Sinclair ZX Spectrum 48kB

IM2FIX - flicker fixer

The ULA generates noise (snowing, flickers) on the screen when the I register is set to values range #40..#7F. (As the I is placed as MSB on address lines while each T4 refresh cycle, ULA interprets this as very frequent screen memory access.) To fix this, the A14 line connected to ULA must be ANDed with /RFSH line. (So the refresh address points safely to ROM. In case of ZX128 compatibility upgrade also protects the slow banks in section D - redirect them to fast section C.)

AND gate can be build from diode and 1kohm resistor or PNP transistor and resistor. The best way to mount the fix is to mount the transistor under Z80 (/RFSH - pin 28 - is near GND - pin 29). The A14 line must be cuted out from ULA in two places. The jumper and edge connector part of A14 must be joined with a wire to Z80 and keyboard part of A14. The ULA part of A14 must be separated from them with resistor. Then, anode of the diode or emitter of the transistor must be connected to the A14 ULA input (pin 36).

Files for download

40892019.jpg 16.3kB (2005-01-30 16:26)

32K rampack - external 32kB for ZX Spectrum 16kB.

4164-pl.txt 1.1kB (2004-12-04 16:27)

[pl] Zamiana 4116 na 4164.

2_sch.gif 508.8kB (2004-11-16 00:42)

Issue 2 schematics (good quality).

3_sch.gif 711.0kB (2004-11-16 00:46)

Issue 3 schematics (good quality).

4ab_sch.gif 888.7kB (2004-11-16 00:50)

Issue 4A/4B schematics (good quality).

3b_sch.gif 710.4kB (2004-11-16 00:47)

Issue 3B schematics (good quality).

6a_sch.gif 710.2kB (2004-11-16 00:29)

Issue 6A schematics (good quality). Bug: /RFSH is not connected to edge slot.

5_sch.gif 482.5kB (2004-11-16 00:50)

Issue 5 schematics (good quality). Bug: dot missed on /MREQ.

service.zip 342.2kB (2002-09-15 16:17)

Service manuals and partial schemes.

Links to other pages

http://members.lycos.co.uk/eightbitula/

The Lil Old Sinclair Computer Technical Information Repository. Schematics and service.

See also

4MB RAM upgrade : ZX Spectrum

ZX Spectrum memory upgrade - 4MB RAM (DRAM).