ZX 48K SPECTRUM VIDEO ISSUE

[Desperado]

Good evening everyone, can someone help me with this ZX 48K Issue 4B? There is a video problem when I turn it on, both with the RF antenna cable and with composite. However, when I touch the board underneath, more or less around the crystal and modulator area, the image becomes clean again. What could it be? Thanks everyone for the help!

 

[Istarian]

If merely touching the board makes a distinctly noticeable difference, you might have a problem with capacitance.

Capacitors are typically used in conjuction with a crystal oscillator to ensure steady, reliable oscillation.

Crystal oscillator - Wikipedia

en.wikipedia.org

Many modern touch screens use capacitive touch tech that detects your finger (or a stylus) touching the screen based on the way that contact affects the electric field.

 

[xp0nentz]

Good evening everyone, can someone help me with this ZX 48K Issue 4B? There is a video problem when I turn it on, both with the RF antenna cable and with composite. However, when I touch the board underneath, more or less around the crystal and modulator area, the image becomes clean again. What could it be? Thanks everyone for the help!

Hmmm, I don't know. Maybe try replacing the crystal or something around that area. That'd be my best guess.

 

[daver2]

The simplest way is to use your oscilloscope to look at various signals as you poke your fingers around.

However, first things first... You know what I am going to say...

Check ALL of the DC power rails for the correct Voltage and low noise and ripple. It looks like there is a reset signal (as I can see the start-up message). Check the clock for the correct frequency and stability. There may be a preset capacitor to adjust the crystal frequency.

After that, you need to look at the video (especially the luminance and chrominance) signals to see if they are changing.

EDIT: Also look for signs of poor soldering, flux residue that should not be present etc.

You are now into analogue circuitry, so have you picked up any theory as to how all of these components (resistors, capacitors and transistors) work?

Dave

 

[Zhang]

Might be a stretch, but when I was building a russian ZX Spectrum clone machine last year, I was having a syncronisation issue with the video. When I touched the crystal outer shell, the video turned normal. I ended up soldering a wire from the crystal shell to the gruond pin. This may be worth trying if other methods don't work.

 

[cj7hawk]

Are you able to see the noise on an oscilloscope?

You might be able to work backwards from the output to the source...

It also might be the display or an impedance matching issue causing noise.