Weird CPC664 Fault

[Bryce]

Pin14 is the 5V supply on the 74HCU04. The schematic is wrong. The correct pin is 8 as far as I know. So your scope measurement was on Pin14? If so measure the voltage on startup at some other point on the PCB (Z80 supply pin perhaps?) and see if the same dip can be seen. If the dip is only around the reset chip area, then something there (most likely a TTL chip) has a failed gate that shorts to GND when the input goes high or it's output is connected to a TTL chip that has an input shorted to GND.

Bryce.

[Audronic]

Hi Bryce

Thanks 

yes it was the power line ??

I have also Replaced the IC 110 74Ls132 and I have replaced the 0.1Mf cap C128 The timer cap with a 0.2 Mf cap
Thats all I had available. and Now it sort of works 
This is weird machine

If I replace the ROM-PC IC103 with a 40009 I am able to at least get a box with white screen.
But if I put the original in 40022 Intermittent Nothing.

Hmmmmmmm

Thanks

Keep safe

Ray

  

[Brocky]

id check around the buffers in/outs..
ls244 and ls373.. 

[Audronic]

Hi All

I am putting this machine on Hold for a while

to many items have been changed.
I am having a break from it

Thanks for all your support - Help

Keep Safe

Ray

[Audronic]

Hi All

Ok After Major Surgery we now have a PARTIAL working Machine (664)

The Ram Chip Bit 4 was intermittent Grrrrr

Removed the FDC chip and this is what I am up to

The photos show a correct startup screen and a faulty screen

I have checked all the lines from the CPU to wherever its connected to ?

PS:- I thing that this was Over Volt-aged at some stage

Photo Below

any help would be appreciated

Keep Safe

Ray

[Audronic]

Hi All

Ok Fixed.

IC 113 Pin13 was missing a Via connected to that pin

Now we have a screen like The photo

Thanks to ALL for various inputs

Keep Safe

Ray

[Brocky]

nice one Audronic

so i was on the right track with one of the mux's gone bad...
it seemed it had to be a mux, the rom was working fine, else you wouldnt see the text, but the memory was being scrambled due to the missing bit...which corresponds to the bad bit shown in geralds tester....

[Audronic]

Hi All


Of course the other half of the CPC664 dis not work (Disk area) Grrrr
I have Never worked in this side of the CPC664 or the CPC6128 before.
A Great learning experience.

So after looking at the schematic for some Time it turned out to be "Yes you have Guessed it"
The IC 203 a 74HCT240 Very unwell, and an other IC213 74HC161 Output Stuck HI

In the future I will look at any thing that has "HC in its Name (as above)

Now the complete Machine is working.
Thanks to all who have contributed.

PS:- I had a second CPC664 which also had a faulty Disk Drive (Area) On the Motherboard
Same fault this time Just the IC213 74HC161 Outputted Stuck HI
This is Now Working

Keep Safe


Ray