C128 Repair

From: Pasi Ojala (albert_at_cs.tut.fi)
Date: 2006-11-06 09:38:35

Hi all,

I've been working on a dead C128, and all socketed chips seem
to be working. I'm going to try logic analyzer next, but wanted
to ask if anyone can suggest something from the symptoms.

- Black screen
- Action replay inserted -> black screen
- CIA's, SID, VIC, VDC, MMU, ROM's do not cause failure in another C128
- Z80 seems to work, as the 128/64 mode goes low when C=
  pressed during boot and stays high when not.
- When measured with a multimeter /NMI is low,
  LORAM (P0) is low, P4-P5 may remain in input state.
  (Does CIA2 even cause NMI's in normal boot operation?
   Is this because the C128 autoboot checks for a drive?)

To me this seems to point to either 8502, PLA, or DRAM
(maybe less, because AR doesn't change things).
Probing the 8502 address and data lines should reveal more.

Any thoughts?

"As well try to understand the sun, Perrin. It simply is,
 and it is not to be understood. You cannot live without it,
 but it exacts a price. So with women."
	-- Gaul in The Wheel of Time:"The Shadow Rising"

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