cpc464 voltage provided by psu?

[deadmeat]

Hi 

I am not so familiar with the Amstrad CPC and I am testing a very badly preserved 464, case was damaged by solvents and was in a very sorry state. I wasn't expecting much but when I powered it up I got the basic screen immediately. I've powered just the motherboard by jumpering 1 and 3 as the tape deck is in a bad shape and need servicing. 

As said I got the blue and yellow basic screen however when checking voltage rails and signals, I noticed that the +5V was actually measuring +4.0V, CPC was connected to my bench PSU which was delivering a stable 0.750A on a setting of 5.02v@1.5A so there does not seem to be a short and current does not seem too high. I can't do too many tests until I reassemble everything chips temperatures seems fine, CPU and the video chip are hotter than others and the yamaha is slightly hot, while all others are cold to the touch. Caps are all ceramic, with only one electrolytic that does not seem damaged ... so, is this nominal? 

Considering at the moment I have no way to load programs into the computer except typing them with the keyboard, is there any kind of diagnostic rom that will perform a more in depth check? 


BR

[Brocky]

the lower voltage on the board is normal.. usually not quite that low..
but if its working..there is no problem!

there is a diag rom you can replace the onboard rom with.. but.. if ya seeing the basic screen.. its working fine!...so the diag isnt going to tell you much...
https://github.com/issalig/amstrad-diagnostics/blob/main/build/AmstradDiagLower.rom is the one you need to replace the onboard rom (you can use the upper rom version if you have a romboard,m4 etc)

[Bryce]

Hi deadmeat,
    I'd suggest you give the power socket and power switch a good clean. The voltage drop is probably happening in one of those.

Bryce.

[eto]

Besides the things explained make sure your bench power supply really delivers the 5V at 1A. 

Or test with another 5V supply if you have one. 

I had exactly the same symptoms and it finally turned out that the bench power supply was malfunctioning and could no longer deliver 5V at 1A. 

It showed 5V and it showed the 0.9A - but there was only 4V I could measure. I tried a lot until I tried a different PSU and voila - 5V again.