ZIP CHIP on //e, //c

[johnmac@fawlty.towers.oz.au (John MacLean)]

I am experiencing the following problem (which is holding up the release
of DOS 3.3 Launcher 2.0):

On accessing the zip chip register $C05A (on a //c without a zip chip
installed), the //c appears to get hi-res colors modified.
Oranges turn green, and blues turn purple.

This softswitch is an annunciator on a //e, but disables vertical blank
interrupts on a //c.
I am suspicious that this may have something to do with it.

The code I am executing (to slow down to 1MHz) is:

	LDA   #UNLOCKZIP
	STA   $C05A
        STA   $C05A
        STA   $C05A
        STA   $C05A
	STZ   $C05A
	LDA   #LOCKZIP
	STA   $C05A

Has anyone experienced similar problems?
Can anyone suggest alternative code?
Can anyone confirm whether this problem only exists on a //c, or whether
it also exists on a //e?
Can anyone confirm whether this problem exists only when a zip chip is
NOT installed on a //c?

John MacLean.
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