ZX Issue 2 mod

ZX Spectrum Issue 2 boards are some of the worst Spectrum boards.

This is mainly due to the fact that

- a transistor (IORQ+A0) is added (somewhere replaced by a diode with a resistor brutally connected to +12V)

- the voltage boost converter does not have a capacitor in the circuit 

- in the colour generation circuit the colour components U and V are connected by DC

- ULA with code designation 5 has high power consumption and gets very hot

After years, the most common problem with this version is the white color changing to yellow. Almost certainly this fate awaits any Spectrum computer with an Issue 2 board.  It won't help, or will help only for a while, if you carefully adjust the trimmer settings for the level of the color-carrying U and V signals that enter the LM1889. The circuitry for adjusting these signals has been redesigned and substantially changed in later versions of the boards.

Unfortunately, there are not many solutions. Either adjust the colors each time you turn it on, or change the wiring to match later versions. Since the new wiring is more complicated, this means adding more components, which would create a "nest" in the computer case.

Since I prefer to keep computer modification to a minimal and unavoidable extent, I decided to implement the modification on a small PCB connected to main board using 3 wires only.

By removing the trimmers, we not only gain space, but also input/output points for four signals.  Y signal, +5V  and 0V needs to be added.

The prototype was built on an 18x18 mm printed circuit board. The connection points are exactly in the position of the trimmer pins. Due to the dimensions, all components are in SMD design.

Other changes concern the computer board. They are described in the following table.

/* these components can be mounted on an add-on board, then you only need to remove them from the main board

/** capacitors are connected to ground on mod PCB as in ISSUE 6, they must either be removed on the motherboard and mounted only on the mod PCB, or replaced on the motherboard, or not deal with at all

/*** only in issue3 and 4, no need to change!!!Boards 2, 3b,  5 and 6  use 10k.

/**** R78 connects pin 3/IC14 to +5V

Assembled PCB, R76 a R77 are in bad position here!)