• Please review our updated Terms and Rules here

Spectrum 48k screen of death, lower memory failure?

L

lockwood77

New Member
Joined
Jun 9, 2025
Messages
4
Hi all. I've got a 48k ZX Spectrum that boots with the image you can see here. I've done all the pre- and post-power tests including lower memory described from here: https://www.lostretrotapes.com/zx-spectrum-repair/

r/zxspectrum - 48k Screen of death, lower memory failure?

All the figures from the multimeter read fine (certainly within the tolerances mentioned in the above guides). However, the screen looks very much like the lower memory IC6 failure (first 4116 RAM chip) shown on this page: https://spectrumforeveryone.com/features/quickly-diagnose-48k-ram-failures-ninja-style/

The voltages for that chip read fine though (-4.7V pin 1, 11.8V pin 8, 4.7V pin 9), as do all the other lower memory chips (very similar figures for each).

Before I go trying to change any 4116 chips, is it likely that one could fail whilst still reading the correct voltages? Could that screen of death represent the failure of anything else?

The regulator does read over 100 as per the pre-power test detailed in the first link above, but it takes a while to get there (well over 10 seconds), perhaps that's at fault and if so, would it cause the first 4116 to look like it's failing?
 
lockwood77 said:
Before I go trying to change any 4116 chips, is it likely that one could fail whilst still reading the correct voltages? Could that screen of death represent the failure of anything else?
Click to expand...
In my experience when the RAM chips fail they usually always still have the correct voltages getting to them. The voltage tests are just to verify that the power supply section is ok, e.g no massive shorts or other major issues.

The screen could probably indicate something else, e.g. more than one faulty RAM IC, but with a lack of further equipment (Diag ROM, oscilloscope etc) IC6 is certainly a good place to start. In fact piggybacking a known good RAM chip onto IC6 - and the others in turn - to see if anything changes could also be a good idea, before you go de-soldering
 
Last edited:
As this is your first post, welcome to VCFED.

The power supply rails would normally be present - unless the voltage regulators have failed in some way OR a DRAM chip has gone 'really' faulty and shorted a power rail (this being much less probable).

The more likely fault (as mentioned above) is that the 4116 DRAM has gone faulty internally - or the logic driving the DRAM (especially refresh) has gone faulty. Although, on a Sinclair machine, this is usually a bespoke IC...

4116 DRAM devices have a high failure rate I am afraid. 4164 devices (+5V only) are much more reliable, but they are younger technology.

The piggyback method will detect perhaps 50% of the failures - it will work for an errant HIGH logic signal from a failed DRAM but not for an errant LOW logic signal. As previously mentioned, you are looking for changes in the display - not necessarily it being 'fixed'.

Dave
 
I wonder in your case if the colors and bars would point you in a specific direction. As I understand it, on the Speccy there is only one nonlinear framebuffer with a separate attribute table.
 
Hey @lockwood77 , welcome to the forum.

That shows that the ram test is occuring, and the video ram is getting written. It's most likely a single ram chip, and a ram chip failing generally causes a failure of the -5v and 12v rails, which causes more 4116 RAM chips to fail... It's a vicious circle.

There are some cheap ROM based spectrum diagnostic cartridges you can get cheap online - these will tell you which RAM chip is likely the cause.

But, since we know the ZX Spectrum ram test loads $02 into all the RAM locations...

RAM_FILL:
LD (HL),$02 ; load with 2 - red ink on black paper
DEC HL ; next lower
CP H ; have we reached ROM - $3F ?
JR NZ,RAM_FILL ; back to RAM_FILL if not.

And we see Magenta lines, which are 2 pixels thick and have the value of $03, we can reasonably assume that the Bit 0 6116 is very likely faulty and is the cause of your problem.

Replace the 6116 that supplies Bit 0, or begin your test process there.
 
Thank you all for the replies, very helpful. I did see the diagnostic boards, I think that's likely to be my best course of action now that several of you have confirmed that my voltage tests are likely to miss the majority of RAM failures (I thought if I was barking up the wrong tree with RAM, then a diagnostic board would be a waste of time - seems like it will probably show me the issue though).
 
Hi all, I got hold of a diagnostic board with a selectable few test ROMs on it. Summary: Screen display is mangled whichever I try. From what I've read of the various diagrom guides, that likely means IC3 or IC4 failure rather than 4116 chips (as it looks like a 4116 failure would allow the diagnostic to display the offending IC)?

Here's a video of the Retroleum diagnostic booting:

I can choose options from the menu, even though I can't read it - this is the Lower RAM Refresh test:

The Dr Logan test ROM boot looks like this:

And this is the Smith & Alford test booting:

Would I be right to think I should be looking at IC3 & IC4, or is something else going on?
 
It's entirely possible to have more than one fault - especially on a spectrum.

First thing I note is all the colors are wrong... You should get green or red lines on the border so I think there's a tint issue? I assume now it's a NTSC Speccy? Is that correct?

Here's the ROM info from the beeps and lines from the Retroleum page.

In case the display is unreadable (or the Spectrum is not connected to a TV) the bad bits are also indicated by a sequence of 8 beeps (from bit 7 to bit 0) A high beep is a good bit, a low beep is a bad bit. The good/bad bit status is also indicated afterwards by stripes in the border, the topmost stripe being bit 7 (green=bit OK, red=bit bad). If the lower RAM is OK, then just 2 short beeps are heard.
Click to expand...

So when I said Magenta earlier, it's also possible that it's red lines on black and I'm just seeing the wrong color I assume now, so seeing the color correct is an important aid in debugging a broken board.

Which revision Speccy is it?

A schematic is the next thing you need.

spectrumforeveryone.com

ZX Spectrum PCB Schematics and Layout - Spectrum for Everyone

Here you’ll find schematic and layout information for various models of the ZX Spectrum. 16/48K Family Issue 1 Schematics and[...]
spectrumforeveryone.com spectrumforeveryone.com

IC3/IC4 are your lower 16K RAM multiplexers.

Replace them first and be very careful to not break tracks since speccys are often hard to desolder due to cheap manufacturing. Easiest way is to get sharp small diagonal cutters and cut the pins where they meet the plastic chip carrier from the top and then remove the pins one at a time - though if you have a good desolder kit you can also just remove the chips in a single pass - but the through plating is often fragile.

Also have a look at the solder on the chips before you replace them - is it the same as other chips? Previously repaired spectrums can have faults in the PCB from bad earlier repairs that could have occurred any time in the past few decades.
 
Thanks for the info. It's most definitely a PAL Spectrum! My original photo is definitely red lines to the naked eye I would say, I think the lighting/phone camera may have reproduced it poorly.

It's an Issue 2 board. This will be my first attempt at any such desoldering/replacement, so I shall be very careful!
 
You must log in or register to reply here.
Back
Top