Oh yes, IC26 should be checked thoroughly. It's a common failure. I've had over 10 Beebs with this multiplexer failed in so many ways. It's a common part with no obsolescence issues, easy to check with the scope. https://www.ti.com/lit/ds/symlink/sn54ls139a-sp.pdf this is the datasheet for that IC.
The CRTC chip select is only used when accessing the registers. To test that change mode whilst observing the scope (use a single shot trigger to capture it) so try MODE4 and press RETURN.
The CRTC chip select is only used when accessing the registers. To test that change mode whilst observing the scope (use a single shot trigger to capture it) so try MODE4 and press RETURN.
Statistics: Posted by maniacminer — Mon Jan 29, 2024 12:55 am