Any way to test GA is working correctly?

[llopis]

I'm on to my next repair project: A CPC 464 that was apparently completely dead.
The first part was easy: the Z80 was overheating and clearly dead. I replaced it with a new one and now I get a black border and noisy patterns on the screen (mostly pink/purple and they sometimes flicker and change for a few seconds).
I started looking into the RAM, and all the Dout pins are pulsing as expected, but pin 3 (W) is always high and pin 4 (CAS) is alway slow. That doesn't seem right, does it?
So I started looking at the Gate Array. It's a 40007 GA with heat sink. The board is a Z70200 MC0002B. When I examined the pins, I see VSYNC and HSYNC going in as you would expect, but INT is always low, which makes me think the GA isn't working. Poking around, it looks like pins 6, 7, 8, and 10 are always low, which doesn't inspire me with confidence.
I replaced it with another GA from another CPC (not known to work) and I get the exact same behavior and screen pattern on screen. It doesn't rule out the GA, but it seems too much of a coincidence that two random CPCs would have the same problem.


Are you aware of other tests you can perform on the GA to verify it's working? Alternatively, are you aware of something else that could be causing the GA not to generate INT signals?

[Bryce]

There are other reasons why the INT might be staying high, such as the computer being stuck in a loop waiting for an answer from something that isn't answering, corrupted Firmware (bad ROM) etc.In your case I would assume that the GA is ok. The chances are zero that two GA's would give identical results if they were damaged.

You really need to get yourself a LowerROM replacement device with Geralds RAMTest installed. This is the first thing (after checking supply voltages) that should be checked and much faster than messing about with meters/scopes on the RAM pins.

Regarding the CPU. Z80's that die of "natural causes" don't tend to be any warmer than a working CPU. The fact that it was getting hot would suggest that the CPC may have had too high or a wrongly poled supply connected to it at some time.

Bryce.

[llopis]

Thank you, Bryce. You're an incredible resource for this community with your encyclopedic knowledge of Amstrad hardware.
That's very interesting about Z80s not usually dying that way! I hadn't thought of that.
I'm still waiting on X-MEM, so I can't wait to use it to test this.

[gerald]

For the INT always low, you just need the Z80 to have IRQ disabled.
The GA waits until the Z80 does an IRQ acknowledge cycle to rise the signal.
IRQ ack cycle only happen when servicing the IRQ.