ZX Spectrum Z80 Keeps Resetting

[Adrian Graham]

> On 31 Dec 2017, at 20:09, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:
> 
> For some time now I have had a couple of ZX Spectrum machines. Neither
> works, each has a different problem. Today I got one of them out for another
> look. The problem with it is that it seems to constantly reset itself, you
> see the copyright screen briefly and then it disappears (the TV screen shows
> the usual "snow" when there is no signal). It just cycles round doing this.

What is your PSU outputting? Sounds like it’s *just* borderline to feed the 7805 on input. At least the 12v coil is OK so that’s one less thing to worry about - if it wasn’t you’d not get a copyright screen at all. Remember the PSU is centre negative, it should be putting out over 12VDC.

> My thoughts are that this must either be a bad Z80 CPU or a bad ROM. Neither
> is socketed and I am reluctant to desolder ICs unless I really have to as
> there is always a risk of damage to the board.

If either of those two are bad you’ll get static bars on the screen, they won’t cause a reset like you’re seeing. I’d be checking power first.

Cheers

—
Adrian/Witchy
Binary Dinosaurs - Celebrating Computing History from 1972 onwards