During the Fall of 2021, my Nissho PDP-11 (11/94 equivalent CPU in a PDP-11/34a BA11-K and DD11-PK chassis set-up) was humming along unmolested when it suddenly stopped communicating over Ethernet.

Installed in that machine was a Digital DELUA M7521 Network Interface board.  And it decided to quit working. 

Ethernet networking is important to me to be able to transfer large amounts of data from my simh PDP-11 2.12BSD (My versioning of a completely refactored 2.11BSD with many extensions) to the real PDP-11 machine for boot testing and performance.  I also use that machine for imaging RL02 drive packs and utilizing SMD disks (CDC Sabre SMD, Mitsubishi SMD drives and a CDC RSD 80 megabyte cartridge drive).

There's really no finding DELUA boards easily, or affordably, any more.

My diagnosis, and resulting easy repair, follows.

The first thing that I noticed was that the power light was not illuminated on the transceiver bulkhead panel for the cable connection to the DELUA board. 

The DELUA User Guide, EK-DELUA-UG-002, section 6.2.1 "Selftest" under "Troubleshooting" on page 6-1, states the following about indicator light at the bulkhead:

"If the DELUA fails its external loopback test, check the LED on the UNA bulkhead assembly that monitors the -15 V supply to the transceiver.  There is also a circuit break on the UNA bulkhead assembly for the -15 V power to the transceiver."

The fuse at the bulkhead assembly was OK.  But a VOM meter shows no voltage at the fuse.

Time to check the -15 V path coming from the backplane.  According to the chart provided on page 2-2,

the -15 V feed is at "finger" F (bottom left-most edge connector #6 when facing component side), side B (aka Side 2), Pin 2 from the right.

Here's the pin on the connector:

I followed that thick trace from pin 2, along the edge of the board, going up to the top corner where the chassis lever is, to its termination at a through-hole junction to the component side.  The component side is a through-hold fuse component which returns back to this side at the "Post-fuse connection" point identified.  This circuit continues to the pin on the berg header to the left to which the bulkhead cable attaches:

I tested the voltage at the top - 15 V connection, and the voltage was there.  The bottom connection was dead.  I had found the failure.

On the component side, this is the -15 V circuit through-hole fuse component (after replacement):

I did not know the rating of the fuse that failed, not having found a schematic for the DELUA M7521 board.  I did have, however, a failed DEQNA (Qbus Ethernet) board that had the same on-board fuse.   I removed it from the scrap DEQNA, soldered it onto the DELUA, and it is now repaired and working.  The full CZUAD diagnostics have passed.

I wish all board repairs were so easy, particularly because I do not have board extenders to diagnose faulty boards in-situ.  Logic analyzers are not usable in the tight confines of a UNIBUS chassis with its board spacing.