I got this Commodore 64 motherboard (Assy 250469 Rev. 3) quite a long time ago. All metal parts was rusty. And the PCB was quite dirty too. I didn’t dare to turning on in this condition. So… Let’s fix this poor motherboard!
First, I removed the cartridge port metal frame and after that I de-soldered the RF modulator. I removed all IC (in this case 3pcs) from the IC sockets. The motherboard was ready for serious cleaning. If the PCB is very dirty like this one I usually use hot water and liquid soap with a old toothbrush. Do not worry about using water. If all metal part is rusted that mean this PCB got a lot of water already 😀 So I thoroughly scoured the PCB and after that I removed all remaining soap with plenty of water. It is very important that no residue of detergent remains on the PCB! Time to dry PCB. I always use heat gun to drying PCB. Keep on mind on the PCB there is plenty of plastic parts which can be melt if the temperature too high. So be careful about set the air temperature. After the PCB are completely dried I could thoroughly examine the PCB. And It was quite nice condition. I put back all IC to the sockets. So I decided to test it the motherboard without RF modulator. For this I used a game cartridge. Without RF modulator the Commodore 64 can not generate video signal at all.
So how can be tested? The answer is the audio out. The SID works perfectly even when the RF modulator is removed. So I chose a game cartridge which start music right away. I put together everything and…. It’s working! I heard game music on TV (of course without picture). Nice! Let’s fix the RF modulator. First I removed the top and bottom metal cover from the modulator. After that I cleaned the modulator inside with compressed air and I cleaned the modulator metal side with a wire brush. Then I soldered back on the motherboard. I didn’t put back the top and buttom metal cover. Those was too rusty. So I put all cable back and tested again the motherboard. The test was a success. I had Commodore 64 welcome screen on the TV. I used the C64 Dead Test cartridge to test all motherboard function. Everything is worked. I got a good working C64 motherboard.