So it’s not a faulty Z80 CPU then.
I presume these are known to work in a ZX81, so it’s not a ROM chip fault then.
With a white screen, I think this unlikely anyway.
I would not worry about this. On the ZX80 the video signal comes from two 74 series chips via resistors, so you will get a different average DC voltage.
Going on what you say so far, I’m still suspicious of poor socket contacts.
The other possibility is that one of the 2114 SRAM chips has developed a problem. First off, try reseating them in their sockets.