ZX Spectrum 16K/48K/+ ULA colour difference pin voltages

[1024MAK]

The following is for a Nebula fitted in a ZX Spectrum 48K issue 3 board.

Voltages in the columns marked (*1) tested with an inexpensive 3 ½ digit multimeter on the 20V DC range.
Voltages in the columns marked (*2) tested with an expensive professional 3 ¾ digit auto-ranging multimeter on the DC setting.

Test program:

Code: Select all

10 FOR a=0 TO 7
15 BEEP 0.1,1
20 PAPER a: INK a: BORDER a: CLS
30 PAUSE 50
40 LET i$=INKEY$
50 IF i$=“” THEN GO TO 30
60 NEXT a
70 GO TO 10

ZX Spectrum 16K/48K/+ IC14 colour difference pin voltages

Voltages in the columns marked (*1) tested with an inexpensive 3 ½ digit multimeter on the 20V DC range.
Voltages in the columns marked (*2) tested with an expensive professional 3 ¾ digit auto-ranging multimeter on the DC setting.

IC14 voltages when a completely white picture (ink, paper and border all white) is being shown:
Pin 1, 9.676V (Chroma Lead)
Pin 3, 5.14V (Chroma Bias)
Pin 5, 0V (Ground)
Pin 6, 0.544V (not used)
Pin 7, 0.83V (not used)
Pin 8, 0.83V (not used)
Pin 9, 0.62V (not used)
Pin 10, 4.608V (not used)
Pin 11, 4.635V (not used)
Pin 12, 12.121V (Video input)
Pin 13, 4.163V (Chroma Sub-carrier, output) #1
Pin 14, 12.145V (+12V)
Pin 15, 12.145V (+12V)
Pin 16, 12.145V (+12V)
Pin 17, 8.768V (Chroma Osc Output)
Pin 18, 9.695V (Chroma Lag)

Note #1 the loading of the test equipment may affect the picture.
All voltages tested with an expensive professional 3 ¾ digit auto-ranging multimeter on the DC setting.

Mark

[hellraiseruk]

Thanks for this, it'll be very useful for those fault-finding colour related issues. If only this had been posted a few weeks earlier

JD