31 Dec
2017
31 Dec
'17
12:39 p.m.
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