CPC 6128 locking up on start up

[MrHorn]

Hi everyone,

My 6128 is locking up on start up and getting some video corruption. I tried running it on 64k mode, but the problem persists. The unit has had an ABBA switch installed at some stage, not sure if this would affect it. Sometimes it shows the ready prompt, and other times it doesn't.
Has anyone encountered a similar problem before?

Thanks.

[Bryce]

Could be either a CPU problem or a bad ROM.

Bryce.

[MrHorn]

Could be either a CPU problem or a bad ROM.

Bryce.

Thanks Bryce.

So either the upper or lower rom 40015/40025 could be bad?
Would the CPU out of my ZX Spectrum work in it?

[Bryce]

Could be either a CPU problem or a bad ROM.

Bryce.

Thanks Bryce.

So either the upper or lower rom 40015/40025 could be bad?
Would the CPU out of my ZX Spectrum work in it?

The 40025, it contains the upper and Lower ROM. The 40015 is AMSDOS and unlikely to be the cause.

Bryce.

[MrHorn]

Ok thanks.

Can a 27256 be substituted for the 40025 23256, or do I need an adapter?

[Bryce]

The pinout is the standard EPROM pinout.

Bryce.

[MrHorn]

Cool, is there a ROM dump of the 40025 I can download anywhere and do I need to split it for the 2 roms?

Thanks.

[Bryce]

You can find all the images you need here: https://www.cpcwiki.eu/index.php/ROM_List

Bryce.

[Animalgril987]

You will need  the 2 files to match, for example: English CPC6128 OS needs to go with English CPC6128 Basic.
Make sure you burn OS file into bottom 16k of EPROM and Basic into top 16k. 

[MrHorn]

You can find all the images you need here: https://www.cpcwiki.eu/index.php/ROM_List

Bryce.

Thanks, the rom looks ok. I tried the CPU from the spectrum and get a blank greenish screen with video distortion around the border. 
Interestingly both CPUs seem to work in the spectrum, I guess it must use less functions than the amstrad.
So the CPU is partially damaged, or both CPUs are, as I get two different results when they are in the 6128.
I am starting to think this unit fell victim to one of those most common errors before I got it, over-voltage.

[MrHorn]

You will need  the 2 files to match, for example: English CPC6128 OS needs to go with English CPC6128 Basic.
Make sure you burn OS file into bottom 16k of EPROM and Basic into top 16k.

Thanks, it  worked.
Is this the case with all 32k Roms in this slot? I've noticed most ROM files seem to be 16k. Does the other 16k always need to be filled, even if it is just a duplicate of the bottom 16k?

[Bryce]

You need both of course. The lower 16k is the firmware, the upper 16k is Locomotive Basic.

Bryce.

[MrHorn]

You need both of course. The lower 16k is the firmware, the upper 16k is Locomotive Basic.

Bryce.

Thanks, any idea how the 16k diag rom works there? Just that in the upper and firmware in the lower?

[pelrun]

The diag rom is entirely selfcontained and doesn't use firmware (or RAM for the initial tests) - you put it in place of the lowerrom.

[MrHorn]

The diag rom is entirely selfcontained and doesn't use firmware (or RAM for the initial tests) - you put it in place of the lowerrom.

Thanks for clarifying.

[MrHorn]

Ordered another cpu... the ram ohms seem ok.
Does anyone know what the typical symptoms of a bad ULA and CRT.C are?

[SerErris]

None of your symptomps. If CRTC or ULA would not work (or Memory C000-FFFF) than you would not see the screen you see. The lockup would not happen after the basic initialization. 

How did you confirm RAM is working? Maybe the variable space or the jump tables are in an area of memory that does not work anymore? This is highly unlikely, because typically a chip would fail, rendering a single bit over the entire address range unstable/not working. 

[MrHorn]

None of your symptomps. If CRTC or ULA would not work (or Memory C000-FFFF) than you would not see the screen you see. The lockup would not happen after the basic initialization.

How did you confirm RAM is working? Maybe the variable space or the jump tables are in an area of memory that does not work anymore? This is highly unlikely, because typically a chip would fail, rendering a single bit over the entire address range unstable/not w

Thanks for confirming. I read on another thread here you could measure the resistance between opposing pins of the ram, they all seemed the same, so I assumed they were good. With the keyboard connected the delete key will beep when pressed.

[SerErris]

If you can, burn an EPROM with the Diagnostic ROM and let it check the basic hardware ... It will point out RAM problems.

It may also help with other things not working correctly, but I think your issue is not that easy. Could also be a cold solder joint somewhere ... 

[MrHorn]

If you can, burn an EPROM with the Diagnostic ROM and let it check the basic hardware ... It will point out RAM problems.

It may also help with other things not working correctly, but I think your issue is not that easy. Could also be a cold solder joint somewhere ...

It will just lock up with alternating green and purple striped vertical bars with the diag. I tried reflowing the entire board, and couldn't find any bad caps.
This would probably make a good episode for Noel's Retro Lab if I lived closer.
I give up on this one for now, thanks for the help everyone.

[SerErris]

You may want to join his discord and consult him. Maybe he can get you some advise. ...