Far more probable is an Apple or NewerTech 100MHz or 117MHz 603e daughter card. If an upgrade is to be made, I suppose that a 3.5x multiplier (on a 33MHz bus) is the better to go for, and that means a 540/540c.
I have three of the latter for two PB500 series. They are all branded NUpowr (for the NuBus oriented upgrades from NewerTech). The first works well; the second had a mutilated RAM-card-to-daughter-card connector; and the third is being a reluctant bride, so far, but I shall persist.
The working PB 540c/117MHz has certainly a new zing in its gait, using OS 8.1. (The nominal limit for PPC-upgraded 68K Macs.) Adequate RAM, meaning addition of a 32MB expansion card, is almost a mandatory co-upgrade.
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I have three of the latter for two PB500 series. They are all branded NUpowr (for the NuBus oriented upgrades from NewerTech). The first works well; the second had a mutilated RAM-card-to-daughter-card connector; and the third is being a reluctant bride, so far, but I shall persist.
The working PB 540c/117MHz has certainly a new zing in its gait, using OS 8.1. (The nominal limit for PPC-upgraded 68K Macs.) Adequate RAM, meaning addition of a 32MB expansion card, is almost a mandatory co-upgrade.
de