NEC 8201-A Portable PC

Advertised on eBay as 'one of the first laptops', the NEC 8201-A is actually a surprisingly nice computer, even today. It's got an 8x40 characters LCD screen, built in Basic, Word-processor and Terminal software and  a decent keyboard. With  three serial ports,  a printer port, Cassette interface and a Barcode Reader interface it also has plenty of connection possibilities. And the most appealing feature of all : it can run for almost 20 hours on 4 AA batteries !

The battery pack seems corroded, though not beyond repair. When cleaning there is a lot of green stuff coming off the board. Probably copper oxide, but it could also be remains of leaking batteries.

But after inserting a fresh set of batteries the computer does not turn on. Checking the connection from the battery to the board I soon found out the contacts were still bad. So I first just soldered the pack directly to the board.
The computer turns on, and the display shows text, although hardly visible. Turning the contrast control does not change this except maybe at one end, where I could see an almost black screen sometimes. So it looks like this control may be bad. The schematic shows its a 50K potentiometer so this might need cleaning or replacing. Looking on the internet for details on the control, I also found some notes from people that found the control to be OK, but a fault in the -5V power which could cause the same problem. So I first measured all voltages on the power supply.

On the picture I indicated the GND point used, how the connector is numbered and the voltages that should be present. All seem pretty good, except maybe number 18, which is not really 9V, but this could be due to the fact that it is battery powered.
Anyway, the -5V (which as far as I can see is only used for the LCD contrast) is present, so it must be a problem with the potentiometer.  Following is a detail from the schematic showing how the control is used.

Since I could not figure out from the footprint of the potmeter how it was connected I first measured connections from the adjacent  components R10 and R77. From that I found that R10 is not 100K but 50K. The two big solder squares are connected to the wiper of the potmeter, and the two other small contacts are the outer ends. Since I did not have a 50K model I used a 10K trimmer to test it. And indeed the screen now was readable. The contrast range was still limited so I added a 20K resistor in series with the trimmer and got a good result from that.
Obviously the trimmer that I used was different from the original so there was no way it would fit into the original PCB.

Also the knob would not fit the trimmer. So I decided to print one on my 3D printer. Then I connected the trimmer to the bottom of the PCB using wires, and glued it on the top using hot-glue. All this worked out really nice so and the computer screen is useable now.

Price [Original]	€25,- [US$699,-]
Processor	Intel 80C85 @ 2.4576 MHz
RAM	64KB
ROM	32KB
Programming	Microsoft Basic 1.0
Why ?	One of the first laptops.