CPC 464 doesn’t boot, shows blocks on screen

[hatman72]

Hi everyone, I'm hoping that someone can suggest what might be wrong with this early 464 with a 40007 Gate Array. 

When I start it always ends up at this screen with alternating blocks of 128 characters in light blue and purple. There is no beep if I press delete. There seems to be lots of activity on the board and no chips seem to be getting excessively warm. The problem is that since only the CPU and GA are socketed I don't want to start desoldering chips at random as I do not have a desoldering gun. 

The computer came to me this way and has been stored in a loft. I have no idea whether it was working when it was put there. 

[Bryce]

Hi Hatman72,
      that's not a classic RAM failure. I'd suggest you start by cleaning the pins and reseating all socketed chips and see if it helps.
What's your level of electronics knowledge and do you own any test equipment?

Bryce.

[hatman72]

Hi Hatman72,
      that's not a classic RAM failure. I'd suggest you start by cleaning the pins and reseating all socketed chips and see if it helps.
What's your level of electronics knowledge and do you own any test equipment?

Bryce.

Thanks Bryce. I've had both socketed chips (GA and CPU) out. I know the CPU is ok but my other (working) Amstrad has a 40010 GA so I can't easily test that. 

I own a multimeter and a cheap oscilloscope (which allowed me to check there's activity on all the RAM chips, but I'd consider my electronics knowledge to be fairly basic.

[salvogendut]

Hi, I am no expert but had recently a similar problem. What really helped me to debug the 464 was the cartridge you see in the pic. I got it off Ebay

[hatman72]

Hi, I am no expert but had recently a similar problem. What really helped me to debug the 464 was the cartridge you see in the pic. I got it off Ebay

I have an M4 card, so installed the diagnostics ROM on that. Unfortunately it doesn't boot from that either   

[Rabs]

Maybe this video may help

.

In this video there is a similar screen pattern, sometimes, but also the usual grey screen black border RAM problem display.

You cannot view this attachment.

In this GadgetUK finds that IC112 74LS32 Quad OR is faulty. Not quite sure how the floating output he finds relates to the issue as pin 11 selects the printer latch but pin 8 does select the data latch (by the way I made pin 8 float on my test board and I get the usual grey screen RAM problem symptoms).

No idea if this is the same as your problem but saw it and thought I would share.

[salvogendut]

Hi, I am no expert but had recently a similar problem. What really helped me to debug the 464 was the cartridge you see in the pic. I got it off Ebay

I have an M4 card, so installed the diagnostics ROM on that. Unfortunately it doesn't boot from that either 

I do not own an M4 but I do own a ULIfAC and tried using it to debug this and that did not help for me. Maybe it has to do with the fact that whatever booted the ULIfAC tried to access RAM (even when I used a CONFIG.TXT with just the diagnostic ROM) where this one cartridge does not

[hatman72]

Maybe this video may help

.

In this video there is a similar screen pattern, sometimes, but also the usual grey screen black border RAM problem display.

You cannot view this attachment.

In this GadgetUK finds that IC112 74LS32 Quad OR is faulty. Not quite sure how the floating output he finds relates to the issue as pin 11 selects the printer latch but pin 8 does select the data latch (by the way I made pin 8 float on my test board and I get the usual grey screen RAM problem symptoms).

No idea if this is the same as your problem but saw it and thought I would share.

That does look very similar, and I have seen mine produce the pattern in different colours occasionally, especially if the cassette deck is plugged in. It does look like a sensible thing to check. 

[Rabs]

That does look very similar, and I have seen mine produce the pattern in different colours occasionally, especially if the cassette deck is plugged in. It does look like a sensible thing to check.

Either way I would follow the advice by @Bryce.

[hatman72]

Drawing a bit of a blank on this. The two socketed chips have been in and out multiple times with no difference. I've been trying to spot any broken traces or anything suspicious but nothing is jumping out other than a stain around the sound chip. Suspect this was from a spillage or damp as there's also a bit of corrosion on the legs on this chip. My assumption is that a problem with the AY would not cause a boot issue but if people think differently then let me know. 

[Bryce]

Drawing a bit of a blank on this. The two socketed chips have been in and out multiple times with no difference. I've been trying to spot any broken traces or anything suspicious but nothing is jumping out other than a stain around the sound chip. Suspect this was from a spillage or damp as there's also a bit of corrosion on the legs on this chip. My assumption is that a problem with the AY would not cause a boot issue but if people think differently then let me know.

Can you post a picture of the stain? If a capacitor has leaked it could be pulling the voltage rail down or could have corroded a solder joint nearby.

Bryce. 

[hatman72]

Drawing a bit of a blank on this. The two socketed chips have been in and out multiple times with no difference. I've been trying to spot any broken traces or anything suspicious but nothing is jumping out other than a stain around the sound chip. Suspect this was from a spillage or damp as there's also a bit of corrosion on the legs on this chip. My assumption is that a problem with the AY would not cause a boot issue but if people think differently then let me know.

Can you post a picture of the stain? If a capacitor has leaked it could be pulling the voltage rail down or could have corroded a solder joint nearby.

Bryce.

Two photos of the bottom and one of the top. Hope they are clear enough.

[Bryce]

Oh. That's not from a leaked capacitor, but there is a lot of corrosion visible. I'd recommend you to reflow all the solder joints. There might not be any failed components at all, just bad connections. Especially the ROM joints (I think Gunhed even suggested ROM issues above) could cause the symptoms you are seeing.

Bryce.

[hatman72]

Thanks Bryce, I'll try reflowing the joints from that area to see if it makes a difference. I'll report back once I've had some time to do it. 

[Bryce]

Hi Hatman72,
          it might just be a shadow, but several pins look like the tin coating on the pins has corroded (Chip pins are usually copper coated with tin) and created a barrier between the pin and the trace. To confirm this, you could measure the resistance between the top of the pin and some other point on the PCB / other pin that it should connect to. Either way, the way they look now definitely merits a reflow. However, you'll need to remove all the corrosion before you reflow them, so it might be easier just to remove and replace the worst effected chips.

Bryce.

[hatman72]

It's not a shadow, the AY and the two chips nearest it (PIO and IC106) do have some corrosion on the legs facing the back of the computer. 

I'll probably have to have a go at removing these. I'm not sure what the best way to remove corrosion on chip legs is. 

[Bryce]

You can use fine sandpaper to remove the oxidated layer.

Bryce.

[SerErris]

Hi, I am no expert but had recently a similar problem. What really helped me to debug the 464 was the cartridge you see in the pic. I got it off Ebay

I would love to see what is inside this cartridge.