Desoldered CIA 2 on the hunch that it was killed by a hotplug, and the system started into basic at first power on post desolder.
Cycled the power and it's back to that screen... scratches head.
Same issue with diag test cartridge initially, but then some garbled cycles and eventually a cyan border and just white background. I can make out during the cycling that some of the screen patterns match what I'd expect from diags so some code is being executed at least.
Exact same issue with...
I have a c64C which is showing a very similar fault to this:
The pictorial fault guide suggested that this fault could be caused by a faulty 7406 (U8) or 2 other 74LS14s - because I have the parts & wishfully thinking these may be relabelled on the 64c I replaced U23 (74LS14) & U22 (7406) -...
6526 is available as brand new part still -> https://www.mouser.co.uk/ProductDetail/Western-Design-Center-WDC/W65C22N6TPG-14?qs=opBjA1TV901DbgJ9rfcBoQ%3D%3D&countrycode=GB¤cycode=GBP
You could work outward from UC1, there's a data in pin where a serial stream comes in from the read head, it's get's differentially amplified by the 2 592's and then more so by the 311. UC1 then essentially deserialises the data stream and pops it out 8 bits at a time to UC2.
I'm not sure...
Got to be worth trying to replace the trimmer pot or the newtronics control board first - there are a lot of drives out there with dead heads so I would've thought it may not be too hard to obtain. I don't know the pot value I'm afraid. I had an alps with a very slower spindle motor, but it...