The End of an Era
The Commodore 128 was the successor to the wildly popular C64 and the final 8-bit home computer to be released by Commodore. While it’s complete backwards compatibility with the C64 was a big selling point, it added a number features geared more towards the business market.
The C128 contained an a second Z80 CPU along side it’s MOS 8502, capable of running CP/M (albeit slowly) and CGA-like VDC graphics chip which added high resolution graphics and 80 column text modes, but had no gaming features such as sprites. Sadly, it could not compete with the increasingly inexpensive array of other 16 and 32-bit home PCs which would put a commercial end to the 8-bit home computing era.
The Power of Poo
This particular C128 had apparently been sitting in a barn or a shed for some time prior to finding it’s way to me. The main unit, disk drive and power supply were very yellowed, and the monitor, while not yellowed was unfortunately covered in bird and insect droppings. This is one of the worst things that can happen to an electronic device due to how acidic and corrosive animal waste can be.
The Good…
Needless to say, everything had to be thoroughly cleaned and disinfected prior to being brought into the shop.
Fortunately, the power supply and disk drive were in perfect working condition and the main computer just needed a new power switch. The C128 can also tend to overheat due to the large metal RF shield on top of the motherboard and its poor connections to the chips below.
One perfectly valid solution to this is to simply remove the RF shield. Instead, I decided to re-bend the aluminum fingers so they made better contact with the chips and replaced the inadequate dried out original thermal paste on each with a modern thermal pad.
The keyboard on the C128 is also a bit of a let down. The key stems are prone to cracking which causes the keys to stick. These parts can be challenging to 3D print and originals are becoming more and more difficult to find.
Retrobrighting the yellowed plastic shells was also easy and straightforward. Since the weather here was cold and overcast for several weeks, I used the heat method. Everything was placed in sealed or well covered containers (to reduce evaporation) and put in an oven kept between 140F-160F.
The Bad…and the Ugly
The Commodore 1902A was the third and final monitor in the 190x series which was released to take advantage of the new high resolution graphics put out by the C128 and Amiga computers. Oddly, the standard DB-9 connector used by most other PCs of the time with CGA or EGA graphics used on the 1902 was replaced by a more proprietary DIN-style round connector on the 1902A.
The C128 however can be used in 80-column mode with most any CGA to VGA adapter available today. I’m currently using mine with an EternalCRT connected to my lab CRT monitor.
Another odd design decision made for the C128 was to use separate connections on the back of the computer for 80 column (labeled RGBi) and 40 column (labeled VIDEO) mode. Curiously, both outputs can be connected to two different displays simultaneously, creating a pseudo-dual monitor setup. Programs of the time almost always used only one display at a time, depending on whether the machine was booted in 40 column or 80 column mode, but users can switch between the displays on demand, by toggling the 40/80 column key and hard or soft resetting the system. Audio (and C64 mode) is of course limited to only the 40-column video output. An RF TV output is also available, but by this point in PC history it was rarely used.
The monitor looked great once cleaned, but turned out to be rabbit hole of damage which only got worse the closer I examined it. In addition, the only schematics for the 1902A I were able to find were very low resolution and almost illegible.
The main power switch no longer latched in the on position. A similar-but not exact replacement-was found at Mouser (part PT-S3WL) and easily installed, albeit with the mounting bracket turned upside down and backwards and a bit of kapton tape to hold it securely in place.
The main circuit board however, had significant areas of corrosion which had begun to remove the solder mask and damaged the underlying traces. All the potentiometers on the board as well as most of the external adjustment knob pots had corrosion on them as well.
I tested all of the traces with visual damage for continuity and to my surprise none had corroded completely through yet. All the areas with damaged solder mask were sealed and protected with clear nail polish.
Thus began the long and tedious process of circuit tracing and troubleshooting various intermittent issues.
At least two dozen solder connections had broken or cracked and needed reflowing. All the large electrolytic capacitors thankfully tested good. Several other suspect component including a couple of transistors were removed and tested out of circuit.
A number of potentiometers that had failed due to corrosion-including all three color signal pots on the neck board-were replaced with modern ones. This allowed at least a basic testing and color calibration of the monitor.
While this monitor does function, it’s still far from completely usable. Despite cleaning and applying deoxit to all the external adjustment pots they still cause flicker and intermittent picture issues. Finding suitable replacements may take some time.
The CRT also shows its age and does appear to be dimmer than it should when properly adjusted. A CRT swap might be feasible if I happen across a suitable donor tube, but given its limited higher resolution applications, this monitor just isn’t worth spending a huge amount of money on to get working perfectly.
All that being said, the restorations and repairs I was able complete on the system were still extremely satisfying. The C128 itself, despite not being a commercial success like it’s predecessor as well as the 1571 disk drive are important parts of Commodore’s history and well worth preserving.
Total Restoration Cost $750