When I saw this Amiga 1200, I immediately felt sorry for it. The case and the keyboard were already heavily yellowed, but even worse was the botched attempt to fit a Gotek drive into the case. To make room for it, the previous owner obviously broke open the area around the floppy drive with pliers or wire cutters, completely ruining the beautiful old case.
Originally, I had planned to bleach the case and the keyboard. I wanted to tidy up the ugly opening at the floppy drive with a rotary tool and then close it with a 3D-printed part as best as possible.
But then I had a much better idea. 😁
The motherboard
First, let’s take a look inside. A Rev 1D.4 board in good visual condition is installed. I replaced the electrolytic capacitors and updated to AmigaOS 3.2.1.
On the underside of the board, I found a copper wire for a so-called “floppy fix”. When Escom produced the last Amiga 1200s, there were no Amiga floppy drives left. Instead, they installed standard PC floppy drives. However, many games and demos didn’t run on the machines modified in this way.
The original floppy drive of this computer was no longer present, and Gotek drives can perfectly emulate Amiga floppy drives. So I decided to reverse the floppy fix. To do this, the jumper wire is first removed. To restore the original RDY signal, a wire is then soldered from pin 34 of the internal drive connector to pin 1 of the external floppy port.
Since I was working on the underside of the board anyway, I also wanted to remove E123C and E125C to improve the stability of accelerator cards. However, the capacitors hadn’t even been populated in the first place, so that was sorted.
That meant it was time for a first thorough test run. Everything went well so far, until I noticed that the right mouse button didn’t work on either port. A separate blog article describes the cause and the fix. In short: you have to replace four resistors with ferrites.
My work on the board was finished after that. It now passed all diagnostic tests.
The extras
As with my other restorations, I don’t just overhaul the machine, but also future-proof it with a few extras.
First up: the yellowed case with the ugly cut. It was clear to me that the case would never look truly nice again, even with a half-decent restoration attempt. Besides, I had always wanted a black Amiga. Therefore, I treated the Amiga to a brand new, black a1200.net replica case instead.
The floppy drive was missing. Instead of the Gotek drive that was originally supposed to take its place, I chose a Centuriontech GoEX drive. Instead of a USB stick, it uses an SD card. It also features a rotary switch that makes selecting the floppy image easier. Matching OLED displays are available as accessories, but for my new Amiga, I wanted a much smaller display that wasn’t as conspicuous.
I used a 0.91" OLED display, the kind you can find in practically any maker shop. Here it is important to swap pins 1 and 2 when soldering the cable, as the power pins on this type of display are swapped compared to the original GoEX display module.
I printed an A600 display module case, which fortunately also fits an A1200. I joined the individual parts together with hot glue. In hindsight, I should have used normal glue, as the hot glue softened the PLA of the print a little. The cable is then routed through the cooling vents of the Amiga and the module is clipped on, gluing is not required.
For the GoEX to use this OLED size correctly, a file named FF/FF.CFG must be created on the SD card with the following line:
display-type = oled-128x32
For a pixel-perfect image via HDMI, I also installed an Indivision AGA MK3. It turned out that the a1200.net replica case seems to have slightly different dimensions than the original case. I therefore designed a modified trapdoor and bracket for this case type.
When I got the Amiga, there was already a Marpet Developments M1207 RAM expansion in the expansion slot. It was treated to a fresh coin cell battery and now provides the machine with an additional 4 MB of Fast RAM, a 68882 FPU and a real-time clock.
And last but not least, the machine got custom LEDs, with the power LED in my signature colour, blue.
Assembly
What’s still missing? The black keycaps, of course, to match the black case. After many years of waiting, they were finally available, and I had a set delivered just in time for Christmas.
Unfortunately, a matching badge is not included with the black case, but Badgeman had something suitable in stock.
After that, the Amiga was ready for final assembly.
If Commodore had given us a choice of case colour back in the 1990s, I would have chosen a black Amiga. And now here it is, a completely black Amiga 1200 with a totally new exterior and a modernised interior.
Whilst repairing an Amiga 1200, I noticed that the right and middle mouse buttons were unresponsive on both ports of this machine. It worked with an original Amiga mouse, but not with my YAMI mouse adapter. However, the mouse adapter couldn’t be the cause, as it has worked reliably for decades on all sorts of Amigas, including another Amiga 1200.
The problem is already known to the community and seems to affect other mouse interfaces as well. The solutions I could find so far were:
- Just use the original Amiga mouse. 😉 Despite all the love for old hardware: the era of ball mice is over.
- Modify the mouse adapter. For some, there is even a “fixed” version.
- Use a “FixRMB” tool. However, the tool has to be started first, so it doesn’t work in games or in the boot menu. It also requires a mouse adapter with internal pull-up resistors. YAMI doesn’t have these, for example.
- Swap the Paula. The soldering work requires experience and suitable equipment. Furthermore, it’s a shame about the otherwise fully functional Paula.
None of these options really appeal to me. I want this Amiga to work like all the others. So I went looking for the cause.
The middle and right mouse buttons are connected to the Paula’s POT pins. These inputs are actually intended for analogue joysticks and use a very simple ADC. The analogue joystick charges a capacitor whilst a counter in the Paula measures the time. The charging process takes varying amounts of time depending on the joystick’s position. Once a certain voltage level is reached, the counter is stopped. The counter value reflects the capacitor’s charging time and therefore the joystick’s position.
For mouse buttons, there is a digital mode. Here, a built-in pull-up resistor pulls the POT line to High. As soon as a mouse button is pressed, the line is pulled to Low. The result can be read directly from the Paula registers.
With an original mouse, there was 0.9 V on the POT line on this Amiga when the button was pressed. With the mouse adapter, it was a mere 1.1 V. This seems like only a minor difference, but on this Paula chip, it was already enough for “button pressed” and “button released”.
Now comes the exciting question: Why does the problem only seem to occur on Amiga 1200 models, even though the affected Paula batch was probably also used in Amiga 4000 production?
If you compare the schematics of both machines, there is a small but important difference. This is a simplified diagram of the joystick or mouse port for both models:
The essential difference lies in the components circled in red. In the Amiga 4000, Commodore installed EMI filters there. Basically, they are nothing more than a bare wire in a ferrite. They filter out high frequencies, but act like a 0 Ω resistor at low frequencies (like a mouse button).
In the Amiga 1200 and Amiga 600, Commodore installed 68 Ω resistors here, presumably to lower the production costs for the low-end models. (Interestingly, Commodore returned to ferrites on the CD³².)
What effect does this resistor have? Together with the pull-up resistor built into the Paula, it forms a voltage divider. The switch in a classic Amiga mouse pulls this divider to ground, resulting in 0.9 V at the POT input. This is just enough to be recognised as Low.
A mouse adapter, however, doesn’t have real switches, but logic outputs. The PIC16F84 used in the YAMI interface, for example, has a Low level of about 0.6 V. The voltage divider then delivers 1.1 V at the POT input, which is interpreted as High by this Paula batch. The mouse button is no longer recognised as being pressed.
I don’t know if the pull-up resistor has too low a value in this Paula batch or if the threshold for Low levels is lower. Both would be possible.
To fix the problem on my Amiga 1200, I replaced the 68 Ω resistors E353R, E354R, E363R and E364R with SMD 1206 ferrites, as used in the Amiga 4000. They are slightly larger than the 0804 resistors, but can still be soldered directly onto the pads.
You also need to replace E353R, E354R, E363R and E364R when working on the Amiga 600.
This is just a minor hardware modification that can be carried out even with little soldering experience. Certainly more so than swapping a PLCC chip.
Following the change, the mouse adapter also worked reliably on this Amiga.
Make sure to replace the resistors with ferrites, not the capacitors next to them! The resistors are typically printed with the number 680. (Nothing would break if you accidentally swapped the capacitors, but the right and middle buttons would then be permanently pressed.)
PS: If you found this article because your Amiga has this problem too, please send me the date code of your Paula chip. Perhaps we can find a pattern of “bad” date codes. Thanks!
Ever since I got my Amiga 4000, I was pondering about if an Amiga 1200 would have been a better choice. I mean, the Amiga 4000 is nice because it has a lot of space for extensions. But on the other hand, it is rather bulky and heavy, so it isn’t much fun to take it to a friend or a party, unlike the compact and light Amiga 1200.
But why not have both? 😉 I had found an Amiga 1200 offer on the Bay that was too good to be ignored, so I bought it.
It’s an Amiga 1200 from the days after Commodore went belly up, and when Escom took over and sold the last Amiga stocks. The good news is that the Escom cases were made of ABS with an anti-UV treatment, so they will never yellow. The bad news is that the keycaps were not treated, and are very visible yellowed by now.
I sent the keycaps to the experts at the CBM Museum Wuppertal for whitening. The case itself is almost in a mint condition, all it needed was a bath in warm dishwater.
Let’s take the computer apart and have a look inside.
There’s a 2.5" hard disk, which turned out to contain a Workbench, a few games, and also many bad sectors. I’m going to replace it with an SD Card solution anyway. The shielding has a bit of flash rust and was bent around the ROMs, probably from forceful prying out the ROM chips with a screwdriver. Besides that, the overall status is quite okay.
Under the shielding, I found the mainboard in a good condition, especially without flash rust on the modulator. To my surprise, it is a Rev. 1D.1 board, which was actually the first broadly sold board revision. In one of the last Amigas that have ever been produced, I had rather expected to find a revision 2 board. Anyway I was lucky because the 1D.1 revision is said to be the most stable one, and it also has a good Lisa chip that was manfactured by HP. On both CIA chips I found traces of flux, so the board seems to have been repaired before.
According to my contact at the CBM Museum Wuppertal, Escom sold everything they could find at the Commodore remainders. Allegedly they also produced “new” Amiga computers with refurbished mainboards. Maybe this is one of them?
I first inserted diag ROMs and checked the hardware, but found no problems, so I upgraded the system to AmigaOS 3.2. The next thing on my to-do list was to replace the electrolytic capacitors, as they tend to leak over all those years, causing damage to the PCB. I have already done that on my Amiga 4000 before, but on this model the space was a bit more limited. I even had to remove a freshly replaced SMD capacitor because another capacitor did not fit next to it any more.
To enhance the stability of accelerator boards, it is recommended to remove the capacitors E123C and E125C on the bottom side of the PCB. The easiest way is to use two soldering irons like a pair of tweezers.
After a thorough wash with IPA, the board was then ready to move back into the case.
I want to modernize the Amiga so it can be connected to a HDMI monitor. The RGB to HDMI solution of the Amiga 500 won’t work on the AGA chipset though, so I decided to get an Indivision AGA MK3 from Individual Computers. It is plugged onto Lisa and one of the CIA chips, and offers an HDMI output even with sound. (Which is quite an accomplishment, as both chips are not connected to a sound line.)
Since I was on it, I also extended the memory with an ACA1211. Unfortunately it turned out that AmigaOS 3.2 is incompatible to the ACA1211, and the system won’t boot in this combination. I had to return to the original AmigaOS 3.0 ROMs again. Eventually I traded the ACA1211 for an ACA1234, which is also an accelerator and works fine with the latest AmigaOS.
To make the wire mess complete, I replaced the floppy disk drive with a GOEX drive from Centurion Tech.
Centurion also offers LED boards with customized colors. I have picked blue as power LED, green as floppy drive LED, and red as harddisk LED.
In the meantime, I got the whitened keycaps back. They were almost white again, but sadly there is still a slight, but visible yellow tint. Maybe I will buy a new set of key caps once they are available. The Amiga would then look as new.
The original Escom label is just a cheap sticker. It looks ugly to me, so I replaced it with a replica Commodore badge.
And then, for the first time after my purchase, I could close the Amiga 1200 case again.
Please welcome the newest addition to my Amiga collection!