ZX Spectrum Z80 Keeps Resetting

[Jon Elson]

On 12/31/2017 02:09 PM, Rob Jarratt via cctalk 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.
>
>   
>
> I put a logic analyser on it as well as a scope. The CPU Reset pin looks
> fine, it goes high and stays high.
>
>   
>
> The logic analyser shows that it is happily reading code from the ROM during
> the boot sequence and it will suddenly start fetching instructions from
> address 0 again, this appears to be somewhat random. I don't have a lot of
> experience with logic analysers, and I have found that some pod/probe
> combinations cause the machine not to work at all, so the logic analyser is
> not, unfortunately, above suspicion. However the behaviour I see appears
> consistent with what the logic analyser is telling me.
>
>   
>
> 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.
>
>   
>
> Has anyone seen a similar problem before? Could this indeed be the CPU or
> the ROM, or could there be a different cause?
>
>
It COULD be, but then it has to do a LOT of things right to 
get the copyright message on the screen.
I'd be more likely to suspect a bad location or region of 
the ROM than the CPU.  Or, it could be noisy power
rails (maybe bad capacitors degrading) or some other 
component in the address or data paths that
is no longer holding good logic levels or speeds or 
something like that.

Jon