Faulty CPC464 Twinkle Twinkle ( FIXED ).

[Audronic]

Hi All


I have an Unwell CPC464.
It passes Geralds RamTest BUT
when I remove the RamTest and power up I get Twinkling Dots (Not Characters) On the screen
It Looks like 2 sets of Twinkles ?
Photo Attached


Any help Pointers would be welcome.


Thanks.   Ray

[llopis]

Hi All


I have an Unwell CPC464.
It passes Geralds RamTest BUT
when I remove the RamTest and power up I get Twinkling Dots (Not Characters) On the screen
It Looks like 2 sets of Twinkles ?
Photo Attached


Any help Pointers would be welcome.

It looks like a bad GA or CRTC.


A few questions:
- Do you get a beep when you press backspace?
- Can you send us a picture after typing BORDER 3? (I'm curious whether you get a normal border of if it's all distorted).

[Audronic]

@@llopis


Ok I got the keyboard to work :- Border 3. (Garish)


Ray

[llopis]

@@llopis


Ok I got the keyboard to work :- Border 3. (Garish)


Ray

It's a bit weird that the initial startup text is all scrambled, but the text you type looks fine (just out of position). Also, you're in Mode 0!


My guess would be bad CRTC, but Bryce or someone else can confirm that. I actually haven't ran up against anything quite like that before.

[Audronic]

@llopis


Ok I am just re assembling the 464, I only had a keyboard  attached.
I will see after I get the screen similar to before I will test the Beep keys


Yes it Beeps after I get the screen up


Thanks


Ray

[Bryce]

If you've already eliminated the GA, then most likely one of the 74LS153 chips has died.

Bryce.

[llopis]

If you've already eliminated the GA, then most likely one of the 74LS153 chips has died.

Interesting. I hadn't thought of that. What's the clue that makes you think it's those chips? Also, is there a way to test them to figure out which one is bad?

[Bryce]

The fact that the CRTC and GA are producing a picture at all, would suggest that they are ok. Neither tend to have a partial fail, they either fail completely or work as expected. Random dots would suggest that the data making it to the CRTC is incorrect, ie: either a floating signal or the random data that the multiplexers contained before switching. Testing these is a bit complicated, but monitoring the multiplexer outputs (pins 7 and 9) can give you a clue as to whether a multiplexer is working correctly or not.

Bryce.

[Audronic]

If you've already eliminated the GA, then most likely one of the 74LS153 chips has died.
Bryce.

Thanks
I have substituted the 40010.
The fault is still the same.
No Stuck outputs on the 74LS153's
I will continue !

Progress to Date :-

IC109 ( 74LS153 ) Changed because Pin 4 and 12 were stuck HI ( Output from HD 6845 )
These lines are fed from the IC108 ( HD 6845 ) Pins 16  ( MA12 ) and 17 ( MA 13 )
Unfortunately I chose the wrong chip to cut out ( IC109 (16Pin) ) as the Fault is the same. Damn.

It looks like the HD 6845 is Unwell.

Where are those snippers ?
Thanks.   Ray

[Audronic]

HI All


@Bryce@llopis


Thanks for your help
Its working.
The 6845 Burned out a Track (Trace). Fixed.
Have replaced the Chip IC 108 (MC6845p) with a Marked Chip HD46505SP/HD4568SPThis works OK so if you purchased any with these markings they are OK

Thanks.      Ray