Starting point

This Commodore 128 looks quite good on the outside, but it is known to be not working unfortunately. The machine is reported to not start up in either C128 or C64 mode. But, aside from not working, the casing and keyboard looks to be undamaged and with little signs of use. Notice that the keyboard layout is Swedish characters «Å», «Ä» and «Ö». In the picture gallery below you can see the machine before refurbishment and the corrupted startup screen for both modes.

Refurbishment plan

The refurbishment plan for this flat C128 (several of them in parallell):

- Refurbish the casing (cleaning, repairing and retrobrighting)
- Refurbish the keyboard (cleaning, reviving the plungers and maybe retrobrighting)
- Refurbish main board (cleaning, checking, repairing, replacing capacitors and voltage regulators, adding heat sinks etc.)
- Recap RF-modulator
- Verify operation by testing

The plan can be updated during the refurbishment process. Sometimes I discover areas that needs special attention.

Disassembly

Opening the Commodore 128 is not complicated, but it is a good idea to be a bit careful to not break the small plastic parts. First the six screws are removed from the bottom cover.

Next step is a bit fiddly. On each side, parallell to the thin line running along the keyboard, there is a small plastic clip. These clips holds the top- and bottom cover together. To release the clips a small "plectrum" from the Pro Tech Toolkit is used to pry the covers apart. Below is a picture showing the position of the clip (seen from above) and a picture of the clip when released (seen from the side).

With the clips released, but the top cover not completely removed, the next steps are to remove the ground screw, the keyboard LED and finally the keyboard connector. The ground screw can be found on the right side of the keyboard, and the LED connector on the left hand side. Same as the ground screw the keyboard connector can also be found on the right hand side. See pictures below.

Now the top cover can be lifted from the bottom cover. The inside is revealed showing a large RF-shield (which is also functioning as large heat sink for the ICs). It looks to be in quite condition

Notice that there is one IC which is visible through the RF-shield. This IC is an EPROM containing the character ROM, but the circular glass window is open! An EPROM is erased by exposing UV light to it. Not that there is much UV light inside a closed Commodore 128, but this window should covered.

The RF shield is removed and at the same time some tape is placed on the character EPROM (U18) to prevent any damage by UV light.

Finally the mainboard is lifted from the bottom cover. The bottom cover looks to be in fine condition. Some dust and dirt, but otherwise it looks good.

Mainboard

This mainboard is version Artwork 310381 (Rev 9). It looks to be in quite good condition. There is a significant amount of dust and grease on the mainboard, but other than that it looks ok. As previously mentioned this mainboard is known to be not working. Below are some pictures of the mainboard before refurbishment.

Initial testing

The mainboard is connected to a known good power supply and a TV via the A/V connector. No big surprise here but the result is:

• A SCREEN WITH GARBLED CHARACTERS (see picture below)

Even if the screen is a complete mess there are something good here. It appears that the VIC-IIe and CPUs are working. Otherwise I do not think it would produce any graphics at all.

Visual inspection

As mentioned the PCBA is full of dirt and grease. But I can not see any sign of damage, corrosion or rework. There are some bodge wires across the mainboard, but this is from factory. All the ICs are present, and the EPROMs are covered with stickers (and some tape on the EPROM U18).

But now I notice something interesting... the EPROM in U18 appear to be loose from the socket. This is the very same IC which I had to place some insulation tape to prevent exposure to UV.

The EPROM IC is carefully removed, the socket cleaned with contact cleaner, and the EPROM is put back. Now the machine is powered on again, and LO-AND-BEHOLD! The machine boots up!

Cleaning the PCB

Although not strictly required, I think it is good practice to clean the PCB. There is a significant amount of grease and dust on the PCB. Before the PCB is cleaned all socketed chips are removed. This is not because the ICs can´t handle the cleaning, but because small drops of liquid can get "stuck" in the sockets and also that the paper labes of the IC will disappear.

The PCB is cleaned with mild soap water (using a soft paint brush), and rinsed with water before left to dry for a couple of days. Below are some pictures of the mainboard after cleaning.

In the table below the main chips are listed (before refurbishment - some may be changed during repair if required). As can be seen from the table above most of the ICs are manufactured somewhere around Q1 1987. So it is a fair guess that this Commodore 128 was produced sometime late winter/early spring in ´87.

Checking the voltages

For the Commodore 128 to work flawlessly the voltage levels needs to be with acceptable levels. The fault symptoms do not suggest that there is an issue with the voltages, but it is good practice to measure these - and to rule out that there is a problem with voltages. In the table below all the measures voltages are listed (this list will also be updated after refurbishment). All the required voltages are present and within tolerances, so there are nothing obvious wrong in that area. NOTE: the +5 VDC is a bit low, but I think this is due to the power supply. But I will keep an eye on that.

Banner picture credits: Evan-Amos