Apple II europlus ram issue

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Apple II europlus ram issue

I am trying to fix an Apple II europlus.

It can't boot and it looks to be related to a ram problem. Or maybe that's just a red herring.

If there is a ram chip in C3 or D5 or E3 it displays a weird screen. See picture 1 and 2.

If there is NO ram chip in C3 and D5 and E3 it displays a more normal screen. See picture 3. But of course then it can't boot without the C3 ram.

I have tried using ram chips which I have tested ok in another machine.

I have tested all the 74LS chips on the board, they seem to be ok.

I have tested the 6502 and the roms in another machine, they work ok.

I tried desoldering the C3 socket, because maybe there was a problem with the socket. Did not find any problem there other than destroying the socket. I put in a new socket, that did no change the behaviour.

 

Any ideas as to what the problem could be? Where I should be looking?

 

Pitcture 1

 

Picture 2

 

Picture 3

 

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Honestly, it looks like you

Honestly, it looks like you may have multiple issues.   But, from the vertical bars, it looks more like the CPU is failing to execute code from the ROM.  Basically, it's stuck in a low-resolution graphics mode and can't get out.   Typically, this indicates a problem with one or all of these chips (CPU and ROMs).  However, since you've already checked the ROMs and CPU in another machine, I'd turn my attention to the sockets.  Try using a multimeter set to continuity mode to check the IC pins in the sockets for continuity to the bottom of the board. 

 

As for the noise in the first pick two pictures,  I'm not sure what to make of it.  Even though they look very different, it could be the same issue presenting in either high or low-resolution graphics mode.  Whatever it is, it has me leaning more towards the CPU end of things than the ROM.  You might try some DeOxit in the sockets to see if that cleans things up.

 

As for the characters in the third picture, the RAM bank in row C is the lowest (1st) bank addressed/accessed by the CPU.  Since most of the pins are linked, any missing chip will effectively disable the bank.  Esentually, as far as the computer's concerned, there was no RAM.  This probably prevented the computer from entering a graphics mode in the first place, thus resorting to characters from the related ROM in row A.  

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Red Herring

Hi Tolderlund,

this looks like quite a challenge!

Like many early home/personal computers, the Apple ][ briefly displays garbage on screen the instant it's powered on, though usually in text mode as in pic #3. The graphics modes in the other pics are indeed unusual.

I do agree however that the RAM issue may be a red herring, and would side with DistantStar in suspecting the CPU, since that should clear the screen on powerup. Otherwise my money would be on the ROMs, but you've already ruled those out. I also assume you've checked for stable power supply output.

If you have a 'scope, now's the time to break it out. For starters, I'd check for basic CPU activity: clock, chip select, address lines, data bus. One neat way to check the addressing logic -- assuming you have pin-compatible EPROMs and a suitable programmer -- is to replace the ROMs with EPROMs filled with NOPs. These will cycle the address bus lines at different frequencies, at least  the end of ROM is reached (though this could contain a JMP 0000 to wrap around). It's a pretty elaborate setup, but it came in handy when I revived my completely comatose Tektronix.

 

Hope this helps. Keep us posted,

--GT

 

 

 

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Found it

Hi GanjaTron,

thanks for your suggestions.

 

I do have a scope. I was so lucky to win a Keysight MSOX3104T  in 2016.

They had a lottery (was free to enter) and my name came out. See https://www.youtube.com/watch?v=W2_ftQp6T3Y

So I got this nice scope completely for free: https://www.keysight.com/dk/en/support/MSOX3104T/mixed-signal-oscilloscope-1-ghz-4-analog-16-digital-channels.html

 

Now I'm not an expert with these things, as I am a software developer and not really a hardware guy.

I prodded around with the scope for along time. Clock signal and other things looked normal to my eyes.

 

Then I decided to do something else.

I removed every IC from the board and replaced them with all the ICs from my other working board, thinking that nomatter the result it would tell me something.

Either a problem with the board or problem with an IC.

And the board worked, it booted into the prompt. So the problem had to be with some of the original ICs.

I started by putting in the original RAM in C row. And the problem came back with the weird screen.

It turned out to be the C5 RAM. I put in a spare 4116 in C5 and all the original ICs back.

Now it boots just fine into the prompt.

Problem solved.

 

So the fact that the screen looked sort of fine when the C3 RAM was removed made me blind and I didn't think about checking all the other RAM chips.

Many hours wasted because of that.

A lesson learned I hope.

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