Zx spectrum 48k screen fault

[RobbieP79]

Hi guys.
I thought I'd fire up my spectrum which I bought years back as I had one when I was a kid.
Well, when I powered it up the middle of the screen is blue with white border and when i load a program on (i use playZX on my phone) the program name is at the bottom of the screen and not the top but the game will load but after a few minutes it will freeze. (I did notice this when I first powered it up when I bought it and thinking that was the reason why the seller got rid of it)

Now, I have read that this could be the Lower ram 4116 DRAM chips so would I just replace IC10 or shall I just replace all of them?

Thanks

[1024MAK]

Hello and welcome to the forum

The type of RAM chips used are 1 bit wide DRAM. The basic 16k machine has eight 1 bit by 16k DRAM chips (often referred to as the ‘lower’ RAM) to provide the 16k bytes. Each chip provides only one data bit for each address. Hence for an eight bit byte (which is what the Z80 CPU needs) there are eight chips. With all eight chips responding to each address in their 16k range and each chip providing one data bit each.

The 48k (and plus) model has a further eight DRAM chips. This time 1 bit by 32k. Combine this is 32k bytes. 16k bytes + 32k bytes gives 48k bytes.

So it’s not always easy to work out which chip is causing problems.

If the machine is accepting command line input from the keyboard, can you please enter this line of code:
PRINT PEEK 23732 + PEEK 23733 * 256
Then post up the number or message that is printed. ‘PRINT’ and ‘PEEK’ are keywords entered by pressing the appropriate key or key combination. If you try typing them in as individual letters, it won’t work. You don’t need to type the spaces.

Mark

[Ast A. Moore]

It’s a little odd that only part of the screen is cyan. I’d like to see the video of the reset RAM test. Still, I’d tentatively suspect ICs 9 and 11 (bits 3 and 5, respectively). Perhaps the ICs themselves are fine, but some of the connections are broken. I’d inspect the traces and at least reflow some solder joints. The data pins are 2 and 14, so start with those.

[RobbieP79]

i did think of cold solder on the pins so will have a look a do a reflow.
Just to be sure, you mean unplugging and re-plugging back in and watch what the screen does?

[1024MAK]

i did think of cold solder on the pins so will have a look a do a reflow.
Just to be sure, you mean unplugging and re-plugging back in and watch what the screen does?

Yes, power down, wait for five seconds, then power back up.
See the videos in this post

Mark

[Ast A. Moore]

Just to be sure, you mean unplugging and re-plugging back in and watch what the screen does?

Yes, although ideally, if your display goes into standby mode as soon as the video signal disappears, you’ll want to solder (temporarily) a reset switch. That way we’ll be certain to see the entire reset/bootup sequence. While the Spectrum’s bootup RAM test is primitive, it can still sometimes provide valuable diagnostic information to the trained eye.

[RobbieP79]

Right guys. I did what you asked. When I put the PRINT code in, I got 0 ok 0:1 assuming I put the code in right.
Second I've done about a minute of recording of unplugging and re-plugging the psu back in so whether I was doing It too quick I don't know.
Thirdly, I've posted a pic of what happens when I load a game and I did notice this Happened on another game. The image will load but then go up to where it is shown on the photo.

Hope this helps and it sheds some light.

Rob.


YouTube video of boot up sequence:
https://youtu.be/K3xIEf4nDtk

[Ast A. Moore]

Hmm. This is most unusual. In the instances when the bootup did get caught on camera, I didn’t notice anything unusual. The fact that the machine rebooted normally only every other time, I attribute to the fact that you didn’t power it down for long enough for the CPU to start executing code from address 0. If, however, after letting the machine sit unpowered for a few seconds you don’t get a stable reset, I’ll be concerned with a few possible issues.

I can potentially see multiple issues, for example, RAM or address multiplexors.

Can I ask you to swap IC6 with IC7 and retest?

[RobbieP79]

No, I couldn't leave it a couple of seconds as the screen would of shown just the copyright screen and not the full boot up and that was why I was quickly cycling the power. I will swap the IC's once I get my soldering iron out. Did you notice anything with the loading image why it would scroll up to a quarter of the picture?
I have just thought I think I can tune the speccy in the dvd/VCR combi as that doesn't make the screen go blank and then do another recording of the boot up sequence if that will help?

[Ast A. Moore]

The VCR trick sounds like a good idea. Do give it a go. Can’t comment on the loading screen yet. Too little information.

[1024MAK]

Did you notice anything with the loading image why it would scroll up to a quarter of the picture?

What do you mean? Do you mean it scrolls up the screen so loosing the top part of the image leaving ‘new’ blank space (lines) at the bottom?

There are ‘system variables’ which is an area in RAN that stores information that the BASIC ROM code uses to keep track of the current state of the machine, including the position of the edit box / input line at the bottom of the normal display area. Also the current foreground (ink) and background (paper) colours are stored in this area.

My thoughts are that either there is a fault with this area of RAM (or the circuitry / wiring that feeds and controls it) or the default values from the ROM are corrupt. Although I don’t think a ROM fault is likely (they are not common faults) it’s still worth checking.

What type is the ROM chip? Is the ROM type wire link set correctly?

Mark