Thank you, Paul, for the tuning videos. I replaced each tensioning spring with two 0.5 inch springs as suggested, then did the whole tuning thing for both axes. Last night it just worked. PHD gave Osc-index values of 0.16 to 0.27 and RMS 0.10 to 0.15 through approx. 400 subs totalling 5 hours exposure time, with round stars.
Contrast was less in the moon-bright sky, but autoguiding worked well, even though missed guiding steps due to star mass variations (twinkling) were frequent. With short (1 minute using my 100mm, f/2.0 lens) exposures due to sky brightness, I didn't need autoguiding at all. The tuned ZEQ25 is a keeper.
Thanks for the update. That sounds like what the mount is capable of producing, given that all the 'variables' are removed.
I have a collection of PHD graph screenshots from the past several months, that most people would frame. The smoothness of the mount when tracking is an indication of how little periodic error the mount really has.
It's all due to the Z Balanced design, which just puts the offset weight where the mount can easily deal with it.
I don't see any need for a mount other than the Z for AP, especially in this payload and price range.