I recently picked up a C128 on Ebay and decided to spend a bit of time getting up and running and perhaps add a few extras to the mix.
The first thing that I did was take it apart and give it a thorough cleaning. The unit does show allot of yellowing but I decided to wait on the retro-brighting until the weather warms up a bit.
The next thing I did was completely take apart the keyboard, clean every key and wipe down the board and back of all of the keys with 70% alcohol. The keys weren't making good contact with the board and this pretty much fixed it.
I ended up getting a Rev. 7 board and here is my workbench setup in all its glory:
COMMODORE 128 MODIFICATIONS
On the board itself I replaced all of the electrolytic capacitors as well as added Jiffy DOS kernel ROMs.
I do own various broadcast quality CRT monitors but wanted to use an LCD to save space on the desk.
For 40 column mode I ended up buying a cheap scan converter that can take S-Video in and spits VGA out. It's not ideal but works well enough for I want it to do.
For 80 column mode I purchased a CGLabs CGA2RGBv2 adapter coupled with a GBS8200 converter.
This power supply seems to work well in 40 column mode but the 80 column display doesn't like it for some reason. The most noticeable difference is in the loss of horizontal sync. Sometime the display becomes garbled, as if vertical sync gets out of whack as well. It's hit or miss every time the power is turned on.
It worked rather well when I was using the original C128 power supply. However, the power supply died and so I ended up purchasing a "modern" C128 power supply off of Ebay.
Sometimes it just works
I noticed the display is a little cleaner when not running Jiffy DOS
Since this wasn't happening with the original power supply I assumed it had something to do with the new one. However, over the years of having to troubleshoot systems I've learned not to jump to conclusions and see where the data takes me.
The first thing I did was to see what's going on with the power rails.
Looking at the 9V AC output of the power supply my meter shows it as closer to 12V.
Since I don't currently own a differential probe (note to self) I looked at the individual 9V AC taps which were not pretty but good enough for the C128 I guess.
Next I looked at the 5V power rail. Nothing special here except the actual voltage hovers around 4.6V.
After that I looked at the output of the bridge rectifier on the input to the 12V regulator. There's some ripple but nothing the regulator can't handle. As you can see it's around 26V which causes the regulator to get rather warm so I slapped a heatsink onto it for good measure.
Now I decided to move my attention over to the 80 column circuitry. First thing was to look at the 16MHz clock that feeds the 8563. It's pretty messy.
The next thing I did was look at the timing between the H-Sync signal and the blue color signal. This is where things started to get a little weird. First, lets look at the signals when the text is centered and appears to be in relatively good shape. As you can see the color signal timing looks OK.
Now for when the video is off-centered. As expected the blue signal shifts towards the left and gets closer to the H-Sync signal.
Here's the "Commodore Basic..." text:
Here's the "Commodore Basic..." title text:
Here's the "Ready" text:
Now for the worrisome part. The leftmost text on the screen stomps on the H-Sync signal. I suppose this makes sense as the text is being written off screen during the horizontal blanking period. I looked at the signals directly off of the 8563 as well as on the other side of the 74LS244. Result is the same:
Checking the timing of the H-Sync vs V-Sync we see the 60Hz V-Sync pulse (in blue). I suppose it looks OK.
I ordered some 64K RAM chips to upgrade the 8563 and since I had to remove the 16K chips decided to also swap out all of the surrounding ceramic caps. This cleaned up the 16MHz clock a bit.