Looking for FDC timing chip or substitute for TRS-80 model 4 Gate Array

[RingingResonance]

I'm in the processes of restoring a TRS-80 model 4 gate array and have located one of the problems causing the system to hang at startup and only show garbage on the screen. The FDC timing chips is dead and sends a hold signal to the CPU which was also bad along with the ROM chips.

I can make it work by removing that chip and jumpering that signal to ground, but I would like to eventually get the floppy drives working.

It's U76 on the board. It's clock isn't even running and I pulled the crystal out and put it in my faithful DT80 terminal to test it. It works fine.

Schematics say it's a VC2070. The Tandy part number is 8040544

Even if it's unobtainium, if I could at least find a datasheet (google has been no help) and internal circuit diagram I might be able to recreate it to some degree or emulate with a micro-controller. This makes me thing that maybe I could gather enough info from timing diagrams to do something like that.

Video series that I've started on the project:

 

[Amardeep-AC9MF]

I understand the service manual gives some data on the various custom logic parts, perhaps enough to reproduce it. Have you checked there?

 

[Kool65]

I understand the service manual gives some data on the various custom logic parts, perhaps enough to reproduce it. Have you checked there?

You are correct there is some info in the service manual, see from page 184, there are timing diagrams, etc. Here is the chip layout from the manual. At some point a replacement for these custom chips is going to be needed as they won't last forever.

 

[RingingResonance]

I wonder what internal registers it has.