Apple II plus - yet another scrambled display issue

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Apple II plus - yet another scrambled display issue

Hi everyone,

I have been following this forum for some time now, but this is my first post. Recently I bought an Apple ][+ from an auction site. After some work on the PSU I was able to proceed to the garbage screen phase :-) What happens is:

1) The power is fine, the keyboard lamp lights,
2) there is a beep,
3) the screen looks as attached, the contents are more or less random, but some 8-column pattern emerges,
4) pushing reset on the keyboard does nothing.

I started to re-socket some of the chips, what's weird is that in some of the IC-s (approx. 1 in 10) connectors were heavily oxidized, almost black, but most of them look brand new. Anyway, I removed the E row of RAM chips, as it was the only one with mixed ICs (E3 lang. card IC was different). But all-in-all RAM looks genuine, chips have little Apple logo on them. In general there is no sign of repairs or tempering with the board.

As far as I understand the "beep" signifies that at least some of the startup routing was executed and that CPU was able to access ROM and RAM(?), so the problem is somewhere else - ?

Thanks for any ideas,

Greg

 

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Try Ctrl - Reset

Try Ctrl - Reset and see if the picture changes and you hear another beep.

 

Thanks,

 

Josh

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Only 32k installed.  Does the

Only 32k installed.  Does the sticker on the bottom indicate a 32K machine?  Either way that probably isn't the problem.  But the problem could be RAM or possibly even a ROM issue.  And you definitely want to add another 16k, as much software requires at least 48k.  If you press reset as suggested above and get a prompt, you may be able to determine if the RAM or ROM are good.  You can list the ROM from the monitor, and a bad ROM is usually pretty obvious.  There are also ways to do some simple RAM tests.

 

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ctrl-reset

Hi,

 

0. I re-soceted most of the TTL chips, which resulted in some problems :) but finally I returned to square one, i.e. symptoms described in the original post.

1. Ctrl-Reset results in a beep, no change on screen, no prompt.

2. top row of RAM missing - this is my work - removed it because one of the chips was different from the rest, but this should be an issue at this stage I think.

Is there any systematic way to debug such issue? (apart from "work from the CPU towards the RAMs" ?

 

Cheers,

 

Greg

 

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Forget the CPU and RAM

If Ctrl-Reset emits a beep, then the CPU and RAM are most likely OK. Concentrate on the video generation circuits.

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some progress...

It just occured to me, that assuming that the system is more or less fine (CPU and ROM/RAM working)  it should start Applesoft.  So, I can try to switch graphic modes with GR, HGR, etc. Of course with text mode broken I can't see what I'm typing but .. anyway ...  I managed to enter GR mode - I get mostly white pixels with some strange coloring, HGR seems to work - I get blank screen with 4 lines of garbage text at the bottom. After issuing 3.4 commands the systems seems to lock-down so testing is hard. Having looked at http://www.appleii-box.de/H198HomeRepairGuide.htm I suspect that it might be an issue with B14 (video) or C12 (adress decoder) chips. I excluded E11-E14, because as far as I understand the memory refresh would be broken with these chips bad (?) Anyway, will try to replace these ICs and see what happens...

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good news!

So, I bought a whole bag of ICs (they sell them bundled in 10s). My first choice was the Video->RAM address generator. And ... guess what! The first chip I replaced did the trick: E14 - 74LS283 (4-bit adder), which is a first stage of this module. In fact, I would rather bet on one of 74LS153 multiplexers, but it made sense to try 283 first. Anyway the machine works, of course there are some problems with the keyboard, etc.

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Hey, that's great luck!

Hey, that's great luck!

 

It's always the last one you try, right? Except in this case it was also the first...

 

My II's and II+'s are "touchy". I turn them on every year or two and always have to reseat chips and fool with them to get them running. I don't think the sockets are too great on them, and 40+ years of slow corrosion does not help them along.

 

 

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iF you dont mind.

If you don't mind. Is that a bundle of the same type or a mixture of chips for the apple ii Plus and if it is. Can you post a link? Also congrats. Don't worry. After I fixed my video issue on my II plus. My keyboard sucks too. Thinking for now on getting a ps2 converter. But still need to up my bank account still.

 

Thanks,

 

Josh

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