put a different drive in an Amiga that is normally used as an external drive make sure you get the data connector the right way round-pin 1 of the drive must match pin 1 on the Amiga.