Inspection didn’t show any particular problems. The power supplies are quite simple and don’t seem to have any of the usual EMI filtering.
Everything fired up without smoke. The rails looked clean, and I adjusted the supplies on both the computer and the expansion unit.
There are a lot of modifications including:
- High resolution graphics
- Lower case mod
- High speed mod
- Joystick
- Termination of control signals on the expansion unit
- Cassette modification
- Reset switch
This machine was obviously owned by an enthusiast.
I only briefly got clear text on the monitor, and then it would not sync. This was traced to Z5 in the sync generator, which is a common failure point. This is a CMOS part and I replaced it with an 74HC02.
Some keys were generating strange character sequences. This was caused by a broken wire ground on the keyboard cable which was somewhat fragile.
The machine uses an ALPS keyboard and the key switches seem to be working well.
I replaced the keyboard cable using the instructions here:
I found the keyboard ic positions too close to the connector so I had to bend the pins up at 45°. I also added some earth lines to try to provide some shielding. The keyboard is mounted using some rubber mounts which had deteriorated. I replaced them with rubber tubing.
The amount of RAM reported by the ROM basic (PRINT MEM) was variable. I had four spare 16k RAM chips, so I kept on swapping the mainboard RAM chips until I got the full 15572. Then I repeated the exercise on the expansion interface. I found 5 dubious RAM chips which were replaced.
| Expansion Interface 48k top | FFFF | -1 |
| Expansion Interface 48k bottom | C000 | -16384 |
| Expansion Interface 32k top | BFFF | -16385 |
| Expansion Interface 32k bottom | 8000 | -32768 |
| 16k top | 7FFF | 32767 |
| 16k bottom | 4000 | 16384 |
| 3FFF | 16383 | |
| 0000 | 0 |
The screen showed artefacts on the two bottom lines and occasionally on the two lines above. These changed as new text was displayed over them but the text was incorrect. Looking at the text it was apparent that the LSB was always set.
This becomes very annoying because the bottom lines are copied up as the text scrolls. Eventually the whole screen is filled with corrupt text. For now, I can keep clearing the screen, but it needs to be fixed. This was due to a faulty video RAM chip 2102 at Z48. It was replaced.
With this last fix the machine appeared to be working.