Amstrad CPC 464 Rev. 2 mobo test & repair

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Amstrad CPC 464 Rev. 2 mobo test & repair

Postby minn0w on Sun Jan 29, 2017 9:28 pm

I just purchased an Amstrad with color display, reasonably good condition for what i paid. Owner said it was left in the attic since its was last used many many years ago.

It powers up with no boot and a grey flickering (~25hz, might be its normal refresh rate) screen. Was no sync or any other signal on the display connector. All the pins were floating around 1.5v / 2v.
So went poking around with a scope while looking at the schematic.
Went through and checked a few signals on the Z80, 5V OK Reset OK.
But the clock was not present and pin is high.

After wondering where all the ICs in the schematic are, i realized i had an older revision schematic.

Found the right schematic and traced the clock from the clock generator circuitry and into the dreaded Gate Array.
Found that the 40010 IC was floating almost all of its outputs at around 1.5V.
I checked the required input on the 40010 (VCC, Reset) they looked OK.
Found that the only output that had any meaningful behavior was the READY pin, which just goes high after the power up reset procedure.

So i'm reasonably sure that i'm going to buy another 40010 :-(
My hunt for one of those begins...
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Re: Amstrad CPC 464 Rev. 2 mobo test & repair

Postby minn0w on Thu Feb 02, 2017 8:37 am

Update

Have purchased a 40007 Gate Array. NZD $40 later.

I will be looking at putting a 40pin zif socket in the 40007 header on the board (if it fits in the case).
Otherwise i will install a normal 40 pin header.

I am assuming that there is no other modification to the board.

Any info on this process would be appreciated.
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Re: Amstrad CPC 464 Rev. 2 mobo test & repair

Postby minn0w on Sat Feb 18, 2017 2:03 pm

Update:

My 40007 arrived.
I installed R138, R139 and a socket for the 40007 (a ZIF wouldn't fit with a heat sink)
I am now getting clocks and activity on pins that were not before, however no boot.

CPU appears to be getting required signals to start up.
I put the logic analyser on the address bus of the Z80 and found it in an undetermined state, wobbling around 2.0V. Not floating either. The pins don't seem to be able to be encouraged high or low with an external potential.

So next in line is a new Z80, which is on its way...

I had also replaced the 74LS244 since i saw it not pulling the bus in the right ways, but when i tested the old one it was fine. So something else on the data bus was causing contention, which i now assume was the 40010 and the CPU.

The 74LS312 looked at me funny the other day too, i might have to show it who's boss.
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Re: Amstrad CPC 464 Rev. 2 mobo test & repair

Postby tezza on Sat Mar 04, 2017 10:30 am

Sounds like you're getting there. Keep at it!
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