C64 Breadbin Repair

Few weeks ago I purchased a new Commodore C64 with Assy. Board 250407 Rev. B from 1983. My guess was that this machine had not been switched on for decades ;-)

Nevertheless the visual conditions from this nearly 40 years old computer were quite good:

Top view

Rear view

Back view with connectors

Side view

After turning on the C64 this output was displayed:

colorful mix of letters 

Mhmm, where could be the error? A good starting point is to measure the power inputs on the board:

Measuring points, Source: forum64.de

Blue = GND
Red = +5V
Yellow = +12V











Nope, the input voltages are OK! Next it is a good idea to visit The Pictorial C64 Fault Guide to visually localize the error. So in my case it was hard to find a similar image, but after some research the best candidates are:

• PLA (U17)
• VIC II (U19)

If you don't have working spare parts but an Arduino Uno there are a nice collection of test programs available (C64 Test Tools from jamarju).

Suddenly the PLA was not socketed and therefor I had to desolder it first to be able to run the Arduino Test (with breadboard) YouTube Video of the setup 

(don't forget to connect the /CE pin of PLA to Ground)

Note: Take the chance and solder in a socket.

So, PLA was also OK. Next I tried to remove the VIC II chip and put it into a working board; Voila, same error. So I finally found the faulty IC ;-)

Ordered a new VIC II (MOS 6569R3) and now everything works like a charm!

Conclusion: 

Find the error in this way was quite hard, so for the future I will use a test kit (e.g. Harness from Sven Peterson in combination with deadtest and diagnostic cartridge)