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No Cursor

Symptom: Startup screen appears, but no cursor.
Target : C-64 (all models)

Possible sources of failure


Before checking anything, remove any modules from the expansion port. If the problem is now solved, your module applies such a heavy load on the /IRQ line that it drops below the trigger level. At the risk of causing you a sleepless night: Your module seems to be seriously damaged!

IRQs present?

Check pin 21 of the CIA #1 (the one next to the keyboard connector) which is the /IRQ output. You should be able to detect a periodic signal with either a logic tester (both the HI and LO LEDs lit) or with an oscilloscope (the signal should be of rectangular shape and the frequency - independant from PAL or NTSC version - 60Hz). Most likely you will not detect correct /IRQ signals. You should now swap the two CIAs, if they are socketed. If you can now see the blinking cursor (and have a defective serial bus or user port :), the CIA is damaged, buy a new one. If the CIAs are not socketed, replace CIA #1. After exchanging the broken CIA your C-64 should work again.

In the rare case that the /IRQ signal at pin 21 of the CIA #1 seems to be alright, check for it at pin 3 of the CPU, too. If you cannot find it there, check for haircracks. If - which is very rare - the /IRQ signal can be detected at the CPU, then it may be that it is damaged, replace it.

IRQ signal well-shaped?

If you happen to have an oscilloscope, check the amplitude of the /IRQ signal and also pay attention to its (hopefully rec- tangular) shape. It may be that the amplitude is too low for triggering interrupts; either the CIA or the CPU may be damaged.

Updated: May 18th, 1999
Created: January 1997

Site copyright © 1997 by Marc-Jano Knopp
This document is part of MJK's Commodore 64 & LCD Page
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