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REPAIR COMMODORE PC40-III
September 2008

The Commodore PC40-III is one of the original IBM-compatible Personal Computers and was made in 1991. Because it has an Intel 80286 microprocessor and a 12MHz clock speed, it may be slow enough to be suitable for use as a 'test bed' for some logic test equipment and microprocessor programming. This PC was given to me a few years ago (thanks Camilla) and has been in storage ever since.

First job was to completely strip it down and clean everything up. Everything looked OK and all the parts were in surprisingly good condition.
Specification

Intel 80286-12 Processor
Commodore PC40-III Motherboard with 4 ISA Slots (3 full and 1 half)
Base RAM 640kb, Extended RAM 384kb, Total RAM 1024kb
Seagate ST351A/X 40Mb HDD
3½ 1.4Mb FDD
5¼ 1.2Mb FDD
PSU 110W +5V at 12A, +12V at 4A & -12V at 0.3A
Commodore AZERTY Keyboard to 5 pin DIN plug
Commodore 2 button Mouse to 9 way D socket
Audio phono socket 
Commodore 1930-II Colour VGA Monitor to 15 way D plug
COM serial port on 25 way D plug
LPT parallel port on 25 way D socket
The PC and Monitor powered up OK but came up with the "Battery Failure" message which halts everything. Ctrl-Alt-Esc calls up a very crude BIOS with a secondary screen on F2. Although the parameters can be changed, they are not saved because of the battery failure. Unfortunately, this Mobo doesn't use a silver oxide disc battery which can easily be replaced. Instead it uses a special IC which has failed.
Pin 1 in lower left hand corner                 Pins 2, 3, 16, 20, 21, 22 are missing
The Dallas DS1287 is a Real Time Clock IC which is now obsolete. It contains a lithium battery, quartz crystal oscillator, timing and address circuitry. The battery should last for 10 years (probably not 17 years), but once exhausted the PC will fail. There are 2 options: find a replacement IC with a working battery; or repair the IC. There are some websites that expain this technique which involves cutting into the IC, disconnecting the old battery, then fitting a new external battery.
Fairly large Mobo at 330 x 320mm showing U201 removed
With all the discrete IC's and ISA expansion slots, this Mobo should facilitate access to data and address lines at a relatively slow clock rate.
24 Pin IC Socket fitted                      Old Dallas DS1287 in Socket
A replacement DS1287 on eBay solves this problem. BIOS set and system now boots up OK to MS-DOS v4.01 and DOSSHELL. Not Y2K compliant though! Need to download a DOS Manual as I have forgotten most of my DOS programming (should have kept all those cupboards full of early disks and manuals on DOS, Windows, Lotus 123, dBase, etc, that got junked).

Lots of useful DOS information on the Web and even some old DOS games which were great fun at the time, but very simple now.
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