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Spectrum - building a 48K issue 3B from scratch
Posted: Sat Sep 18, 2021 3:33 pm
by Derekjdc
A question: I am building a 48K issue 3B from scratch, and all I am waiting on is the lower RAM chips. I couldn't wait, so I plugged the speccy in and connected the monitor, and nothing happens on the screen (as if nothing connected). Just wondering if I should be worried there is something else wrong, or is it normal that the speccy will not display anything if the lower RAM (4116 chips) are not present at all.
Re: Spectrum - building a 48K issue 3B from scratch
Posted: Sat Sep 18, 2021 4:57 pm
by Tiger
@Derekjdc - the lower RAM (IC 6 - 13) is essential for the ZX without them you'll get no picture, no BASIC - nothing...
Re: Spectrum - building a 48K issue 3B from scratch
Posted: Sat Sep 18, 2021 8:25 pm
by 1024MAK
With no 4116 or equivalent ‘lower’ RAM, when the ULA tries to read screen data, it will instead read garbage. Whatever random voltages happen to be visible to the ULA on it’s data bus pins. So the ‘paper’ area of the display will be filled with constantly random colours and pixels. This assumes that the Z80A is active (although it too will be doing nothing useful).
You should still get a stable border, which, if the Z80A, the ROM, and the PCB are okay, should be white.
If you are not getting any picture, then there is a problem with either the power supply system (no +5V or no +12V), the ULA or the video circuitry.
On Sunday, I’m going to move these recent posts in this topic to a new topic, as it’s generally better for a different fault to have it’s own topic. It saves possible confusion.
Mark