Troubleshooting Astro Spectra: board swapping order ?

[Leadenwah]

I have 2 identical Astro Spectra Plus radios. One is fine the other recently came up with " FL 01/90 ".

No leaks, damaged components or misfitting boards found.
Swapped out known good W4 control head, still get FL 01/90
Swapped out known good interconnect boards with no change.
Antenna has always been connected.
Power supply trick will halt cycling display, but it returns when normal power is applied and will not allow reading of radio.

Without proper test equipment it seems that swapping boards would be the next step.
Which should I swap first?
Is there an order of priority in which I should begin trying known good circuit boards or other components ?