treesh@bach.helios.nd.edu (10/07/90)
With the help of some of the technical users of this newgroup, I would like to make up some kind of technical repair guide to the commodore 64 logic board. It seems to me that there really is only one thing that ever goes wrong wiht the commodore 64 system. (well, not one thing, but one symptom that alaways seems to come up). SYMPTON: power light comes on, tv set clears static to black, video monitor shows black screen. Serial disk drive in-use lights come on, then turn off at power up of the commodore 64. No video image, no keyboard responce and system seems completely dead. If you look in books written for reapir, sams photo-fact, or any other technical assistance journals, they all seem to saddly overlook a lot of possibilitys of things that can cause this symptom. Lets look at this with some study. First of all, we know the power supply is working because we are getting a power light. Granted, it may be at the wrong voltage, but this is rare right at power up...cold. If your using a TV set you can also verify that the internal fuse is good if you notice that the screen clears to black, and the sound of static goes to silence. This also tells you that the 9 volts ac is proabaly working in the power supply since the RF modualtor will not work if the AC volts from the extermal power suppy is bad. After verifying the power suppy is good. (perhaps by trying another??) these technial reference manuals will tell you to next check the RESET circuit. This is a 556 chip connected as a 'one-shot' pluser. When power is frist applied to the logic board, this cicuit should generate a logic zero for about 1 second, and then jump high, and stay high for the rest of the time. They have you use a logic probe to make this test. I have found that this is another part that hardly ever causes this symptom, and you can check it much easier just by looking at your disk drive in-use light. If that drive light pops on, then off when you power up your 64, then you can be 99% sure that your reset is not the cause of the problem. With a know-good powersupply and RESET circuit, your now getting into some rahter complex crap! A logical place to start next??? VIDEO CHIP. This 40 pin chip is alaways scoketted, so changeing it it quick and easy. But, this part does a lot more then just video, so it could account for why the keyboard has no responce, and the enter system is dead to the eye. The clock signals generated by this chip drive other parts of the system, and if they are messed up, then the entire system can be messed up. CPU. The 6510 is NOT compatable with the 6502, and can not be replaced wiht a 6502 on the commodore 64. The reason is because the 6510 has an I/O port on it, that the 6502 does not. This I/O port controls the tape drive, and the ROM/ADDRESS selection control lines wich feed the PLA. This chip is only scoketted about 1/2 the time, so if you can change it easy, do it. If not, then you will have to probe it out. PLA. The PLA is one of the hardest, and most expensive parts in the commodore 64. Its a strage chip that works kinda like a ROM, but what its real function is, is to look at the control lines of the CPU, cartridge port, and most of the address bus. Using all those singals as input, five (I think) out puts are connected to the Cart port, and all three internal ROMS. This chip desideds wich rom is sleclectd and in use when. This chip is nearlly imposible to scope out, or probe out and I have found that only by replacment can you determin if this part is bad. It can easly cause the above symptom. ROM. Roms are also hard to scope out. BASIC and KERNAL can cause the above black-screen symptom. CHRSET usualy wont if its bad, the symptom is screwed up video, but not usualy no video at all. RAM. If any of the 8 ram chips are bad, then you will get a black screen in most cases. This is because if ram has a failing address, then the system inintaliztion at power up will hang, and not get to the video chip. SCOPE/PROBE techniques: A good place to start with your logic probe is the CS lines of the roms. You should observe that no two roms should have this line low at the same time, if they are, then the PLA is proabaly gone. The the CS lines are pulsing on the kernal, and basic, then this is proabaly normal, but does no rule out a bad rom. If your getting pules on the CS line, then you should also get pluse on ever address and data line. If any are stuck high or low, then there is a bus problem. Since all roms are connected to the bus, only changing them one at a time will tell you which rom is hogging down the bus line, either data or address can fall victim to this. RAM is the same way, the CS lines should pules, and if not, then the PLA is proabaly the problem. If CS lines look ok, then all data and address lines should also pulse. If one is stuck, then it could be any of the eight chips doing it. Well, I have typed enough for now, but dont forget about the CIA's!! Since they too are connect to the address bus, they can hog it donw. w