At this point I felt I was running out of options and basically started replacing chips ‘Kamikaze style’. I basically started swapping chips that may or may not be directly connected to the serial port. In this context, the 556 logic timer at U20 generates drive resets at powerup and was therefore also swapped with a fresh new one.
The 7406 chip was still burning hot. I therefore suspected that there was a fault in the female plug and exchanged it with one from a spare board I use for parts. This did not help either.
The transistor at Q3 may also affect the serial port if it is shorted. A closer inspection showed that transistor Q3 was indeed shorted! In with a new one from the spare board.
Still a hot 7406 chip at U8. After some serious neck scratching, I unplugged the 64 Doctor serial port tester and inserted my original Commodore 1541 diskette drive to the serial port to see if this would change anything. Trying to access the disk drive yielded a ‘DEVICE NOT PRESENT ERROR‘.
Now I was getting somewhere! I found the schematics of the serial port. Besides it’s close connection with the CIA chip at U2 and the 7406 chip at U8 it also contains 9 diodes that serves as a protection circuit of the serial bus. If the 7406 chip is broken, it may be caused by a broken protection circuit (more information can be found here). I knew the CIA chip was not faulty (nor the power plug that I’d also swapped), I therefore started measuring the diodes with the voltmeter of my multimeter. As the diodes are placed on the PCB, the values cannot be used for much more than faultfinding. However, I was looking for some kind of outlier in my measements and guess what? The diode above resistor R29 (CR100) was almost zero volts compared to the rest of the diodes (they had values of 1.2-0.5 V). Before starting up my soldering iron, I double checked the voltage of the same CR100 diode on another Assy 250245 board that I knew was working – and the diode seemed to be shorted! I therefore exchanged it with one from the spare board I only use for parts. The shorted diode (1N4148 or IN914) was soldered on top of resistor R29.
…and finally, everything was working again! I had access to the serial port (the diskette could be reached with a simple ‘LOAD”$”,8’ command) and the 7406 chip at U8 did not get burning hot anymore! I then put back the MPU, the SID, the PLA and the VIC-II one by one to see if they were faulty and they were all working.
The repair included three RAM chips, a fuse, a 556 logic timer, a transister, a 7406 output driver and a diode.
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