Amstrad CPC 6128 start-ups with with weird screen colours

Started by J2112Z, 11:59, 12 January 23

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J2112Z

Hello,

New member, and my first post.

I was wondering if anyone could help me? I've just purchased an Amstrad
CPC 6128 from FleaBay as "untested" (I know, glutton for punishment :)
).

When I switch on (with an new PSU, tested for correct voltage &
polarity), the start-up screen, instead of being the standard yellow
text on blue, is yellow on purple / pink! (see attached photo).

My SCART adapter cable is new, from Retro Computer Shack, & I've tried
everything out on two TVs, with the same result.

Bizarrely, if I run a BASIC program to change the BORDER colour every
few seconds & print the colour number, when I check them against the
colour list Amstrad thoughtfully stuck on the case, all the colours
are present & correct! If I change colours on the backgound from say,
purple / pink to blue, it all works! The 'glitch' only occurs at
switch-on.

I've not got any software to try out. I was intending to replace the
disc drive with a Gotek, at some point. As of writing, I've not opened
the case or performed any refurbishment / cleaning on it.

Is this 'colour glitch' something I should be concerned about (RAM
failing / video chip failing)?

Thanks in advance.

J2112Z

OOPS! :doh:  Very sorry. Just realised I've posted this in the wrong section! Canit me moved to the CPC Hardware section, please? 

eto

If you can switch the background colour back to blue and it stays like this, it's really unlikely that there is a fault. I am also not aware of any mod that has that effect except for a manipulated ROM. 

Can you open the computer and check if they have replaced the ROM?

abalore

It looks like a corrupt firmware. A single bit change in the ROM can produce this. If everything else works fine, the hardware is ok.

J2112Z

OK. Thanks for the swift responses, people. Much appreciated!  :)

Thought I'd posted in the wrong section, so it looks like haven't, after all? 

Can someone tell me how to replace the BORDER command I use to test the colours with the one to change the background colour to blue (PAPER? PEN? INK?) Sorry, I'm not familiar with the CPC dialect of BASIC (had / used PCW8256, ZX81, Speccys & BBC Micros back in the day).

Hopefully over the next day or two, I'll crack it open and check the ROM, and report back. I'll need to give it a clean with contact cleaner & isopropyl alcohol anyway. Bizarre, if someone's changed the ROM. Why would someone do that? If it's corrupt, then I hope it can be replaced easily, or not deteriorate any further?

Also, the '+ ;' key doesn't work, so I'm hoping it's just 'muck' under the key, or needs a new spring, but if it's the membrane then that's not a dealbreaker as I understand replacements are readily available?

I've only spent a total of one hour so far on it, but apart from that one key and the 'pink' screen, and the disc drive, it appears to work fine. The SOUND command works. The disc drive, I'm going to replace with a Gotek, so I'm not worried about that. If it turns out that all the drive requires is a clean & a replacement drive belt, then I may use it on my PCW8256, as I still have discs for that (if they still work after all these years).


TotO

"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

eto

Quote from: J2112Z on 14:19, 12 January 23I hope it can be replaced easily, or not deteriorate any further?
If you can desolder a chip and burn an Eprom, it's easy. 

Quote from: J2112Z on 14:19, 12 January 23Also, the '+ ;' key doesn't work, so I'm hoping it's just 'muck' under the key, or needs a new spring, but if it's the membrane then that's not a dealbreaker as I understand replacements are readily available?

The 6128 membrane is pretty robust and unless there is physical damage, it's most likely just dirty. Clean it softly and check again. 

For the drive it's most likely the belt.

TotO

If it is confirmed, I can take a look to see if I have the original IC in stock.
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

SerErris

You can programm an EPROM .. but you actually should socket it anyhow :-)
Proud owner of 2 Schneider CPC 464, 1 Schneider CPC 6128, GT65 and lots of books
Still learning all the details on how things work.

J2112Z

Hello.

Thanks again for the advice, people. It's all appreciated.  :)

I hope to crack the 6128 open this weekend to examine the ROM, give it a clean & run a 'soak test' with the background set to blue to see if it stays that way.

I don't have an EPROM blower, I'm afraid, so if the worst comes to the worst, I'll have to replace the ROM with a known good one (& fit a socket, or course). Having said that, my soldering skills are woeful, to put it politely.  ::)


SerErris

Then you probably want to find someone near you, that can help with the desoldering. It is not just the chip that can get damaged, more probably you will damage the mainboard and pull of traces of the board.

Desoldering a chip requires some practice and at best some tools to be successful and not damaging the pcb.

Depending on where you live I could provide a Eprom for you that contains the 6128ROM+BASIC. 
Proud owner of 2 Schneider CPC 464, 1 Schneider CPC 6128, GT65 and lots of books
Still learning all the details on how things work.

J2112Z

Hello again,

I've opened it up. Not as bad as I feared. Wasn't stored in a barn, by the looks of it.  ;D

I've attached a photo of the ROMs. They look OK to me but, I'm no expert. Couldn't see anything else obvious, like leaking or blown capacitors etc.

After a quick glance at the manual re. colours, what I've have now noticed is this:

After switch-on, if I enter: INK 0,0 the background turns bright blue (colour 2?). Shouldn't it be black?

INK 1,0 turns the text colour black. I assume that's correct?

Now check out the following:

INK 0,0,26 flashes the background colours between bright blue & bright white.

INK 0,1,26 flashes the background colours between magenta (I think) & bright white.

INK 0,2,26 flashes the background colours between bright blue & bright white.

INK 0,3,26 flashes the background colours between mauve (I think?) & bright white.

??? ??? ???

Compare this with:

INK 1,0,26 flashes the background colours between black & bright white.

This surely can't be right ???

pelrun

The GA uses hardware palette numbers, so you want to use the table here: https://www.cpcwiki.eu/index.php/CPC_Palette

It definitely looks like bit 0 of the ink 0 register in the GA is bad (stuck high). Blue is 44h and purple is 45h; black is 54h, and bright blue is 55h. Colours that already have bit 0 set are unaffected.

It'd be interesting to see if the other inks are affected; it looks like ink 1 isn't (text is correctly coloured) but I'd like to see what happens if you do a PEN 2 and then modify INK 2,x.

SerErris

Yep looks like a dead (partially) Gatarray. There is nothing else to do then replace it.
Proud owner of 2 Schneider CPC 464, 1 Schneider CPC 6128, GT65 and lots of books
Still learning all the details on how things work.

J2112Z

Hello. Just a quick update:

As I had nothing to lose by doing this, I gave the PCB a good clean with contact cleaner, removed the 40010 Gate Array, and cleaned the socket with contact cleaner. The pins on the chip were shiny and clean, so no sign of corrosion. Re-inserted the chip, switched on and... no change. (To be honest, I would have been very surprised if there was, but it was worth a shot.)

So, for now, I'll box it all up and wait until I can get hold of a replacement 40010.  :(

Anyway - a big THANKS to everyone for their help. Much appreciated. All the best.

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