How to challenge a technician...

     I don't often take on seriously damaged C64 boards simply because few people are willing to pay for the time and parts it takes to restore one. If a board has been damaged by a failed power supply, many chips can be affected and troubleshooting takes a lot of bench time. Such boards are usually swapped out and the dud used for parts. 
     This C64 mother board is one of the later versions, board number is 250466. It has only two RAM chips but otherwise is very similar to previous boards. When I got it, some of the parts were missing and one socket (CIA2) was installed, which indicated someone probably tried to fix it, gave up and started parting it out. 
     I replaced the missing parts (Clock oscillator, VIC, SID, PLA, CIA, and 2 regulators) and powered up the board. Of course, all I got was a blank screen. None of my diagnostic carts including the Dead Test Cart were of any use. No chips were overheating (shorted IC's will often get very hot quickly) and I was really starting from scratch. I have a meter to measure the current on the +5V line and it was normal.
     I began troubleshooting by measuring the internal supply voltages... all were OK. Next, I used my oscilloscope to check the clock, reset, IRQ and signal lines of the CPU. I found several address lines reduced in amplitude and one very low with no signal on it. Since there are several IC's tied to those lines, I couldn't know which IC was pulling the line down. A resistance check of those three lines showed a lower resistance to ground and one was very low, so I concentrated on that one. To isolate which IC was faulty, I desoldered individual pins of the three ROMs and the CPU itself. (Some techs cut pins or board traces, but I don't like to do that.) The address line was clear until I reconnected the CPU... a clear defect inside the chip. I replaced it but still got no boot screen. 
     I then used my dead test cart again which produced one blink every two seconds... a RAM problem. Replacing one RAM chip cleared that error but I still had a blank screen. The Dead Test Cart was the only one that did anything at that point, but it didn't blink the standard error code. All it did was show a bright white screen after about 10 seconds, back to blank in another 10 seconds, bright, blank, etc. Replacing the other RAM didn't help... same symptoms. 
     At this point, I dug another diagnostic cart out of my stash and tried it: C64 Doctor. It has several rows of LEDs that show which IC is faulty. The entire top row of LEDs stayed lit which indicated a RAM problem... but I knew the ones I installed were good. What now?
     Experience has taught me that stubborn RAM problems can be caused by one or both of the interface chips: 74LS257, so I replaced them both. Presto! Opening screen normal and my diagnostic cart and harness showed no other problems. Only one of the RAM chips was actually bad and only one of the interface chips needed to be replaced. I always re-test chips that I've changed to ensure each is good or bad so I don't waste anything. This board now has half a dozen sockets. I never replace an IC without installing a socket, so that part of the board never needs to be soldered on again. I confess I damaged a board trace when changing one of the RAMs, but I repaired it before installing the socket. Imagine what would happen if a broken trace was not repaired and a chip or socket installed over it. 
     This many bad chips is a clear indication of a failed power supply (the Commodore "brick"), and as you can see from the above, it's not really worth all the work and replacement parts to try to fix such a board. I do it once in awhile just for the experience but it's not cost effective to go that route for a user.