After 30 years the BigBoard II PCB comes to life.
I took pictures from different progress states.
For all ICs i use sockets. After feeled 1 million solder joints i wash away the residue from the flux.
Then i carefully inspect the solder joints for shorts.
I populate the board with all ICs and measure the resistance between the supply voltages and compare them with an knows good board.
When i first power up the board, the buzzer sounds forever. When working corectly, the buzzer beeps short after about a second after reset.
That means, that the DRAM ist test partial successfully. But the buzzer beeps immediate after the board gets power.
Yesterday i poke around with the scope to find the fault, but in the evening i gave up.
The address and data bus signal are all OK. The system clocks are also perfect. The Z80 do something and the monitor ROM is in access.
Today i look closer in the circuits for the I/O device address decoding. After some time i found a grounded output from an LS138,
which normally enables the data bus driver from the keyboard port. I looking along the trace to find the short, but it seems difficult.
Then i found the short to the GND along the 8" floppy connector. A little tin bridge, hard to see.
After removing the bridge, the board works perfect. :D
Frank
I took pictures from different progress states.
For all ICs i use sockets. After feeled 1 million solder joints i wash away the residue from the flux.
Then i carefully inspect the solder joints for shorts.
I populate the board with all ICs and measure the resistance between the supply voltages and compare them with an knows good board.
When i first power up the board, the buzzer sounds forever. When working corectly, the buzzer beeps short after about a second after reset.
That means, that the DRAM ist test partial successfully. But the buzzer beeps immediate after the board gets power.
Yesterday i poke around with the scope to find the fault, but in the evening i gave up.
The address and data bus signal are all OK. The system clocks are also perfect. The Z80 do something and the monitor ROM is in access.
Today i look closer in the circuits for the I/O device address decoding. After some time i found a grounded output from an LS138,
which normally enables the data bus driver from the keyboard port. I looking along the trace to find the short, but it seems difficult.
Then i found the short to the GND along the 8" floppy connector. A little tin bridge, hard to see.
After removing the bridge, the board works perfect. :D
Frank
