A500 plus black screen

[Steve_w]

Hi,

I have an a500+ board that is showing a black screen. The custom chips are known working chips, I've socketed and replaced the following chips, u10,11,12,13,37,34,35.

I've tested the reset circuit, CPU has RST and HLT high, on reset the signals go low then high, the address bus from 1-11 is low, 12-16 floating, data bus is also floating.

OVL is low and stays low which I believe means the ROM is not being mapped to the start of memory and maybe why the CPU is not doing anything.

The CAS signals are not right, but I'm not sure if this is the cause of OVL staying low or whether the OVL issue is causing the odd CAS signals.

Any help would be much appreciated.

Thanks

[stephen_usher]

Given this video...



Check the left CIA?

[Steve_w]

Thanks for the video, I have tested continuity for all pins on the odd CIA, all fine, the CIA is a known working CIA.

[terriblefire]

Thanks for the video, I have tested continuity for all pins on the odd CIA, all fine, the CIA is a known working CIA.

Swap them then.

[Steve_w]

Yep done this, same issue.

I'm not sure why OVL is not being set, it stays low on power up, when I do a soft reset using the keyboard, it stays low. On a working board on power on and after soft reset OVL goes high after reset as its asserted and ROM is moves to 0000h ready for the PCU to start executing ROM code.

[terriblefire]

Yep done this, same issue.

I'm not sure why OVL is not being set, it stays low on power up, when I do a soft reset using the keyboard, it stays low. On a working board on power on and after soft reset OVL goes high after reset as its asserted and ROM is moves to 0000h ready for the PCU to start executing ROM code.

Swap Gary over from a working machine.. if that makes no diff measure OVR when both the CIA and Gary are out. it should float high.

[Steve_w]

Thanks

I have tested the Gary chip I am using in the faulty board in working board and it works fine in that board, so I know its not Gary.

I will test OVL later when I get in, I have tested continuity on all pins for all custom chips so I am pretty confident the connection between Gary, (RP101) and Odd CIA is okay.

I see no activity at all on the Address bus, if I put the scope probe on A1, or A2, there is nothing at all on startup, I would have thought there would have been a bit of activity before OVL is set, or maybe I'm wrong.

I am wondering if its a RAM issue, I have replaced the RAM logic chips (LS373's and LS244's) with new tested chips.

Thanks for your help

Steve

[terriblefire]

Thanks

I have tested the Gary chip I am using in the faulty board in working board and it works fine in that board, so I know its not Gary.

I will test OVL later when I get in, I have tested continuity on all pins for all custom chips so I am pretty confident the connection between Gary, (RP101) and Odd CIA is okay.

I see no activity at all on the Address bus, if I put the scope probe on A1, or A2, there is nothing at all on startup, I would have thought there would have been a bit of activity before OVL is set, or maybe I'm wrong.

I am wondering if its a RAM issue, I have replaced the RAM logic chips (LS373's and LS244's) with new tested chips.

OK so i was assuming you're testing with DiagROM? If not do that. It doesnt need any RAM to output on serial or screen what the issue is.

OVL should be high with the machine held in reset. If its not then there is no way anything else will work. Start there before changing anything else.

There are 2-3 ways to do this. I personally tie off the reset pin to ground but You can also hold down the required keys on the keyboard or tie the KB_RESET line on the A500 keyboard connector.

If OVL is not high then no matter how sure you are of a connection there is a fault. Remember it will go low very very early in the boot cycle so you need to measure all this will reset HELD.

After that use DiagROM rather than doing potentially more damage to the machine.

[Steve_w]

Yes I am using diagrom.

I will try scoping OVL on reset as you suggest, to recap OVR is high on reset then goes low.

Thanks

Steve

[terriblefire]

in that case OVL is fine.

What do you see on the serial port