The Lil Old ZX Spectrum 48k Service Manual

SECTION 3 - SETTING UP AND SYSTEM TEST

LIST OF CONTENTS

• General Alignment
• Introduction
• Voltage Check
• Colour Adjustment
• Sub-Carrier Oscillator
• 14MHz Oscillator
• System Test

GENERAL ALIGNMENT

Introduction

The Issue 3 Spectrum modification removes the need for adjustment of the colour tuning using potentiometers and trimmers. Therefore, the setting-up procedure for this board is confined to checking that the internally generated power supply voltages and two oscillator frequencies are correct. Adjustment instructions apply strictly to the Issue 2 Spectrum boards only but are generally relevant to the Issue 1 Spectrum as well.

To carry out the setting up procedure it is first necessary to remove the ZX SPECTRUM from the case (Section 2, Assembly/Disassembly). For convenience the keyboard may also be disconnected.

Voltage Check

Check that the internally generated power voltages are as follows:

Colour Adjustment

Potentiometers VR1 and VR2 are used to effectively null the voltages between pins 4 and 2 (for VR1) and pins 2 and 3 (for VR2) on IC14 (LM1889). To allow for thermal drift, the potentiometers are set for non-zero voltages; furthermore these voltage off-sets are set to satisfactory but non-optimum levels in production, and optimum values may be used to advantage in servicing. The relevant figures are given in the following table - voltages are given relative to pin 3 of LM1889.

Sub-Carrier Oscillator

The chrominance sub-carrier oscillator frequency should be 4.433619 Mhz +/- 50 Hz. This frequency may be checked/adjusted by using one of two methods, listed in (1) or (2) below.

Apply power to the power socket using a bench power supply set at 9V. Current consumption will be approximately 500mA to 700mA for the 16k Spectrum and 700mA to 900mA for the 48k Spectrum.

1. Feed the Spectrum modulator output via a co-axial cable into a standard colour TV receiver. Measure the frequency of the locked TV chroma sub-carrier.
2. Connect pin 17 of IC14 (LM1889) via a 4.7pF capacitor and a lead to the frequency meter. It is recommended that this is done using a jig made up from an IC test clip. Pin 18 of this chip must be removed to minimise stray capacitance. Connect a 10k ohm resistor between the input terminals of the frequency meter. Measure the frequency and adjust trimmer TC2 if the measured frequency is outside tolerance.

14 MHz Oscillator

This frequency is not readily set by working to a specified frequency and tolerance. On Issue 2 boards trimmer TC1 should be adjusted to minimise the effects of beat frequencies which cause waves of distortion to flow across the screen. These are particularly apparent with certain colour combinations, eg red INK or green PAPER, and the phenomenon is sometimes referred to as `dot crawl`. It is stressed that adjustment of this frequency is not straight-forward, as deviations due to temperature change are easily visible; tuning should be adjusted to minimise rather than remove this effect.

Proceed as follows:

1. Set up the Spectrum to display green PAPER with red INK
2. Type-in three or four lines of characters and monitor the screen.
3. Adjust trimmer TC1 to minimise the effects of distortion.

SYSTEM TEST

The system test for the Spectrum may be undertaken using the ZXTP taped program loaded conventionally; this requires that the keyboard is connected. The test should be carried out with the Spectrum connected to its own power supply.

The test program exercises all of the Spectrum circuitry with the exception of the SAVE function. To test this function a small, possibly one line program should be typed in, SAVED and the VERIFIED as described in the INSTRUCTION manual.