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