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Apple ][+ white screen issues

J

Juror22

Experienced Member
Joined
Jul 26, 2011
Messages
211
I recently got two Apple ]['s the older one works fine, but the ][+ shows a white screen, but otherwise boots normally. The cursor shows as a blinking black square on white and entering characters on the keyboard advances the cursor on the white screen and system beeps, resets, etc seem to function normally, except the screen is always white.

The Apple ][+ has an 80 col card in slot #3 (a viewmax 80) and typing PR#3 (blind 'cause the screen shows white) changes the screen to 80 char normal black&white. From there, everything appears normal There is a "]" prompt, until I press reset, which sends the screen to white ( PR#3 again sends it back to 80 chars, black&white).

I have reseated most of the chips, and all of those referenced in the repair docs that I have found. Does anyone know what the likely cause is, so I can switch/replace the appropriate chip(s)?
I did find a single trace that looked iffy, but checking for continuity confirmed that it was fine.

A2 service notes:
Blank white screen
A3-74166
B5,8-74LS174
F14,9334
6502

a2 troubleshooting
Chip Swapping
A2 74LS00
A8 74LS257
A9 74LS151
A10 74LS194
B2 74S86
B10 74LS74
B13 74LS02
C2 74LS195
C11 74LS04
D11,12,13,14 74LS161
 
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The all white screen can be caused by a few things.

To eliminate some of the potential causes, boot the machine to basic (no floppy).
If you hear the speaker beep type gr and press enter.
Let me know if the screen clears.
If most of the screen clears then the problem is in the character generation circuit, if it stays white then look to the video source multiplexing.

edit: Given that you have a flashing cursor and stable sync, D11,D12,D13,D14, B2,B10,B11,B13 are likely to be fine.
I'm leaning towards the problem being character generation, that would indicate that the likely culprits are A3 or A5 in that order.
A quick test is to take the ROM from F11 and fit it into the A5 socket. If you see random dots then you have a faulty A5 chip.

Try those things and let me know the results, I'll be able to narrow it down further.
 
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Thanks for the responses, I thought that I had a login at applefritter, but I don't, so I applied for one - I am now awaiting for them to determine that I'm not a robot.

Anyway, moving on to the practical next steps. typing in gr <enter> yields a mostly black screen, so that confirms the chargen circuit is the issue.
The chip swap produced no change - booting with the F11 ROM in the F5 holder still yielded a white screen. Reseating F3 likewise did not help.

Thanks for the assistance.
 
Juror22 said:
Thanks for the responses, I thought that I had a login at applefritter, but I don't, so I applied for one - I am now awaiting for them to determine that I'm not a robot.

Anyway, moving on to the practical next steps. typing in gr <enter> yields a mostly black screen, so that confirms the chargen circuit is the issue.
The chip swap produced no change - booting with the F11 ROM in the F5 holder still yielded a white screen. Reseating F3 likewise did not help.

Thanks for the assistance.
Click to expand...

Then its 90% likely to be the shift register (74ls166) at A3.
 
The current chip is a National DM74166N, is that likely to be the original chip? I have seen where it is a replacement for the ls166, but I'm curious what would be original to the machine, since both are available. I'm looking around to find if I have a spare about, without disassembling my working ][.

Finding no other spare, I took the donor chip out of my ][ and placed it in the 2+, but it still produces the white screen. Putting the donor chip back and booting the ][, it still works fine. Where should I look next?
 
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From Winston Gayler's wonderful book, the path for text video is A3,A5,B3,A9. You've eliminated A3 as a possibility.
I'm not so sure you've eliminated A5 but maybe B3 or A9 ?

video.jpg

Larry G

PS - my gut still tells me bad character ROM A5. A white screen is all bits on as if it was read out that way from ROM.
 
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Its less common but possible that the shift register is not getting the load signal.
There could be a problem with the 74ls00 at A2 or the 74ls02 at B13.

Keep in mind that chip swapping won't always find a problem, with these old boards the problem can be open circuit tracks or sockets faulty beyond the help of reseating chips.

What test gear do you have available to you?
 
I have an ESR meter, a newer and a few older multimeters, a couple of old 2 CH oscilloscopes (Hitachi V209, BBC D-8600) and a Hantek 6022BL PC Oscilloscope & logic analyzer (none of which I have yet learned to use, but this seems like a good project to start with if they are up to the task).
 
Arm yourself with one of the oscilloscopes and the schematic here...
http://www.freeinfosociety.com/electronics/schemview.php?id=1472

In the top left of the schematic you will find A3.

Check pin 15, you should see a signal mostly high with short duration evenly spaced low pulses. This is the signal that latches each line of character data into the latch.
Check pin 13, this should be mostly low but with very sporadic very short high pulses. These are the lit pixels of the characters. If it is mostly high (white screen) then we need to look at why.

That will get you started in the right direction.

Juror22 said:
I have an ESR meter, a newer and a few older multimeters, a couple of old 2 CH oscilloscopes (Hitachi V209, BBC D-8600) and a Hantek 6022BL PC Oscilloscope & logic analyzer (none of which I have yet learned to use, but this seems like a good project to start with if they are up to the task).
Click to expand...
 
It took a few days to get comfortable with the oscilloscope and pin 15 of A3 is as you described, with short lows followed by long highs. Pin 13 is basically all high. If I understand what is going on, then 15 is a (correct) input and combined with the other parallel inputs, outputs to 13. Since swapping A3 and A5 had no effect on the issue, an input(s) to A3, other than pin 15 is/are causing the issue?
 
I just double checked and I've given you bad advice. I incorrectly assumed the character generator (labeled 2516) on some boards had the same select lines as the 2516 Applesoft ROMs.

The character generator uses the same select scheme as a standard 2716 unlike the ROMs. So a ROM in the character generator slot will give you the white screen. (I tested it in a working board here)
I've just read the character generator from a board here and confirmed the selects are standard 2716. I've programmed a 2716 with that data and it works perfectly as a replacement.

In light of that it appears your character generator ROM is faulty. If you have access to an eprom programmer here is an image file I copied from the ROM on my board.
https://drive.google.com/open?id=0B8aVm3oIO3icYnRHbXozczdsWHc
 
Thanks for your continued help, for taking the time to re-check against your own equipment and for passing along the file. I will let you know how it 'burns out'.

Looks like it will be a bit, I only have 27C256 eproms lying around (I searched through some old ones that I had stored, which were software revisions to a CAT scanner, those were 28 pin too). I will have to order up some, probably from eBay.
 
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Juror22 said:
Thanks for your continued help, for taking the time to re-check against your own equipment and for passing along the file. I will let you know how it 'burns out'.

Looks like it will be a bit, I only have 27C256 eproms lying around (I searched through some old ones that I had stored, which were software revisions to a CAT scanner, those were 28 pin too). I will have to order up some, probably from eBay.
Click to expand...

What area are you in, there may be a nearby member that can supply an eprom quicker than from ebay.
 
I'm in central Illinois, (US) and I doubt that there is anyone nearby, but on the off chance there is, let me know.

Otherwise, I should have something in hand by next week.
 
Success! My ][+ lives!

I received the 2716 chips today, erased and burned them, using the provided file (only took two tries to get the settings correct) and it now boots and runs correctly. I will put it all back together tomorrow and start giving it a proper workout, but the white screen issue is history! (Root cause was the bad chargen ROM)

Thanks to all that wrote and of course a special "Above and beyond the call of duty", thanks to David_M for his expert assistance and sacrifice, just to help me sort this out. If there is ever anything I can do, let me know.
 
Made a couple of disks, after hooking it up to my Mac with ADTPro. Playin' Castle Wolfenstein on the A2+, duodisk and a color composite monitor, WooHoo!
 
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