Another Noob, another 48k issue

[Jeepface]

Hi there,

I've just inherited my uncles 48k zx spectrum and zx81. I had both these models in the 80's but my most fond memories are of the Spectrum.

I was hoping to get into some retro gaming but I can't get the spectrum to register on the TV.

The TV is an old 14" portable that has no trouble picking up the zx81 using the same lead. The spectrum is making audible key clicks so assume is working.

Is this problem likely to be one of a thousand things or is this a common problem?

I've tried googling it but can only find issues where they're hooking up to an modern TV.

Thank you for listening,

Bod

[1024MAK]

Hello Bod and welcome to the forum

With the TV tuned in to broadcast channel 36 (same as the ZX81), does the noise displayed by the TV change when powering up or down the ZX Spectrum?

If yes, it could be that the internal DC/DC converter that generates a regulated +12V supply for the video circuitry has developed a fault. If this appears to be the case, don’t leave the ZX Spectrum powered on for more than ten seconds at a time. Because the -5V supply to some of the RAM chips may also no longer be working, and with this supply missing, it is possible that further damage may occur.

Can you also check to see if it has been composite modified.
The easiest way, is to connect what appears to be the UHF/TV output to the composite input on a TV (normally the yellow coloured photo socket) and switch to the appropriate AV input selection. Power up and see if you get a picture.

Do you have a multimeter? If yes, you can test the +5V, -5V and +12V supplies on the edge-connector. The black probe goes to the 0V ‘pin’. Be careful not to slip and short anything out. And do not bother trying to test the -12V (as this is not a DC supply).

Mark

[Jeepface]

Thank you so much Mark, it had indeed been modified, but unfortunately between me writing the last post and the testing with a yellow lead the key clicks have stopped and I'm not just getting a black square that flashes quickly with a yellow screen with black dots.

Have I fried it?

Bod

[1024MAK]

You have a white border, which is good. That means that the Z80 CPU and the ROM are mostly likely okay.
The bad news is that it is likely that you have a RAM fault, or the internal DC/DC converter has been damaged (which supplies some of the RAM chips).

Mark