So here we are, one week in to RC2016-01 and what do we have to show for it?
As you saw in my previous post, I have taken the lid off the VT180 and it sits as a bare metal cage while I try to figure out what’s wrong.
The screen display is no longer in sync and rolls very quickly.
In order to help with the repair I have taken the VT100 board out of the card cage and I have it plugged in to the cable that was powering the card edge connector on the card cage. It’s clearly straight through so should be fine.
That means that I can get to the board with my meter and ‘scope etc. I have also removed the AVO (advanced Video Output) board and the small inter-connect board. This hasn’t changed the symptoms at all.
The first real diagnostic work was to measure the voltages from the PSU. They all seem fine.
I have been able to use a logic analyser (borrowed – I really want one of these) and I’ve been taking a look on the bus.
have attached three screen shots taken of the logic analyser screens. I have 18 channels and so I have the data bus, the lower 8 bits of the address bus and the synch pin. All signals are from clips on the 8080.
I have the triggers set to stable high on sync (start of new instruction) and 0 on the address bus. I put the analyser to wait for trigger and turned on the VT180.
The three shots are from the same run and I’ve just stepped through the time a little to show what happens at start up.
I get this with or without the keyboard. With the keyboard, all of the lights are lit. When I first started on this repair, a few, normal looking lights came on first and then, after a while, they all came on. It’s a funny old world.