CPC-464 freezes on start-up with black background colour

[McArti0]

AY3 Pin21-28 buzz Loud 50Hz is const at normal work. BUT when e.g. Pin 14 join to ground, buuzz at Pin28 is noticeably quieter.

[Bryce]

Just swap the AY. They are known to fail and every other chip involved is magnitudes more reliable than it. The AY is directly connected to the Joystick port and is highly exposed to static discharge.

Bryce.

[Tezz]

Thanks all, I've bought the replacement AY now so will swap that next once it arrives and see how it goes.

[Tezz]

The replacement AY arrived this morning, the removal of the existing AY from the board went well so I've now installed a socket but sadly there's no change with the replacement AY. Still starting with a black background and no keyboard response. So guys what's the next move with the diagnosis? 

[McArti0]

AY3 Pin21-28 buzz Loud 50Hz is const at normal work. BUT when e.g. Pin 14 join to ground, buuzz at Pin28 is noticeably quieter.

[Rabs]

The replacement AY arrived this morning, the removal of the existing AY from the board went well so I've now installed a socket but sadly there's no change with the replacement AY. Still starting with a black background and no keyboard response. So guys what's the next move with the diagnosis? 

I still don't understand the black screen and white text. Further I don't understand how it gets to a Ready prompt then freezes. What is even more confusing is the changing inputs on the monitor gets you back to a blue screen yellow text.

Going back, the CPC was fine, you then got the grey screen, typical memory failure, found a faulty RAM chip, then the black screen and white text fault appeared?

If the RAM was still faulty then surely the CPC would not get to a Ready prompt?

If the ROM was faulty would it get to a Ready prompt?

The screen output looks OK, at least after changing monitor inputs, so the CRTC and GA are working?

Now I know the AY is generally the problem with keyboard input and indeed I have fixed at least two with an AY fault but the last CPC I repaired actually had a faulty 8255 PIO.

[Rabs]

Also what type of AY did you buy the AY-3-8912A does not have any internal pullups on the IO port.

[McArti0]

remove AY3, turn on CPC and by resistor 22k?? Pins 21-28 join to ground. Make PA null when PA is Input. But only whithout pullup, PA should be null at all pins?

[Tezz]

Also what type of AY did you buy the AY-3-8912A does not have any internal pullups on the IO port.

Yes, I bought the AY3-8912A.

It was 10 years ago (not long after it was bought) when the cpc died during loading. All was fully working prior.

It's an odd one now that the identified single faulty RAM IC has been replaced that it has this behaviour.

[Rabs]

Also what type of AY did you buy the AY-3-8912A does not have any internal pullups on the IO port.

Yes, I bought the AY3-8912A.

It was 10 years ago (not long after it was bought) when the cpc died during loading. All was fully working prior.

It's an odd one now that the identified single faulty RAM IC has been replaced that it has this behaviour.

See I go back to that, you replaced a RAM IC and it started this odd behavior.

[McArti0]

Where is 8912A on your photo...

https://www.cpcwiki.eu/forum/hardware-related/cpc-464-freezes-on-start-up-with-black-background-colour/msg236668/#msg236668

[Audronic]

Perhaps Replace the IC 125 "MT RAM" as these have been known to be not reliable
Good Luck

Keep Safe

Ray

[McArti0]

how reset works? IC110 pin2 to ground.

[Tezz]

Perhaps Replace the IC 125 "MT RAM" as these have been known to be not reliable
Good Luck

Keep Safe

Ray

Quote from: Tezz on 15/03/2024, 12:21:

I received some 4164 RAM this morning so I've swapped out the single 4264 for good measure with MT RAM well known reputation although it was tested.

Where is 8912A on your photo...

I've now reinstalled the original AY into the socket.

how reset works? IC110 pin2 to ground.

It works, IC110 pin2 to ground does reset the CPC.

Thanks.

[McArti0]

remove AY3, turn on CPC and by resistor 22k?? Pins 21-28 join to ground. Make PA null when PA is Input.

[Tezz]

remove AY3, turn on CPC and by resistor 22k?? Pins 21-28 join to ground. Make PA null when PA is Input.

Thanks, I'll try that in the morning. Could you explain the expected result of the test?

Thanks again.

[McArti0]

print chars on screen.

and you can test wired LK1 and run CPC. 

You will see Orion on the screen instead of Amstrad.
This will confirm that PB in 8255 is working.

[Rabs]

One idea may be to replace the onboard ROM with the Diagnostics ROM. This will at least test the RAM and eliminate the ROM.

Here is a pic of the Diagnostics ROM installed in my 464.

You cannot view this attachment.

And a pic of the screen after it tests the RAM.

You cannot view this attachment.

[McArti0]

Next chip to desoldering. 8255 next etc. maybe something for an expansion port

[Bryce]

Next chip to desoldering. 8255 next etc. maybe something for an expansion port

Why? It's the most reliable chip in the entire CPC.

Bryce.

[McArti0]

I have an idea that 8255 was shot from the tape. 

[Bryce]

I have an idea that 8255 was shot from the tape.

It would be the first time that happened. It's still much more likely, that the ROM is corrupted.

Bryce.

[McArti0]

Dandanator would come in handy

[Rabs]

Dandanator would come in handy

A Dandanator within the Lower Diagnostics ROM loaded would be a good option as the Diagnostics ROM would checksum the onboard ROM as well as test the RAM.

[McArti0]

ok how to test AY3 as gate.

pin2 LS145 permanently connected to READY Exp port pin40.

Then cpc will stop at address 08A1 (0864 for 464) and 8255 PA will be as input and AY3 as output DA, input PA.

AY3 pins 21-28 has 5V voltage. if Pin 7-14 short to ground then Pin 21-28 has 0V (Pin 14 in - Pin 28 out  e.t.c.)

ps. works great!