Introduction
This Commodore 64 came in a lot of six I purchased off eBay. All were listed as not working. This c64 booted up just fine but had some key that did not work (e.g. space and return) and had no sound. Otherwise, it seemed like it was in good working order and the case was in good condition. It also passed multiple diagnostic tests (except for sound). Shown below is the common 250407 Revision B motherboard with all chips socketed except the Revision 8 VIC-II video chip. I detail below the work I did to get this nice c64 back to full working condition.
Case Study
Visual inspection of the board revealed a system in good condition. No broken traces or solder joints were observed. All capacitors looked healthy. Unfortunately, all the main ICs except the VIC-II were not in sockets. The good news was that I only needed to desolder the SID 6581 chip (U18).
The first order of business was to fix the keyboard. I first sprayed some contact cleaner on the keyboard connector and the motherboard pins to make sure everything had a good connection. This did not change the keys that were not working or only intermittently working. Visual inspection of the keyboard revealed quite a bit of dust and debris (not shown). Bad key contacts were thus the likely cause of the nonworking keys.
Disassembly of a Mitsumi mechanical keyboard is pretty easy. Start by removing the set of screws holding the keyboard to the top of the case.
Then flip the keyboard over. There are a large number of very small screws that need to be removed. Store those in a safe place as they are easy to lose.
The get the brown backplate off you will need to disconnect the two wires from the grey plastic unit shown above to the left. I simply heated the solder with my soldering iron and gently pulled the wire out of the unit with some small pliers.
This will reveal the circuit board on one side with the black contacts on the remaining unit with the keys.
I then used some 99% isopropyl alcohol (I buy this one from Amazon) and a soft cloth to gently clean the green circuit board and contacts as well as very gently with one or two wipes the black contacts. The green circuit board was quite dirty before cleaning.
That’s it! Time to reassemble the keyboard pieces. You will need to resolder the two wires by gently pushing them through the holes with a pair of pliers while melting the solder.
I then tested the keyboard, and it was now fully working. There are several keyboard testers that can be downloaded and booted from an SD card device such the Kung Fu Flash. Here is one.
I next turned my attention to the SID chip. I first desoldered it using my Hakko FR-301 (I bought mine on Amazon) desoldering iron and my Yihua hot air rework station (also bought this on Amazon).
I then soldered in a new 28-pin dual-wipe socket.
I then inserted a newly made SIDKick Pico SID emulator that I purchased on eBay for around $25 plus shipping.
I tested the new “SID” using a SID player. Video below. You can compare this emulated rendition of Cybernoid II to the one I posted at the end of Case Study #2 that was generated using a real SID in good working condition.
Addition improvements to the board could include adding an RF replacement for S-video out. This would improve the quality of the video along with the desirable stock Revision 8 VIC-II that is already installed. I went ahead and clipped capacitors C1 and C2 in the RF module. As I previously described, this improves the sharpness of the video signal. I also removed the metal bracket around the video circuit as it is not needed. This makes it easier to remove the VIC-II when necessary. Finally, I added heatsinks (I buy these from Amazon) onto the VIC-II. This will prolong its life.
Comments
This was a straightforward keyboard repair that only required cleaning with alcohol. I also replaced the SID with a new SIDKick replacement. With some other minor enhancements and cleaning, this c64 is ready for a new home! I have someone in mind to pass it on to.
