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avatar_Rabs

Black Screen 664

Started by Rabs, 21:23, 20 December 22

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Rabs

I may need some help getting this 664 back on it's feet. I was lured in by the colour of the keys.

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But inside is a different story.

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And yes, this ROM is dead.

But the Z80 and Gate Array are alive, tested in a 6128.

So at the moment I have bus activity M1, CLK, MREQ, Address etc but a black screen.

I have nothing on VSYNC of the CRTC. In fact nothing much on any pin. CLK is ok.

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So I am thinking dead CRTC and maybe ROM.

But look at the PÇB. Does not look good.

Any advice greatly appreciated.

WacKEDmaN

start with cleaning the board down!... give it a good wash even if ya have to wash it under a tap!.. just make sure its perfectly dry before booting it back up!..maybe even use some IPA to displace the water under chips etc...

next try the rom...try the one from ya 6128... 
...see if you get anything on the data and address lines at the z80....
check the V and H sync coming from CRTC... (check it going into gate array aswell)

CLK working suggests that the gate array is still ok...but it could also be causing black screen (as the RGB signals are generated from it)

if you have a eeprom burner.. you may want to burn the Amstrad Diagnostics and drop it in the rom socket and boot from it

Rabs

Quote from: WacKEDmaN on 22:40, 20 December 22start with cleaning the board down!... give it a good wash even if ya have to wash it under a tap!.. just make sure its perfectly dry before booting it back up!..maybe even use some IPA to displace the water under chips etc...

next try the rom...try the one from ya 6128...
...see if you get anything on the data and address lines at the z80....
check the V and H sync coming from CRTC... (check it going into gate array aswell)

CLK working suggests that the gate array is still ok...but it could also be causing black screen (as the RGB signals are generated from it)

if you have a eeprom burner.. you may want to burn the Amstrad Diagnostics and drop it in the rom socket and boot from it
Thanks, board cleaned with ÌPA. Gate Array works in 6128 so should be ok. 

But will the CRTC Still generate VSYNC even if the ŔÒM is dead?

Will desolder the ROM, socket and try the lower diagnostics ROM first. I guess given the state of the board that other things will be wrong but encouraged by the fact that  the z80 and Gate Array work.

WacKEDmaN

dont quote me...but i dont think CRTC works right without the ROM working as it setups up the required registers for the CRTC that are stored in the ROM... maybe @Bryce can confirm?..

try the lower rom diagnostics..if you dont get any display id look at replacing the CRTC...

Bryce

Hi Rabs,
    yes, to everything WacKEDmaN just said and yes, the CRTC will only setup properly if you have a working ROM and CPU.

Bryce.

WacKEDmaN

Quote from: Bryce on 08:18, 21 December 22Hi Rabs,
    yes, to everything WacKEDmaN just said and yes, the CRTC will only setup properly if you have a working ROM and CPU.

Bryce.
thanks Bryce....
yay i got something right for once! LOL

Rabs

Quote from: Bryce on 08:18, 21 December 22Hi Rabs,
    yes, to everything WacKEDmaN just said and yes, the CRTC will only setup properly if you have a working ROM and CPU.

Bryce.
Thanks for that.

I am a bit concerned about the oxidisation (or what looks like) under the solder mask and the rust on some of the pins but guess there is nothing I can do about that. I will trace some tracks and check for continuity.

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Bryce

Yes, that's a little worrying. You need to remove some of the solder mask and see how bad it is.

Bryce.

eto

was that 664 found in a swamp?


Rabs

Quote from: eto on 14:31, 21 December 22was that 664 found in a swamp?


You would think so looking at the pcb but it is alive. Drawing about 0.8A for the pcb alone, so no shorts but probably not everything is running. The fact that the z80 and Gate Array have survived whatever happened is a good sign.

Rabs

Quote from: Bryce on 14:11, 21 December 22Yes, that's a little worrying. You need to remove some of the solder mask and see how bad it is.

Bryce.
Thanks I think it may be OK. Removed a bit of ground plane and it looks to be mainly in the mask. The copper underneath looks clean.
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Rabs

So ROM out and socketed but stuck for now as I dont have any 27C256 compatible EPROMS only 28C256 EEPROMS.

Tried to read the original rom using my TL866-II, selecting a 27C256 device and un-ticking Check ID, which I guess should work? But pin detect error.

Rabs

Some progress while I wait for an EPROM.

I found a couple of broken data tracks around the CRTC and put a temp fix in place.

I connected an external ROM card with the Lower Diagrom and now I hear one high beep then after a pause 3 low beeps. 

What does this mean?

Still no activity on the CRTC sync pins.

WacKEDmaN

the first beep is start of ram test.. the 3 beeps is bad ram.. 
you should be getting some display..i guess the CRTC is toast if its not putting out syncs...

Rabs

Quote from: WacKEDmaN on 03:03, 24 December 22the first beep is start of ram test.. the 3 beeps is bad ram..
you should be getting some display..i guess the CRTC is toast if its not putting out syncs...
Thanks, guessed as much. But think this also means the ROM code is running, which means that most of the tracks are ok and because I can hear the beep, the PIO and AY are OK, which is good news. Also as you indicated, if the ROM code is running and setting up the CRTC, then with no pin activity it must be dead. I will double check ÇS etc but think it needs replacing.

Bryce

Quote from: WacKEDmaN on 03:03, 24 December 22the first beep is start of ram test.. the 3 beeps is bad ram..
you should be getting some display..i guess the CRTC is toast if its not putting out syncs...

It's more likely that there are more broken traces, rather than broken IC's. I wouldn't start changing any IC's until I'd done an awful lot of continuity checking.

Bryce.

Rabs

Quote from: Bryce on 12:19, 24 December 22
Quote from: WacKEDmaN on 03:03, 24 December 22the first beep is start of ram test.. the 3 beeps is bad ram..
you should be getting some display..i guess the CRTC is toast if its not putting out syncs...

It's more likely that there are more broken traces, rather than broken IC's. I wouldn't start changing any IC's until I'd done an awful lot of continuity checking.

Bryce.
Thanks, I will double check and scope all the inputs. I can't see any breaks on the traces I found but don't know what damage has been done under the resist. Are the vias a potential breakdown point?

Bryce

Vias are no more or less likely to have an issue, but unfortunately traces UNDER IC's are more likely to have issues because they have the potential to trap moisture and dry slower than exposed areas.

How do I know? Because I was mad enough to do this: https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/hardcore-floppy-drive-restoration/

Bryce.

Rabs

Quote from: Bryce on 15:03, 24 December 22Vias are no more or less likely to have an issue, but unfortunately traces UNDER IC's are more likely to have issues because they have the potential to trap moisture and dry slower than exposed areas.

How do I know? Because I was mad enough to do this: https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/hardcore-floppy-drive-restoration/

Bryce.
I am impressed.

Rabs

Some progress, CRTC replaced, traces fixed and now I have a display. Looks like RAM passes but DIAG jumps to ROM test, not sure why. Anyway will attach a keyboard next.

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SerErris

Diag always runs a ROM test on the startup. ..

For obvious reasons if you have the DIAG rom in there, there will be no system rom and no basic ROM. 

To be able to test both, you would need to put the DIAG rom in an external ROM module (as an upper ROM or as a lower rom replacement with disablement to test the normal OS and Basic ROMs.
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.

Rabs

So on with keyboard, stripped, cleaned and membrane replaced with a new one. But some keys still do not work.

I think the problem is the contact pads because I am seeing a higher resistance across the bad keys, about 1.5k, but only 300 for a good key.

I have cleaned the pads with isoprop and some have recovered but struggling with the rest. Any suggestions?

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Rabs

Forgot to say, testing the keyboard in a known working 664, so either the keys or the membrane and the membrane is new.

Rabs

Resolved, I took a very very sharp scalpel and very very carefully scrapped the top of the contact pads. All keys are now working, some a bit sticky so may have to go over them again but maybe later, need to get back on with the pcb and connect the keyboard to see if it works.

GUNHED

Looking at the first pictures...  :o :o :o
I will never understand how people can treat computers so mean, and especially such a blue keyed beauty! Persons doing that should be put in jail for sure!  :-[

But it's great to see that you got this 664 up and running again!  :) :) :) :) :) :) :) :) :) :) :) :)
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