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Spectrum 128k garbled chars on screen

Posted: Sat Nov 30, 2019 9:59 pm
by gmc
Trying to repair a Speccy 128k (Z70500 Issue 3)

First issue was a broken DC power jack. Replaced and it now powers on but I get garbled characters on booting up.

Starts up and I get the white border so after reading through various threads sounds like the Z80 and ULA is fine and problem might be ram.


I've checked 5V/12V/-12V. All present and stable on the scope.
Pressing reset 9/10 times doesnt change anything. One or twice its reset with a while border and completely black in the middle.

Image

Found a post were you can disable the upper 32k by shorting a pin to 5V. That's for the 48/16k Spec. Is there a trick like this for the 128k?

Re: Spectrum 128k garbled chars on screen

Posted: Sun Dec 01, 2019 6:14 pm
by 1024MAK
gmc wrote:
Sat Nov 30, 2019 9:59 pm
Found a post were you can disable the upper 32k by shorting a pin to 5V. That's for the 48/16k Spec. Is there a trick like this for the 128k?
No :(

I recommend using a test ROM. If the Z80 is able to run code from a test ROM, that should be able to tell you more ;)
For example https://github.com/brendanalford/zx-dia ... i/Firmware

Mark

Re: Spectrum 128k garbled chars on screen

Posted: Mon Dec 02, 2019 5:36 pm
by 1024MAK
If you don’t have any suitable hardware to run the test ROM, if you have an EPROM programmer and blank (unprogrammed) EPROM or are able to program a suitable EEPROM / flash ROM, then you can put the EPROM / EEPROM with the test ROM code in the ROM socket in the ZX Spectrum 128k.

A suitable test ROM is the zx diagnostics ROM:
https://github.com/brendanalford/zx-dia ... i/Firmware

https://github.com/brendanalford/zx-dia ... s/releases

viewtopic.php?f=22&t=1064&p=15158#p15158

Note that as the test ROM (file “testrom.bin”, a plain binary file) is only 16K bytes long, I strongly suggest you either double it up (make a file comprising of one testrom.bin combined with another copy of testrom.bin making the file a total of 32K bytes long) using your PC before downloading it to your programmer, or download to 0x0000 on your programmer, then copy the block 0x0000 - 0x3FFF to 0x4000. Then you don’t have to worry about getting the test ROM image in the correct part of the EPROM chip (as it will be in both ‘half’s’). This also means you don’t have to worry about the state of the bit in the bank switching register.

Mark