• Please review our updated Terms and Rules here

C64C fault diagnosis

B

BennehBoy

Experienced Member
Joined
Nov 2, 2021
Messages
69
I have a c64C which is showing a very similar fault to this:
u8_1.jpg

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) - my tester showed both as being faulty but the issue persists.

I suspect U8 was probably amalgamated into U18 on the C64C (the Large Sharp IC). Am I on the right track?

This is a machine I had gifted to a friend, he has been using it without issue until this occurred after a power cycle - I'm not sure if anything got hot plugged.
 
Whenever you get a garbled screen like that, first check the BASIC/KERNAL roms, then the PLA. These are more likely to be the culprit than any LS logic chip.
 
Do you have any cartridges? Some will bypass the ROMs, which will help rule those ROMs out as being bad. One thing you can try, is remove the CIA chips and SID chip (if they're socketed) just in case these are bad and messing with an address line.
 
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 SID removed.

The CIAs are not socketed, I'll desolder and swap them about.
 
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.
 
I think there must be a broken trace or dry joint, if I flex the board then the system boots... These things are sent to test us
 
No progress on this :(

Might be a sixty clone candidate when I get time, but that said, is there a c64 equivalent of amiga pcb explorer anywhere?
 
You must log in or register to reply here.
Back
Top