C64 250407 > This one wont be easy I suspect.

[Vipersan]

Ok..
powers up ok ...some corrupt text on screen...no cursor....
Test harness hooked up.
powers up U1 does not flag bad but U10 does.
I swapped out U1 first just in case..
no different.
SID shows all voices ...but no white noise ?
I removed SID for now whilst I'm working on it.
Second image is confusing.
..with U2 removed.
I suspect another C64 with trace damage ?
certainly one I haven't seen before.
comments welcomed

 

[ClassicHasClass]

The screen matrix looks like some bits are stuck, which is why you get DAAG instead of DEAD. Bad RAM, perhaps?

 

[Vipersan]

Looks like we have another winner...
Checked all ram in my retro chip tester pro.
All tested ok....so moved onto the logic chips...and the first suspects had to be the MOS logic chips.
this C64 had an 8712..which checked out fine as a 74LS08.
and a 7709 which read bad..
swapped out with a 74LS258 U14....and normal service was resumed.
Even SID got his white noise voice back.

 

[ClassicHasClass]

Excellent!