ZX Spectrum internals (Was: ZX81 kits) - test-drb@ccmp.vtda.org

1 Oct 2001

On Oct 1, 21:31, Tony Duell wrote:

...
   I've been
thinking about SZ Spectrums all the time.
 Oh well, I might just ask anyway, since I've opened it up...
 What is that chip next to the Z80? It's labelled PCF1306P. 
 If it's another 40 pin chip, then that's the custom ULA chip that
 includes most of the glue logic. 
If it's a 28-pin chip it's the ROM, a 23128, IC5.  The ULA is the other
40-pin chip, IC1, near the modulator.  It may have different numbers
depending on the revision.  There's more than one ROM version as well.

...
  > And why is there a switch glued on top of the Z80
with wires going to the
...
  > character ROM? Since it's a localised
character ROM piggybacked on top of=
...
    the
 one soldered to the motherboard, could it be for switching charsets? 
 Does the Spectrum really have a separate character generator ROM. Doesn't
 sound right to me... 
It's not a character generator, it's just code.  The screen memory is part
of the main DRAM and accessed directly by the ULA to drive the video.  I
imagine the switch is to select different code versions.

The thing I like most about the Spectrum service manual (apart from the
inclusion of a proper schematic and other diagrams) is the fault-finding
list.  Most such lists begin with mundane things like checking the fuse.
 The Spectrum begins with:

"TV appears dead.  Smoke appears"  and describes additional symptoms as
"TR4 shorted.  TR4 blows again".

-- 
Pete						Peter Turnbull
						Network Manager
						University of York