Another faulty ZX Spectrum 48K 3B

[Olipix]

Hello guys

I've just repaired a ZX Spectrum+ thanks to you, but now I need your help on a classic ZX Spectrum 48K Issue 3B

When turned on, sometimes it hangs on white screen, or yellow borders
or sometimes it boots, but displays weird stuff at the bottom, and when i press a key it beeps or displays random instructions

I have checked the voltage on 4116 RAMs, and +5v, -5v and 12v are delivered correctly

Here is the video of its behaviour :
https://youtu.be/L4qp5CkJQjA

Can you help me diagnose the problem ?
Faulty RAM ?

Thanks guys

[Firefox]

The display seems good, without weird repeating patterns that often go with a ULA or lower RAM fault.

I'd lean towards a faulty ROM or maybe Z80...

Are any of the chips socketed?

[1024MAK]

Looks like a RAM fault to me, it could be one of the ‘upper’ 32k bit DRAM chips, or a multiplexer chip failure (these feed the address lines to the DRAM chips). It could still be a failure of one of the 4116 ‘lower’ DRAM chips (a failure mode that is not showing up on screen).

In any case, BASIC is having difficulty due to a lack of working RAM (hence the buzzing/rasping noise from the speaker).

If the ROM or the Z80 CPU were defective, I don’t think it would get to the stage of being able to write characters to the screen.

Also keep in mind that the RAM check that is performed at power on / reset is only designed to work out how much RAM is fitted. It is not designed to properly test the RAM. So some RAM faults can confuse it.

Mark