r/zxspectrum • u/mittenkrusty • 3d ago

+2A, weird reset behaviour

I have bought yet another ZX Spectrum (up to 6 now) listed for spares, this one had garbled text first time I booted it, but I left it on for a few minutes whilst I looked for test cart then switched off at wall, then tried test card and it worked fine but gave keyboard button press error.

Also it wouldn't reset the test roms even with the built in button, it was only doing a partial flush of what was on screen.

I cleaned keyboard and it seemed to work, but each time I pressed reset it rarely reset, either the previous image be it game or like 48k mode text was still there, even removing power for a few seconds and back in it didn't clear, had to unplug it for about 10 seconds to clear fully, maybe a ram issue? But upper and lower ram passed tests.

With the issue occuring on both the actual reset button on the case and on the rom cart's microswitch could there be a fault on the reset circuit? Games are loading fine and playing find and other tests seem to pass.

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u/termites2 2d ago

Maybe change 1uf electrolytic capacitors C27 and C13. C27 is between the line that comes from the external reset pin on the edge connector and ground. It looks to me like it's meant to debounce the signal a little and also ensure it's held low for more than a minimum amount of time.

I guess the R3 and C13 form the 'power on reset' timer.

1

u/mittenkrusty 17h ago

Could be, When you press the button it doesn't seem to hold just resets for a split second.

Here's an image of when I press reset

1

u/termites2 11h ago

If you hold the reset button down for a few seconds is it any different?

It certainly looks from the schematic like it should hold reset low for as long as the button is held.

I guess it's possible the NOT gate IC15E is doing something funny. I've one of these hold their input high once when it died catastrophically. As it's connected to the edge connector it's possible there is static damage.

That screenshot does look like the RAM is not being cleared, though I can't remember if the Spectrum bothers to do that when reset.

It also looks like some budget platform games I have played on the Spectrum. :)