CPC 464 No video signal

[Cribbyrhymes]

Hi

I bought myself another 464 last week after being without one for a few years.  Its great to be back on the forum and back as a proud owner of a 464 again  
It was sold as untested but did state that it powered on.  
I opened it up and stripped it down when it arrived and gave it a good clean with a soft brush for the dust and isopropyl and a soft toothbrush for the board and all the connectors etc.
I bought a power supply and scart adapter lead from cool novelties.
It powers up fine with a nice strong red led, the fast forward and rewind work fine on the tape deck but I am getting no display output at all on the tv (same tv I used with my previous 464 a number of years ago).  I put the volume right up on the 464 and pressed DEL but Im getting no beeps at all although at full volume Im hearing quite a bit of crackle through but Im not sure if this is normal or not.
Any ideas as to what to do to narrow down the fault? I have a multimeter and some basic soldering skills.  Would love to get this 464 up and running again, my 9 year old son was so disappointed when it didn't work!
If its beyond my capabilities to troubleshoot and repair I may have to send it to Bryce if he's still providing this service.

[eto]

no signal at all or is there at least a gray rectangle on screen?

[Cribbyrhymes]

No signal at all unfortunately  

[Rabs]

No signal at all unfortunately 

Black Screen could be a number of things unfortunately. Where are you based?

[Cribbyrhymes]

Im in Northern Ireland

[Rabs]

Im in Northern Ireland

Ah shame, bit bit far to help.

[SerErris]

Do you have a multimeter?

It is worth to check if you have power at all (does the power LED light up?).

And then start measuring if the chips get power. Just a first indicator of broken solder joints or even short circuits.

If you do not measure any voltage between +5V and GND, you have either a dead power supply, or a short somewhere.

[Cribbyrhymes]

Yes I have a multimeter.

The machine powers up ok, power led is on and tape deck rewinds and fastforwards but no display.

Im new to testing voltages so bear with me, I take it Im testing the +5v and ground pins on each chip? Im assuming there are pin out schematics on the wiki here?

Thanks for all the replies so far

[eto]

Im new to testing voltages so bear with me, I take it Im testing the +5v and ground pins on each chip? Im assuming there are pin out schematics on the wiki here?

Schematics:
https://www.cpcwiki.eu/index.php/Schematics

[Cribbyrhymes]

Thank you, I'll do some testing tonight with the multimeter and report back

[SerErris]

If you press the "DEL" key, do you get a beep (turn up the speaker).

This will verify if your CPC actually works, and you just do not have any screen, or if it does not work at all (e.g. CPU not processing correctly).

It can be still a lot of things.

GateArray is the obvious candidate. If that is dead, you will have no output at all as it creates the RGB signals and the SYNC signals.

One last question:
It looks like you are using a "TV".
Did you CPC come with the normal Green Monitor or Color Monitor?

I am just asking, because if you connect it to a modern digital monitor, there might be some other reasons why you have a black screen and nothing is wrong with the CPC.

[eto]

GateArray is the obvious candidate. If that is dead, you will have no output at all as it creates the RGB signals and the SYNC signals.

I had CPCs with fried RAM and ROM ICs with exactly the same effect. If e.g. a dataline is always low, it will not work at all.

If you press the "DEL" key, do you get a beep (turn up the speaker).

He tested that already. See first message. 

[Cribbyrhymes]

OK so checked some voltages with the multimeter tonight and I'm getting a solid 5v across all of the following:

IC116 40010 gate array
IC117-IC124 ram
IC111 Z80 (although its the only chip on the board that is getting hot)
IC103
IC108
IC102
IC107

Are there any others I should be checking? My initial thoughts are the Z80 is the culprit or am I on the wrong path?

Thanks for all the replies so far

[McArti0]

Im hearing quite a bit of crackle through but Im not sure if this is normal or not.

This may mean that the Z80 is working.

Do you have a frequency meter?
The multimeter is a bit too small as a diagnostic tool.

[Cribbyrhymes]

I dont have a frequency meter unfortunately just the multimeter 

[SerErris]

A digital probe also may work. You can check the address lines if you can see them going up and down (actually working) and the datalines. For instance if you would see a data line allways high or allways low - that is a fault somewhere in the path to RAM/ROM.

But really, without OSC or other equipement you are more or less without clue what is going on. 

[McArti0]

Z80 M1 PIN27 the meter will treat it as PWM. It should be around 1.5-2V (0V or 5V is bad)

[Cribbyrhymes]

So with the multimeter on pin 27 M1 and pin 29 GND I'm getting a reading of 4.87v

[McArti0]

IC112 74LS32 PIN8 ?
CRTC PIN 21 ?

[Cribbyrhymes]

CRTC PIN 21 - 1.52V
On IC112 is pin 1 GND?

[McArti0]

GA work...
No 74LS32 has GND on PIN7

[Cribbyrhymes]

Thanks

IC112 Pin 8 is reading 5.03v

[McArti0]

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.

[McArti0]

Does your ROM have socket?

[Cribbyrhymes]

Pin 21 CPU is reading 0.02v

My gate array and CPU are socketed, nothing else