My first PC was an Amstrad PC1512. Amstrad led the popularization of the PC as a computer for the home and small businesses in Europe in the mid to late 80's. IBM and other established brands were very expensive. Amstrad was able to greatly reduce the price using smart cost cutting measures. For example, their design was based on a few specialized ASICs instead of large amounts of discrete logic as used in IBM machines. They used plastic instead of metal cases, and the power supply of the bundled monitor also powered the CPU. They also offered attractive improvements relative to the IBM machines: standard ports were included in the board instead of in optional expansion cards. It included a full 512K of RAM (expandable to 640K), the CPU was a step up from the IBM PC/XT (a 8Mhz 8086, instead of the 4.7Mhz 8088). It included a keyboard, mouse, a graphical interface (Digital Research GEM), and both Microsoft and Digital Research versions of DOS. The PC1512 had CGA compatible graphics (with extensions similar but incompatible with medium resolution EGA).
The PC1640 launched later and included a few improvements: standard 640K of RAM, and more flexible graphics compatible with MDA, CGA, and EGA. The graphics available depended only on what was supported by each bundled monitor. I remember being upset when the PC1640 appeared in the shops soon after I had bough the PC1512, and that it supported superior EGA graphics.
My original PC1512 is collecting dust in a village in Portugal. Last time I powered it up it complained about faulty memory. It should be an easy fix, but I don't have the necessary equipment around to repair it when I visit.
For a few weeks, I followed a PC1640 in a US auctions site that no one seemed to want. In a moment of late 80's nostalgia, I made an offer and I got it for half price.
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| Amstrad PC1640 double drive version (DD), it originally came with two 5 1/4 floppy drives, drive B was replaced by a 3 1/2 unit. The depression on top of the case fits the base of the original monitor. |
Once the PC1640 arrived, I confirmed that it was in good physical shape, with only some corrosion in the battery compartment. This is easy to fix and poses no risk of damage to the motherboard. The computer came with a 3 1/2 floppy drive in place of the original drive B. Interestingly, I had made the same upgrade to my old PC1512.
I had to solve the usual problems before I could try the computer: 1) How to power it, since the original power supply was in the monitor (that I do not have), and used a proprietary connector. 2) How to connect the video to a modern display. 3) Get a keyboard (the first Amstrad PCs used a proprietary keyboard, built on Amstrad technology developed for their earlier PCW series of CP/M machines).
I started by disassembling the computer to the bare motherboard, keeping the power LED, and the speaker connected (I need these to help figure out if it is working). I also removed the floppy drives.
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Bottom of the case with motherboard encased in a metal shield. The ROMs, math processor socket and ISA slots are accessible through the shield. The shield was probably a cheaper way to achieve RF emission compliance than a metal, or metalized plastic case.
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Motherboard, the 8086 cpu is located in the center, next to a socket for a 8087 math processor. The board includes four 8-bit ISA slots, the top 3 are accessible without disassembling the computer. Power is provided using a din-14 plug (top center-right) from the power supply located in the monitor. The ports to the left of the power connector are: a CGA/EGA video connector, a parallel and a serial port. The floppy data and power cables are soldered to the motherboard.
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To power the motherboard, I used an old AT power supply from a late 90's Gateway Pentium II computer. I cut off one of the female molex drive power connectors and replaced it by a male connector. Then I plugged it to one of the motherboard floppy drives power connectors. The idea is that this provides power to the 12V and 5V power rails (the -5V and -12V most certainty are not required).
It worked! The motherboard turned on and I heard a beep.
Later, I decided to make this change more permanent by desoldering the din-14 plug, and soldering the molex connector to the motherboard in its place. If I ever need the negative voltages, I'll either install an internal positive to negative DC conversion circuit, or a pico PSU.
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Power connector. The pin numbers are printed on the motherboard silkscreen making it easier to avoid errors when replacing the connector. I connected the molex connector to pins 3, 2, 4, and 11 (5V, gnd, gnd, and 12V respectively.
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Desoldered power connector.
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Microscope view of the underside of the power connector. Picture was taken halfway through desoldering, it needed another pass with the desoldering gun.
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Now I needed to connect a monitor. This may be challenging because the video output is digital TLL instead of analog as required by VGA. Furthermore, most modern VGA LCD monitors are unable to sync at the lower frequencies produced by CGA or EGA video cards. Luckily, I have a cheap generic MAG 22-inch LCD TV from 2009 that is amazing at accepting vintage computer video signals. Most brand name TVs and Monitors are more limited.
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| DIP Switches used to configure various combinations of video mode (MDA, CGA, EGA) and monitor (either Mono, Color CGA, or Color EGA). |
I decided to try a simple adapter cable connecting the respective signals between a 9-pin CGA plug and a 15-pin VGA plug. I connected Red, Green, Blue, H Sync, V Sync, and Ground (I ignored the CGA intensity signal). I configured the motherboard to a CGA output, using the dip switches in the back.
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| CGA and EGA pinouts. |
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Video connector, and a makeshift adapter to connect to a VGA monitor. On the left we see the din-14 power input from the monitor.
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I turned the computer on, and, to my surprise, the monitor was able to sync displaying the booting messages (I was expecting having to use resistors to reduce the CGA TTL to VGA voltage levels)!
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Booting after many years!
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