This machine is an Assy No. 250425 long board. At startup it had a black screen.
First I did a visual inspection of the board and immediately spotted the missing fuse. Adding a fuse did not fix the black screen.
On this particular board, a lot of the large chips are socketed and I therefor replaced them with known working ones. This included the VIC-II, the PLA, the CIA’s and the MPU. I also removed the SID as this is not needed for faultfinding of a black screen.
The black screen persisted. I then ran my Dead Test Cartridge which indicated a fault in the RAM chip at U12.
Unfortunately, a trace was released from the board when removing U12. The memory chip at U11 was therefore also removed. Red arrows show the broken trace.
Two sockets were inserted and the broken trace was repaired using a piece of blue wire on the backside of the board.
…and two fresh new RAM chips were inserted.
Nevertheless, the Dead Test Cart kept givning me a memory fault. After powering up the machine, the RAM chip at U9 would get really hot, so I also replaced this chip.
This removed the memory fault indications by the Dead Test Cart, but the black screen remained. I left the board powered on and started touching all the chips and noticed that the 7406 logic chip at U8 also got very warm. According to Ray Carlsen (link), the 7406 logic chip at U8 may also cause a blank screen. I therefore replaced the chip at U8 – this chip is called a MOS 7707 on this particular board and is known to have a high failure rate.
This chip is not a standard part of my spare parts, so I harvested one from a spare Assy 250425 board that I only use for parts. Using a air gun made it quite easy to remove the chip without bending any of its legs.
…and finally the sweet blue screen! However, the feeling of self-success only lasted a short while.
At first everything seemed to work perfectly until I mounted the serial plug tester that came with my 64 Doctor cartridge. This would make the freshly installed U8 7406 chip get extremely hot after just a few seconds. This chip is closely related to the serial port which is used for connecting the diskette drive. This is an image of the 64 Doctor serial port test plug.