Sinclair ZX80 / ZX81 / Z88 Forums

What type of interference?

If the electrolytic capacitors are life expired, replacing them will make a difference.

Mark

Rapidly moving faint black lines across the entire screen, mainly horizontal, like someone’s scratched the screen over and over again.

Are you sure you mean horizontal? If you were describing vertical lines, also know as “jail bars” then that’s normally insufficient decoupling of the +12V supply to the ‘lower’ DRAM chips.

Mark

It's like this
https://youtu.be/KVqAeTOdlKI?feature=shared

But yes looking at it there are jailbars too

That looks like an analogue fault.

I’m assuming this is via composite video and you are NOT using a modern switch mode PSU to provide the +9V.

That’s either insufficient decoupling on the +12V and/or +5V, or it’s picking up interference/noise from elsewhere.

Replace capacitors C25, C28, C34, C44, C45, C46, C50, C65 C74, C79 and C80.

To reduce (hopefully remove) the jail bars, replace C5, C6, C7 and C8 with 220nF or 470nF ceramic types (I recommend fitting two 220nF and two 470nF, so C5 220nF, C6 470nF, C7 220nF, C8 470nF).

While working on the board, visually check all the other ceramic and other capacitors, replace any that show any signs of damage.

If this was via RF/UHF, test via a composite video modification first, as it may be a modulator fault.

Mark

1024MAK wrote: ↑Sun Oct 01, 2023 10:26 pm
If this was via RF/UHF, test via a composite video modification first, as it may be a modulator fault.

Mark

It was rf, so I’ll do a comp mod and see

Comp mod done (simple, just the 100uF capacitor). The interference is still there, maybe a little less, but still there. Cap kit is on the way from Retro Revival, so will get that done soon.

Is there any benefit in doing the transistor comp mod over the capacitor? the display was a little dim with just the cap in place - does the transistor boost the brightness?

The transistor output stage in the ZX Spectrum does not provide the correct impedance or voltage levels (that’s why the capacitor is recommended) for a proper composite video signal. But it works okay on enough TVs.

Adding extra circuitry can improve matters, how much depends on which modification is used (there are various versions…).

Mark

looking at the second option here:

http://blog.retroleum.co.uk/electronics ... ideo-mods/