sinclair spectrum eprom swap gone wrong

[1024MAK]

External expansions/interfaces do their own address or I/O decoding, they don't use the control signal(s) from the ULA.

Further, to prevent the internal ROM from being accessed, they apply a logic high to the /ROMCS line. This overrides the ULA control signal to the internal ROM.

Mark

[jojo]

Mark , thanks !!

so more inversigation work to do .

[jojo]

unfortunately , i have tried 3 working ( and still working ) ula's but no change in the symptoms .

i will remove the eprom-socket and try again

it is a mess to work on ( the frankenstein spektrum with all extensions soldered to the spektrum pcb ) ,

picture taken before all repair works .

[1024MAK]

ZX Spectrum 16K/48K ROM socket

Code: Select all

Pin Function Destination
 1 N/C
 2 A12 Z80 pin 2
 3 A7 Z80 pin 37
 4 A6 Z80 pin 36
 5 A5 Z80 pin 35
 6 A4 Z80 pin 34
 7 A3 Z80 pin 33
 8 A2 Z80 pin 32
 9 A1 Z80 pin 31
10 A0 Z80 pin 30
11 D0 Z80 pin 14
12 D1 Z80 pin 15
13 D2 Z80 pin 12
14 0V/GND 
15 D3 Z80 pin 8
16 D4 Z80 pin 7
17 D5 Z80 pin 9
18 D6 Z80 pin 10
19 D7 Z80 pin 13
20 /CS ROM selection links
21 A10 Z80 pin 40
22 /OE0 /RD Z80 pin 21
23 A11 Z80 pin 1
24 A9 Z80 pin 39
25 A8 Z80 pin 38
26 A13 Z80 pin 3
27 /OE1 ROM selection links
28 +5V/VCC

The other side of the ROM selection links go to:
Z80 /MREQ pin 19
and
R33 (680 or 560 ohms) then ULA pin 34.

Note 27128 EPROM has:
Pin 1 VPP - tie to +5V
Pin 27 /PGM - tie to +5V

Note 27256 EPROM has:
Pin 1 VPP - tie to +5V
Pin 27 A14 - tie to 0V if ROM image in bottom half or tie to +5V if ROM image in top half.

Mark

[jojo]

Thank you Mark ,

i verified everything , and noticed the following ;

- resistor left side for eprom selection carries 5 volt !!
if i take out the resistor , that mounting-hole carries 5 volt

ground pin of the ( removed 7805 ) carries 5 volt ...............

so there is a lot more going on than what i saw at first glance .

strange is that the unit still powers up , and the retoleum card still works .


absolutely puzzled now .


preparing to take out the spektrum main board by unsoldering the pins to the multiface to work on the spektrum alone .

[1024MAK]

Not sure what's going on with the 7805 0V/GND Pin

Has a 0V/GND line been broken somewhere? That kind of fault sometimes can cause a 0V/GND to float up via leakage from the +5V rail.

One end (the links side) of R33 also goes to the expansion/edge-connector (25 lower, signal called /ROMCS). Expansions that have their own ROM/EPROM will pull this line high when they switch in their own ROM. Hence disabling the internal ROM.

Maybe there is a fault on one of those expansions.

Mark