1992 Amstrad NC100 Notepad Computer Repair

A couple of weeks ago, I spent the bank holiday at two car boots sales – one at Stokesley Showground and one at Thirsk – and ended up buying quite a few nice pieces of kit.

Amongst my purchases was a 1992 Amstrad NC100 notepad computer and its original carry case. This was in reasonable physical shape, however it was in unknown operating condition – the case had some minor cracks, and the RTC battery cover was missing.

It didn’t come with a power supply (6Vdc @800mA centre-negative), so I tried to power it up from four AA batteries – however, the unit was completely dead (no response at all to the soft power switch). It therefore required repair.

I wanted to disassemble the NC100 to inspect the mainboard for damage, which is a simple process: the ROM cover is held in place by one cross-head screw; the upper case is held in place by four cross-head screws, including one in the main battery compartment.

It was clear that the unit had suffered some kind of impact – two of the case standoffs had been snapped off, the keyboard surround on the upper case was cracked, the cover had been cracked off the filter choke, and some of the larger electrolytic capacitors seemed to have sustained physical damage.

The keyboard is attached to the mainboard using two ribbon cables, and the LCD display using one ribbon cable – these are push-fit and can be carefully removed.

The mainboard can then be removed, including its RF shield and insulating layer – this is held in place by the four hex nuts on the serial/parallel ports.

With the mainboard removed, it’s easily accessible for a visual inspection, to try and determine any obvious issues or faults (i.e. burns, blown capacitors, physical damage). Aside from the scuffed capacitors and damaged choke that I mentioned earlier (which were probably not the cause of this symptom), there was nothing else obvious.

After doing some research online, I found that the NC100 mainboard has an onboard fuse, which is apparently a common problem – the DC power supply is centre-negative, and using an incorrect power supply blows the fuse, leading to a dead machine.

There are two major mainboard variants: one has a through-hole mounted fuse, the other an SMD fuse, both rated for 800mA. In my case, the mainboard had an SMD fuse – marked F301 on the mainboard silkscreen.

I tested the fuse using my multimeter on continuity mode, and sure enough, it was open-circuit; it’s possibly that someone had tried to power this unit on using an incorrect power supply. When it comes to DC PSUs, you have to be very careful to get the right one – voltage, polarity, and current rating matter when selecting one!

Removing the old fuse is easy enough using a soldering iron – I then used my desoldering station to clean up the pads, and installed a replacement. In this case, I used a 1.1A self-recovering 1812 SMD fuse, which automatically resets once a fault condition has cleared – this specific part is rated up to 8V, and although it’s current limit is 300mA greater than the original, I can’t imagine that causing any problems in this application.

I also replaced the three larger electrolytic capacitors on the right-hand-side of the mainboard, two of which had scuffs on top from whatever impact the unit had been subjected to – one was leaking physically, so this was a good idea.

I also repaired the snapped case and two damaged standoffs using superglue, and thoroughly cleaned the mainboard, inside of the case, and the keyboard.

I fitted four AA batteries to test the repair, and sure enough, the computer started up! The RTC battery (CR2032) was missing, so it just gave a battery warning.

With a replacement RTC battery installed, the computer seemed to work OK.

The computer and its carry case cleaned up quite nicely.