As written earlier, the first tests learned that address bit 6,7 and 8 were always on and bit 9 always off on the front panel lamps. I fixed that with replacing two 7474, one 7420 and one 7430 on the M725 board. But that wasn't enough to get it back online. To see what is happening on the bus I wanted to have a
Unibus probe and ordered one from Joerg Hoppe. Since I don't have a memory module in this machine I was already emulating that with the
Unibone. This is all on an external DD11CK panel together with a serial card.
With the 11/20 connected and powered on, it immediately showed that not only the address flip flops were broken but also the bus drives for address bit 6...9. So I had to replace two 8881 bus drivers as well. Now all the bits on the bus behaved normally.
But it was still impossible to let the machine do anything. So I borrowed a board set with the idea to fix both sets. When you have the opportunity to swap boards you can narrow problems down to specific boards. So I removed the user added stuff from my boards and wrapped the backplane back to original. Great these wire lists in the documentation!
Swapping boards learned that my M820 and M725 boards were broken. On the M725 I already replaced 6 chips, but there was another 7402 defective. And on the M820 was a bad DEC8815.
M725 broken chips:
2x 7474
1x 7420
1x 7430
2x 8881
1x 7402
M820 broken chips:
1x DEC8815
Especially the chips on the M725 were broken in a strange way. Inputs were just short circuited, and outputs just blown out. Another thing that got my attention is that the broken boards were next to each other in the machine. And the broken chips were all connected to finger D. So I think that someone had the machine running in the past and dropped something ugly on the backplane when it was running...
Or is it possible to damage a machine in this way when someone just reversed the unibus cable?