If you have an Issue 1 or 2 machine that has a strange yellow or blue tinge to its video output, then don’t panic – it most likely needs a simple adjustment to cure this.
There are a few different ways of accomplishing this, depending on what equipment you have. You’ll also need to open your Spectrum, so make sure your keyboard membrane is up to scratch (or have a new one to hand…)
Method 1: Basic adjustment
Locate VR1 and VR2 on the PCB, these are skeleton potentiometers just to the left of the ULA (on the Issue 1 machine these are reversed, with VR2 the topmost of the two).
Make fine adjustments with a screwdriver (non-conductive if possible) until the yellow tinge disappears. If the output turns blue, your TV now thinks the Spectrum is outputting a SECAM signal, so reverse the adjustment bit by bit.
Method 2: Adjustment using voltage measurement
For this method, you’ll need a multimeter along with a couple of grabber probes (or a second pair of hands to hold the probes in position while you make the adjustments).
The method is as follows:
- Set the multimeter to voltage display, mV range.
- Attach the ground probe to pin 3 of the LM1889 chip, just south of the RF modulator, taking care that it isn’t shorted to any adjacent pins.
- Attach the positive probe to pin 4 of the LM1889, and adjust VR1 until the meter display shows as close to 50mV as possible.
- Now attach the positive probe to pin 2, and adjust VR2 until the meter display shows as close to -50mV as possible.
- Check the display visually to make sure it is satisfactory – if not, go back and readjust VR1, then VR2.
It goes without saying to switch off or remove power when moving meter clips between the LM1889 pins!
Method 3: Using an oscilloscope
This is by far the quickest method available, however most people won’t have access to a scope. If you do, congratulations!
Adjustments are carried out by setting up the scope attached to the composite input wire into the modulator, and adjusting VR1/VR2 until the video output looks as ‘flat’ as possible, with no noise on the signal. It’s easiest to do this at the copyright message, as the back porch voltage drop against the white signal output is much easier to trigger against.
It’s possible to achieve good results in a matter of seconds using this method.
What if I can’t achieve a good video output?
Sometimes even given the best equipment available, the colour tinge cannot be removed from the signal. In this case, the following can be tried:
- Recap the machine (this is recommended anyway, but faulty caps can cause noise on the video output)
- If you have a frequency meter or a multimeter that can do basic frequency measurement, adjust TC2 to 4.433619MHz or as close as possible (probe pin 17 of the LM1889).
- Installing a 4K7 resistor between pin 14 of the LM1889 and ground
- Replacing the LM1889 chip.