cpc464 repair black screen newbie question

[cuberdon]

Hi forum ,


coming from sinclair and commodore the cpc464 is a small step aside ;-)


Now , knowing there is no voltage regulator on board the input voltage must be rather precise .

So a little more than 5 volt goes in, BUT ..................

I do measure 3,5 volt on chips the board ,

Do i wait for smoke ? or is there a known culprit which might be the number-one to look for ?

[Bryce]

Hi forum ,


coming from sinclair and commodore the cpc464 is a small step aside ;-)


Now , knowing there is no voltage regulator on board the input voltage must be rather precise .

So a little more than 5 volt goes in, BUT ..................

I do measure 3,5 volt on chips the board ,

Do i wait for smoke ? or is there a known culprit which might be the number-one to look for ?

Hi, if the rail is dropping to 3.5V then something is getting hot. Find the component that's hot and you'll have found the problem.

Bryce.

[cuberdon]

i will need to verify my powersupply 

[SerErris]

The 5V Powersupply needs to provide >1Amps better 2Amps. If it cannot provide 1Amp it will probably drop the voltage to much for the CPC to work, or shutdown completely. 

However as Bryce pointed out, one or more chips might be shorted and that drawin all the power the PS can deliver and then dropping the Voltage (the PS cannot keep it up).

[cuberdon]

current powersupply can do up to 5 amp , but its output is under 5 volt ( i noticed at no-load ) .

so i need to verify if i can source a better one 

[pelrun]

Be careful cranking up the powersupply until you've identified what is pulling down the 5v rail. 

Although there is a oldschool technique for finding shorts where you turn up the power until the offending component explodes.

I wouldn't recommend it

[Rabs]

current powersupply can do up to 5 amp , but its output is under 5 volt ( i noticed at no-load ) .

so i need to verify if i can source a better one

Depending what you want to do after this, further repairs or small projects, I would buy a small bench power supply with current limiting capability. Think mine cost around £50. Of course you would also need to make up a power lead.

[Rabs]

This video may help Noel's Retro Amstrad CPC 6128 Black Screen Repair

[cuberdon]

as soon as all video components have arrived i will be able to do some testing .

in the mean time all info is helpful.

[cuberdon]

scart to hdmi converter has arrived , scart cable has been made , now to wait for the new powersupply and testing can begin .

[cuberdon]

So i switched on the 464 and nothing heats up , it is the GA which becomes warm-ish .

So time for the logic-probe , and maybe more .


All help highly appreciated .

[Bryce]

Try removing all socketed chips and see if the rail goes back up to 5V.

Bryce.

[cuberdon]

a small delay , need to partly re arrange my workspace ;

my workdesk is gone ..................

report back when i can do some testing .

[cuberdon]

slowly getting there ;





still need some time to finish the desk .

powersupply has arrived too .

[McArti0]

Your desk is badly damaged, there is no any Amstrad CPC on it.

[cuberdon]

wait .......................

[cuberdon]

like this ?? ;




poor 464 in tha background just after another zeddy

[McArti0]

we need a narrower spectrum to investigate this 

[cuberdon]

mmm , if you look very closely you could see 5 !!

those speccy's are the last in for surgery , after that the spectrum activities wil close .

Needles to say there is a cpc waiting to be repaired ( with this forums help ) 

And a commodore 128 , and a rather sad amiga 500 . 

[Bryce]

mmm , if you look very closely you could see 5 !!

those speccy's are the last in for surgery , after that the spectrum activities wil close .

Needles to say there is a cpc waiting to be repaired ( with this forums help )

And a commodore 128 , and a rather sad amiga 500 .

Well at least you can use some of the speccy parts to fix further Amstrads! And the rubber keyboards are useful as a mat to stop the CPC from slipping off the table.

Bryce.

[cuberdon]

Bryce , 

speccy's are multi purpose !!


Nowto find time to re-connect everything , and start testing the cpc.

[Bryce]

[McArti0]

Many doors are waiting for Sinclair ZX

[cuberdon]

as the zx81 is only made with 4 chips , one of them being an ULA ( expensive !! and hard to get  ) every enthusiast has a doorstopper laying around

[SerErris]

There is a replica without the ULA.  This one is called ZX81Plus38. The Plus38 stands for 1981+38=2019, the year when the original author started the project. However it did not come to live in 2019 ... 

As always the ULA is just the one chip that made the ZX81 commercially viable (cheap). However it does work completely without it.

Very interesting read on the development of the whole replica here:
https://revspace.nl/ZX81plus38_simple_to_build_ZX-81_clone

The first schematics were done in 2019 - even Rev1.3 however the first actual hardware has been done in 2020 with Rev1.4 was still problematic and required bug fixing and other changes, so that we are now at Rev1.10 at github repo.

Just if you want to build one. I do have a kit lying around here as well as a spectrum+ replica kit. I just need to find the time to solder everything in place. As now the dark nights are coming ... I will probably find some time.