ZX Spectrum 48k version 2 not booting

[Jenkins]

Hi

I am trying to fix a version 2 board ZX Spectrum that is not booting and would appreciate any advice on how best to progress.

Initially the failure mode on turn on was a brown border, not changing and thick black and white horizontal bars in the paper area with a few coloured squares flashing. Because the border was not changing to white, I suspected the Z80 so have removed, socketed and inserted a new one from a reputable supplier. I also replaced the ULA with a new one. After doing that I get the failure mode below.

Now on turning on, the border is white but seems to now continually reset. In addition the image now flickers every second or so with a yellow tint to the whole screen, then reverting to normal colour.

I have a diagnostic card from ByteDelight, on turn on that immediately says RAM FAIL.

Here is what is displayed with the current failure mode:
https://youtu.be/iVlFDd3I0BM

Here is the result of the diagnostic card:
https://youtu.be/RJtsDD3M71Q

Here is a picture of the board:


Before I start removing the ram chips I wanted any advice to pinpoint the error either to a particular ram chip or if the fault could be elsewhere.

For testing purposes I have a multimeter and the diagnostic card mentioned above.

I have checked voltages to the ram chips and all appear to be normal.

Many thanks in advance, appreciate any help.

[kegnkiwi]

hi mate,

There are plenty of people who will give you good advice on how to test ect but I know that I bought a load of 4116 chips from china and waited about 5 weeks for them. I socketed them and put them in but the fault remained. I have no way to tell if the chips were good. I took a punt and bought a replacement lower ram board from rertrolium and it was easier to install as it uses just the first and last sockets.

https://www.retroleum.co.uk/zx-spectrum-chips

Don't get me wrong i still have an issue somewhere but as least the screen boots into the zxspectrum 1982 screen.

[cj7hawk]

What voltages do you have going to the 4116 rams measured between Ground and pin 1,8,9 and 16?

Regards
David

[Jenkins]

hi mate,

There are plenty of people who will give you good advice on how to test ect but I know that I bought a load of 4116 chips from china and waited about 5 weeks for them. I socketed them and put them in but the fault remained. I have no way to tell if the chips were good. I took a punt and bought a replacement lower ram board from rertrolium and it was easier to install as it uses just the first and last sockets.

https://www.retroleum.co.uk/zx-spectrum-chips

Don't get me wrong i still have an issue somewhere but as least the screen boots into the zxspectrum 1982 screen.

Many thanks. I will order one of those and try that.

[Jenkins]

What voltages do you have going to the 4116 rams measured between Ground and pin 1,8,9 and 16?

Regards
David

Hi David

The voltages I am getting on the 4116s are:
Pin 1 = -4.70v
Pin 8 = +11.89v
Pin 9 = +4.97v
Pin 16 = 0v

Many thanks.

[cj7hawk]

When the voltages are all OK, then it's usually the lower 16K ram chips, with a slight possibiilty of the multiplexers for either bank. The ULA doesn't affect the computer booting in most cases.

David

[kegnkiwi]

hi mate,

There are plenty of people who will give you good advice on how to test ect but I know that I bought a load of 4116 chips from china and waited about 5 weeks for them. I socketed them and put them in but the fault remained. I have no way to tell if the chips were good. I took a punt and bought a replacement lower ram board from rertrolium and it was easier to install as it uses just the first and last sockets.

https://www.retroleum.co.uk/zx-spectrum-chips

Don't get me wrong i still have an issue somewhere but as least the screen boots into the zxspectrum 1982 screen.

Many thanks. I will order one of those and try that.

ive just remembered that it does use slightly more that the first and last slots. It uses about another 5 pins out of the whole bank. The reason I forgot was that i had already socketed all the chips and mine just plugged into the sockets. if you intend to socket all the chips its easy if you did not want to do that you can just solder it to the board. It just makes it harder to remove though.

[zx64]

I don't have a diagnostic card, but isn't it supposed to say which of the RAM bits is failing?

[Jenkins]

When the voltages are all OK, then it's usually the lower 16K ram chips, with a slight possibiilty of the multiplexers for either bank. The ULA doesn't affect the computer booting in most cases.

David

Many thanks David. I'll give the lower ram chips a go and see what the result is and report back.

[Jenkins]

Many thanks. I will order one of those and try that.

ive just remembered that it does use slightly more that the first and last slots. It uses about another 5 pins out of the whole bank. The reason I forgot was that i had already socketed all the chips and mine just plugged into the sockets. if you intend to socket all the chips its easy if you did not want to do that you can just solder it to the board. It just makes it harder to remove though.

Thanks, I'll get one of those as it seems a little easier to install. I'll report back results.

[Jenkins]

I don't have a diagnostic card, but isn't it supposed to say which of the RAM bits is failing?

I believe the card I have does, but on this spectrum it seems just to say "RAM FAIL" before isolating which chip is bad for some reason.