CPC 464 No video signal

[SerErris]

Z80 does not read anything from RAM and does not download instructions (M1 should often be 0V).
Also check PIN21 CPU. It should be much lower than 5V but probably won't be.

Looks really like CPU does not get anything useable .... most likely running NOPs? 

Can you check PIN 20 on the ROM ? That should be ROM_EN coming from the GateArray. If that is 5V it is always disabled. And all datalines will be 0 all the time if the CPU tries to read from ROM. 
Same for PIN 22 on the ROM. That also should be 0V. If it is 5V, ROM is disabled and will not work. 
GND on ROM is PIN 14

Working with a multimeter, I find it best to have ground connected to a stable ground (not to a pin on any chip), and then connect just the other probe to any pin you want to measure. 

[McArti0]

CPU frozen in read state ...

You would have to remove it, give power and clock and check the M1 and the others again.

[McArti0]

many signals are open collector or open drain. Sometimes you need to check if there is a low state by checking the 5V between PIN and VCC, not GND

[Cribbyrhymes]

Z80 does not read anything from RAM and does not download instructions (M1 should often be 0V).
Also check PIN21 CPU. It should be much lower than 5V but probably won't be.

Looks really like CPU does not get anything useable .... most likely running NOPs?

Can you check PIN 20 on the ROM ? That should be ROM_EN coming from the GateArray. If that is 5V it is always disabled. And all datalines will be 0 all the time if the CPU tries to read from ROM.
Same for PIN 22 on the ROM. That also should be 0V. If it is 5V, ROM is disabled and will not work.
GND on ROM is PIN 14

Working with a multimeter, I find it best to have ground connected to a stable ground (not to a pin on any chip), and then connect just the other probe to any pin you want to measure.

PIN 20 and 22 on the ROM are both reading 0V

[Cribbyrhymes]

CPU frozen in read state ...

You would have to remove it, give power and clock and check the M1 and the others again.

Ive taken out, reseated the CPU and turned it back on.

What do you mean by give clock? sorry I appreciate the help but I'm new to board diagnostics and repair

[McArti0]

You must first answer the question do you want to be a service for him from start to finish? Do you want to learn everything you need and have the satisfaction of fixing it no matter how long it takes? Or would you rather give it to someone for repair?

[Cribbyrhymes]

I definitely would like to learn and have the satisfaction of fixing it.  Sending it away for repair would be a last resort but if I have to do it I will.  I used to repair mobile phones as a second job but it was all just part swapping no board level diagnostic and repair.  Ive a growing collection of old consoles/computers and handhelds so learning this stuff and then having the skill to carry out the repair will be invaluable in the years to come.

I really do appreciate all the continued help trying to get this 464 up and running again.

[McArti0]

http://z80.info/zip/z80cpu_um.pdf    Pages 5-11 (19-25 pdf) Figure 3-7

You must to understand how z80 read instruction from memory. (for ex. why we check M1 line).

https://www.cpcwiki.eu/imgs/6/6d/Schaltplan_cpc_464.jpg

https://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/

you should buy a logic analyzer

[Rabs]

I definitely would like to learn and have the satisfaction of fixing it.  Sending it away for repair would be a last resort but if I have to do it I will.  I used to repair mobile phones as a second job but it was all just part swapping no board level diagnostic and repair.  Ive a growing collection of old consoles/computers and handhelds so learning this stuff and then having the skill to carry out the repair will be invaluable in the years to come.

I really do appreciate all the continued help trying to get this 464 up and running again.

If you have not seen Noel's Retro Lab can I suggest you watch the videos, for example Amstrad CPC 6128 with Black Screen. This is where I started and found them very helpful. Noel takes you through the basics of fault finding.

[Cribbyrhymes]

http://z80.info/zip/z80cpu_um.pdf    Pages 5-11 (19-25 pdf) Figure 3-7

You must to understand how z80 read instruction from memory. (for ex. why we check M1 line).

https://www.cpcwiki.eu/imgs/6/6d/Schaltplan_cpc_464.jpg

https://bread80.com/2021/06/03/understanding-the-amstrad-cpc-video-ram-and-gate-array-subsystem/

you should buy a logic analyzer

Thank you I've a lot of reading and studying ahead of me!

[Cribbyrhymes]

I definitely would like to learn and have the satisfaction of fixing it.  Sending it away for repair would be a last resort but if I have to do it I will.  I used to repair mobile phones as a second job but it was all just part swapping no board level diagnostic and repair.  Ive a growing collection of old consoles/computers and handhelds so learning this stuff and then having the skill to carry out the repair will be invaluable in the years to come.

I really do appreciate all the continued help trying to get this 464 up and running again.

If you have not seen Noel's Retro Lab can I suggest you watch the videos, for example Amstrad CPC 6128 with Black Screen. This is where I started and found them very helpful. Noel takes you through the basics of fault finding.

Thank you for the recommendation.  I will watch the one you linked above tonight.  Hopefully there are a few more simple things I can check to try and narrow down the issue.  Looks like I might need a logic analyzer or oscilloscope to diagnose further though by the looks of things 

[Cribbyrhymes]

OK so a change of immediate plans. I've started reading and went down a complete rabbit hole with noels retrolab videos, enjoying them a lot and learning a lot. 
Its a steep learning curve though which will take time and I do have to consider spending some money on equipment down the road to compliment my multimeter.
My son though is pestering me to get this 464 up and running ASAP so I think I will send it away for repair this time if its not too expensive in the hope that in the future if it fails again I'll have better knowledge and equipment to hopefully fix it a home. 
So my question is are there any recommendations for any UK based repairers? I found a few through google but any personal recommendations would be appreciated. Happy to pay for a repair but not looking to break the bank.  
Thanks

[SerErris]

I cannot do any recommendation - to be honest as I am not in UK.

However most likely you will need to pay for the hour as no one can tell you at this point in time how much efford it will be to get it back up and running (any fried chips to replace, diagnosis etc.).

To be able to really diagnose it you would need the following:

1. Oscilloscope
2. Diagnostic ROM from Noels Retro Lab
3. Seomthing to put the ROM on (like my very basic hardware to run it)
4. An EPROM with 32k
5. An EPROM Programmer
6. Optional: (multichannel) Logic Probe is helpful, but Oscilloscope is obviously better.
7. Optional: Bench Power
8. Soldering Ion
9. Optional: Desolerding Gun, or any other method to desolder chips.

If you would have the Diagnostic ROM and any form to run it, you could remove the RAM and the ROM in the first place and see if you then can see anything. It would be esp good to run it from the expansion port, as the ROM socket also can have issues.

For your analysis:
Okay ROM does get both enable singales right, so it should be active and output data.
Now you would really need a logic probe, to see if any of the data lines are continously low or high or even floating. Maybe the ROM is fried.
You may want to test the datalines of the ROM against GND to see if they are fixed, or varying. However that is very hard to do with a Multimeter, as it could not identify floating lines.

[Audronic]

@Cribbyrhymes 

As a Staring piece of Diagnostic hardware Just a Logic Probe
These are less than $10.00 Australian .

PS:_ Don't purchase the Cheapest  but get a Medium price one.

These can be used to show "ACTIVITY" on various pins which will indicate the state of the Pins

Good luck

Stay With the Project you will learn a LOT

Keep Safe

Ray

[Cribbyrhymes]

I cannot do any recommendation - to be honest as I am not in UK.

However most likely you will need to pay for the hour as no one can tell you at this point in time how much efford it will be to get it back up and running (any fried chips to replace, diagnosis etc.).

To be able to really diagnose it you would need the following:

1. Oscilloscope
2. Diagnostic ROM from Noels Retro Lab
3. Seomthing to put the ROM on (like my very basic hardware to run it)
4. An EPROM with 32k
5. An EPROM Programmer
6. Optional: (multichannel) Logic Probe is helpful, but Oscilloscope is obviously better.
7. Optional: Bench Power
8. Soldering Ion
9. Optional: Desolerding Gun, or any other method to desolder chips.

If you would have the Diagnostic ROM and any form to run it, you could remove the RAM and the ROM in the first place and see if you then can see anything. It would be esp good to run it from the expansion port, as the ROM socket also can have issues.

For your analysis:
Okay ROM does get both enable singales right, so it should be active and output data.
Now you would really need a logic probe, to see if any of the data lines are continously low or high or even floating. Maybe the ROM is fried.
You may want to test the datalines of the ROM against GND to see if they are fixed, or varying. However that is very hard to do with a Multimeter, as it could not identify floating lines.

Thank you for taking the time to respond and provide so much useful information.
I really appreciate it.
I've a lot to learn and a lot of equipment to buy but who doesn't love buying new toys  

[Cribbyrhymes]

@Cribbyrhymes

As a Staring piece of Diagnostic hardware Just a Logic Probe
These are less than $10.00 Australian .

PS:_ Don't purchase the Cheapest  but get a Medium price one.

These can be used to show "ACTIVITY" on various pins which will indicate the state of the Pins

Good luck

Stay With the Project you will learn a LOT

Keep Safe

Ray

Thanks Ray, your time is Much appreciated as is the time of everyone who has responded so far.
I will definitely pick up a logic probe to start with.
Its a steep learning curve but with time and determination I'll get there!

[Cribbyrhymes]

Thanks to all who responded, I appreciate it.

I found a UK repairer who has a good reputation and is reasonably priced so I've sent it away to him to diagnose and potentially repair it depending what the issue is.

I have a logic probe on the way and once I've done a lot more reading and bought some more equipment I'll definitely purchase another cheap faulty CPC to work on at my leisure and hopefully repair. This current one I'll keep as my main machine.

I'll update the post once I have heard back from the repairer as to what the fault was.