ZX Spectrum 48k DIsplay fault

[roadrash]

I have been trying to sort a old Sinclair ZX 48k Spectrum that when i recently turned it on has acquired a display fault. It was working fine last year so gathered its probably the badcaps. Well I have replaced every single capacitor and even a couple of transistors that (TR4 & TR5) which were a common one to cause display faults but while the display is definitely less noisy than it was, still has the same fault. When turned on the system does seem to go through initialisation and then displays this corrupt blocky screen and I can see a couple of pixels flashing on & off and that I would say is the cursor flashing which shows it is working. None of the chips are getting really hot. Any ideas anyone? Here is a picture of the screen.

[stj]

Re: ZX Spectrum 48k DIsplay fault

bad ram wouldnt surprise me.
only the 4116's get hot when bad, the 4132's dont.

what pcb version is it, and are the rams socketed?

btw, your lucky the keyboard membrane didnt snap!

[roadrash]

Re: ZX Spectrum 48k DIsplay fault

bad ram wouldnt surprise me.
only the 4116's get hot when bad, the 4132's dont.

what pcb version is it, and are the rams socketed?

btw, your lucky the keyboard membrane didnt snap!

Wow your a real busy guy STJ your answering all my posts.
The PCB is version 3. I was careful trying to take it to bits. Not always so lucky when with a Acorn electron the pins broke off the ULA socket when removing the chip.
I Kinda guessed it might be a Ram chip from reading forums. Its going to be finding which one. I looked to see if anyone else had a display like mine as a clue but nothing even close. Looking at a normal screen it looks like mine is displaying the Sinclair message at the bottom but its all distorted. The screen flashes through a few different colours while booting so I guess its working fine just the distortion on the screen is the problem. Do you know roughly which bank of Ram is likely to be faulty?

[stj]

Re: ZX Spectrum 48k DIsplay fault

can you actually read text?
i can post a rom that does diagnostics.

[roadrash]

Re: ZX Spectrum 48k DIsplay fault

can you actually read text?
i can post a rom that does diagnostics.

Thanks thats interesting and a useful tool, Dont I need a flash rom board to load the rom image into like the one Retroleum do here http://www.retroleum.co.uk/smart-card-for-zx-spectrum/

[stj]

Re: ZX Spectrum 48k DIsplay fault

i wouldnt know, i burned it into an eprom to replace the bootrom.

you can load the wav file from an audio-player or pc headphone socket.

[redwire]

Re: ZX Spectrum 48k DIsplay fault

Something I've done with older gear is to take a good DRAM IC and swap out IC's one at a time until the dud is found. Maybe this can be done with the memory IC's. I know 4116's are really unreliable if this where VRAM is vs upper bank.
http://store.retroleum.co.uk/zx-spectrum-chips
Using SRAM as a replacement: http://zx.zigg.net/LRR/