464 video corruption.. CRTC ?

Started by retro_4_ever, 14:53, 26 October 21

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i have a strange fault with a 464, it has intermittent video corruption (ghosting of text, screen interference etc characters showing corruption) very intermittent can be fine for hours then just starts or can start from cold boot as well .. also noticed that when tape deck is played it has a similar effect, but it does it without the tape deck running as well..
also very occasional crash .. i have replaced the ram as they were faulty,, i have partially checked them and swopped them in the belief that if one is faulty it would have a different effect in a different bank .. was suspecting the CRTC but now  unsure .. tried different Z80 & ULA (40007) .. changed the rom as well , but it does it when running from rom board as well and passes the diagnostic ram test...
any ideas .. ? i was going to pull the CRTC but then realized it did it with the tape deck as well made me re consider i also believe the few times it has crashed was when i pressed the play on the tape  but this could be coincidence


Something like ghosting of text makes me think of the kind of problem a slightly faulty Gate Array could create. I wonder if your power supply has trouble getting enough current and voltages are running slightly low (which would be even worse when you run the tape deck). Are you using the monitor as power supply or are you using an external one?


Have you ruled out dc power issues?
CPC 664

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Quote from: ajcasado on 21:54, 26 October 21
Have you ruled out dc power issues?
Yes, first thing i would check.
6128 for the win!!!


Dirty power switch resulting in too much voltage drop across it?... you can bypass it for confirmation...


thanks for the replies ,, i am using a ATX power supply that provides 5v, 2amp on its stand by line and have used it on CPC's without problem, also used it yesterday on another cpc without problem  . i did replace the smoothing capacitor  but not the dc barrel (i will do this today) ..  i also have a more appropriate power supply that i will try just to be sure. i did clean the power switch contacts but i think i will bypass it to rule it out as suggested ..  unfortunately it has the tape deck with the later connector and i don't think i have one another one that works..
will update with the results .. many thanks mark.


ohh thats interesting .. i was running the dc through a cheap volt/amp meter to monitor the current draw .. this perhaps is a mistake .. when i measured the voltage at the power switch it was showing 4.55 volts with the computer active .. and when i removed the volt/amp meter it was showing 4.73v  (5.15 volts when the switch is off in both cases)  and 4.72 volts & 4.92 volts respectively at the actual power jack with & without the meter ..and so far with the meter removed all faults have gone .. oops silly me ...


Ah, as you have discovered, current meters cause a voltage drop to the load that they are connected to.

You don't get anything (much) for free, including with test equipment...

Looking forward to summer in Somerset :-)


Quote from: 1024MAK on 15:53, 27 October 21
Ah, as you have discovered, current meters cause a voltage drop to the load that they are connected to.

You don't get anything (much) for free, including with test equipment...


I got weird mode artifacting for a long time due to dirt in the DC power connector and power switch - also re-seated all socketed chips and cleaned them to guarantee a good connection and my 464's been rock solid since, even with the usifac2 connected to it.
Amstrad fan! | CPC464 + USIfAC II + Revaldinho 512k(universal cpld ver) | Administrator of Amstrad Discord : https://discord.gg/ksWvApv

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