CPC 6128 - white screen, black border

[mmldrm]

Hi,

Complete newbie here trying to resurrect a CPC 6128.

It seems to boot to a white screen with a black border. Digging through previous posts here, some seem to think that this means the Z80 is intact?

Suspecting the RAM, I did the "RAM bank swap trick" by removing the IC118 and replacing it with some jumper wires in the appropriate positions (triple-checked!). This made no difference - still the same white screen with black border.

At this point I'm thinking I could:

1. Replace the Z80 for around £10
2. Replace the 40010 for £35
3. I would also be tempted to replace the RAM banks, but with my soldering skills I'm concerned that I could cause more harm than good. (So that might be a last resort...)

Anyway, before I try any of those options I thought I'd see what you experts think. Help?

Thanks
M

[McArti0]

CRTC working
ROM and Z80 sometimes working.

Press DEL. Do You hear sound?

40010 Pin 32 conect to Pin1 expansion port.(sound) Do You hear sound 300Hz? if no GA not work.

74LS145 pin 15 conect to Pin1 expansion port.(sound) Do You hear sound 50Hz? if no interrupts not work.

[mmldrm]

CRTC working
ROM and Z80 sometimes working.

Press DEL. Do You hear sound?

40010 Pin 32 conect to Pin1 expansion port.(sound) Do You hear sound 300Hz? if no GA not work.

74LS145 pin 15 conect to Pin1 expansion port.(sound) Do You hear sound 50Hz? if no interrupts not work.

I knew I'd forget to detail everything. No sound on pressing DEL.

Thanks for the diagnostics suggestions.

I got the 300Hz tone nice and clear, so the 40010 seems to be good.

With the 50Hz tone, it did make a sound but I needed to keep moving the connecting wire over the expansion port pin for the sound to continue. And it sounded more like static. Not sure if that's a pass or a fail!

[McArti0]

Check this:
https://www.cpcwiki.eu/forum/general-discussion/cpc464-no-video/msg237132/#msg237132

Not sure if that's a pass or a fail!

Fail. 50Hz must be strong.

[mmldrm]

Check this:
https://www.cpcwiki.eu/forum/general-discussion/cpc464-no-video/msg237132/#msg237132

Thanks. Although I must admit that table doesn't mean much to me. Are those some other locations that I can use with the expansion pin to test for a signal/sound? I'll happily give those a try if that is correct.

Fail. 50Hz must be strong.

Damn. I wonder why moving the wire caused it to make the sound.

Interrupts not working - would that be a faulty 40010 chip?

(Thanks for your help!)

[eto]

The black border and white/grey screen show, that the CPC started to boot and initialised the CRTC but as soon as the CPU tries to access RAM it stops. Therefore you can't hear a beep as that requires that everything works.

However due to that screen we can also  expect that ROM and Z80 are almost certainly okay. Also CRTC and GateArray are most likely working. 

But this error is a typical result from a RAM fault. Probably one of the RAM ICs or one of the ICs that isolate the RAM from the bus (the mixers and latches) is faulty.

If you have a Lower ROM board, Dandanator or M4 you can try to run Amstrad diagnostics. If all RAM ICs seem to be dead, it's probably a mixer IC and if it's single ICS then the respective RAM IC or the latch is the culprit. 

[eto]

you could also try to remove the PAL IC and insert a socket where you bridge a few pins to force the 6128 to use the secondary RAM bank as it's primary bank. If it's just a malfunctioning RAM IC (with no permanent short) this can give you a working 6128 - with just 64KB of RAM of course. but then you know for sure that it's the RAM.



https://www.youtube.com/watch?v=LLVHJO-aj3Q

of course, this will only work, if it's only an IC of the primary RAM bank that is failing and nothing else. 

[McArti0]

Although I must admit that table doesn't mean much to me

Do You have a multimeter?

[McArti0]

No. Pin32 and 300Hz sound showing that GA 40010 work.
Z80 not run properly code in RAM.

ps. check sound on pin7 and pin9 all 74LS153. Another but obvious sound on all outputs. muliplexers are good

[mmldrm]

you could also try to remove the PAL IC and insert a socket where you bridge a few pins to force the 6128 to use the secondary RAM bank as it's primary bank. If it's just a malfunctioning RAM IC (with no permanent short) this can give you a working 6128 - with just 64KB of RAM of course. but then you know for sure that it's the RAM.



https://www.youtube.com/watch?v=LLVHJO-aj3Q

of course, this will only work, if it's only an IC of the primary RAM bank that is failing and nothing else.

Thanks. I tried that with a few jumper wires I had, but got the same white screen with black border. Was hoping that would narrow it down to a single faulty RAM bank, but sadly not.

[mmldrm]

Do You have a multimeter?

I do, but to be honest I've used it once about 3-4 years ago and never since. Would have to find YouTube instructions first, to set it up correctly.

But, in short, yes I do.

Edit: this is the multimeter I have, in case it helps to suggest tests.

[mmldrm]

ps. check sound on pin7 and pin9 all 74LS153. Another but obvious sound on all outputs. muliplexers are good

This was interesting. Should the output noise be noticeably different across the pins? All were loud, if varied, except pin 7 on IC105 (the bottom one of the four).

(I'll try to get the multimeter set up today so I can see actual numbers as outputs. Forgot I had it, tbh.)

[McArti0]

20V bottom-left is yours and now  https://www.cpcwiki.eu/forum/general-discussion/cpc464-no-video/msg237132/#msg237132

probes hole mid and right.

[McArti0]

Should the output noise be noticeably different across the pins? All were loud, if varied, except pin 7 on IC105

All differents like bad piano 

What about this Pin7 of IC105? died?

[mmldrm]

20V bottom-left is yours and now  https://www.cpcwiki.eu/forum/general-discussion/cpc464-no-video/msg237132/#msg237132

probes hole mid and right.

Sorry, I see the table of voltages on the GA/Z80 chips but I'm not sure what you are suggesting to try.

Not sure what you mean by 20V (literally 20 volts?) or the probes line. (My lack of technical skills causing trouble here I think!)

Happy to test anything with the multimeter, but imagine you are instructing a guy who sees it as two pointy sticks. (I don't want to fry a chip by pushing current across the wrong pins.)

[McArti0]

imagine

[mmldrm]

Thanks. Where is best to ground on the motherboard?

[McArti0]

watch on image

[mmldrm]

watch on image

Oh man... 🤦�♂️ Can't believe I missed that. I wasn't even being lazy. Did lots of searching for 'best ground on cpc motherboard, etc.

Dziękuję. I will try those tonight/tomorrow.

[mmldrm]

What about this Pin7 of IC105? died?

So, I tried Pin7 of IC105 with the multimeter and measured 1v. So the pin is not dead.

Also here are the results from the 40010:

Code Select Expand

        Pin   Me       Expected
CCLK    4     1.76v    ~1.2v
RAS     34    1.01v    ~0.9v
RAMRD   29    4.93v    ~3.5v
nCPU    14    2.57v    ~2.5v
ROMEN   27    2.84v    ~2.8v
PHI     19    2.06v    ~2.1v
READY   22    1.33v    ~1v
244en   23    4.95v    ~4.5v

CCLK, RAMRD, and maybe READY all look too high.

Happy to test anything else, now that I used the multimeter and didn't blow anything up (so far).

[McArti0]

This means that the RAM is not read very often.
Now You must have 4 hands.
ROMDIS on expansion port. signal to ground  And check RAMRD voltage.

Ground is on screw at flopy drive too.

[mmldrm]

This means that the RAM is not read very often.
Now You must have 4 hands.
ROMDIS on expansion port. signal to ground  And check RAMRD voltage.

Ground is on screw at flopy drive too.

Just to confirm:

1. Connect ROMDIS to ground first
2. Measure RAMRD voltage

I'll get one of my kids to help me. 🫣😅

[McArti0]

Sorry. ROMDIS to ground is nonsens 

[mmldrm]

Sorry. ROMDIS to ground is nonsens 

Haha, no problem.   Are there more diagnostic tests I can do? Are the RAM chips a likely cause?

[McArti0]

Ok ROMDIS to 5V pin28 of ROM. check if anything on the screen changes color.

and monte carlo REFRESH Z80 pin30 to BUSREQ pin37. check if anything on the screen changes color.