This Apple II+, Rev 7, tagged (19)80(week)42. 48K plus Language Card and single disk drive and Apple Controller. Video seemed ok in all modes, until random white blocks started to appear here and there on the screen. I had replaced the power supply (new), but hadn't really cleaned up the computer. Pulled ALL the chips; de-ox'd the pins; and reseated them.
Computer is now functioning well .... with one exception: NO GRAPHICS.
I started with the Sam's™ "Troubleshooting and Repairing Apple II Plus" by replacing (with new) the 74LS194 at B4, 74LS194 at B9, 74LS257 at A8; then the 74LS257 at C12. They wanted to replace the 74LS08 at A11, but the 74LS08 was at B11; there was a 74LS74 at A11. I replaced them both.
Referencing the video circuits and other research on Applefritter, I replaced (with new):
74LS166 at A3 74S86 at B2 74LS04 at C11
74LS257 at A8 74LS194 at B4 74LS257 at C12
74LS151 at A9 74LS174 at B5 74LS51 at C13
74LS194 at A10 74LS174 at B8 74LS32 at C14
74LS74 at A11 74LS194 at B9
74LS02 at A12 74LS74 at B10
74LS02 at A14 74LS08 at B11
74LS11 at B12
74LS02 at B13
74LS02 at B14
The memory check (C050 C053 C054 C057 N 265:FF N 266<265.BFFEM 266<265.BFFEV 265:0 N 266<265.BFFEM 266<265.BFFEV 34:14 (Return)) had run successfully in the past, but now just produces a screen full of questions marks.
At this point, Sam's™ says, "Service Center Action", but there has to be something not soft switching. Any insight would be appreciated.
would be the 74LS151 at A9. But you say you replaced that.
Have you checked that all pins have good connection to the mainboard?
Are there any bad/broken solder joints?
Thanks, tolderlund ....
Rechecked the seating of the '151 at A9, and all the chips, I had replaced. Pulled the motherboard and checked the tracks and soldering of area in question.
Thought, "That's a lot of jumpers off A14", until I realized there are no tracks/traces connected to A14. Earlier REVs and the Apple II don't have a chip at A14. Have investigated, but cannot seem to locate circuit block/schematic diagram for this board, 820-001-07. ©1979.
Checked pin 14 on all the 7400 series chips and found the Vcc at 4.96-4.98.
Lack of graphics remains.
Still at a loss.
Apple II+ Jumpers on A14.JPG
Looking for a schematic of the II+ that has the 74LS02 at A14. The II didn't have a chip; my II+ Rev 7 has a chip with jumpers; later Revs had the chip with traces. The jumpers connect to the '02 at B13 and the '11 at B12.
74LS02 at A14.jpg
OK ... looking at Table 5: Screen Soft Switches ....
On the Apple II, in monitor, entering
*C050 returns LO-RES Graphics screen with four lines of text
*C051 returns a full screen in text mode
On this Apple II+, in monitor, entering
*C050 returns "C050- A0"
*C051 returns: Machine hangs and has to be rebooted from power up
Just saying ... have no idea ... Have replaced the 74LS151 MUX at A9.
My IIe did something very similar (text when you put it in graphics mode) shortly before a RAM chip completely failed, causing boot failure with a screen full of garbled text.
There is no dot on the motherboard to indicate pin 1 for the 74LS04 at A14 (it is 90° from the rest of the chips). The best/worst of my investigation to determine its orientation caused me to put the chip back in backwards. Turning the chip around so the notch faces outboard resolved the problem of no graphics modes.
Thanks to everyone for their assistance in this matter. I am humbled.
For revision specific information, including schematics, I would suggest Winston G. Gayler's The Apple II Circuit Description: I found a PDF at mirrors.apple2.org.za:
https://mirrors.apple2.org.za/Apple II Documentation Project/Books/W. Gayler - The Apple II Circuit Description.pdf
Appendix B focuses on the Apple II motherboard revisions, including the early Rev 7 which did not use A14 and later Rev 7 boards which do use A14 (for improved text mode color killing) . Pp 151 also notes, under "Revision 7 Non-RFI Mother Board", that:
"Note that A14 is inserted into the circuit differently on Rev 7 boards that it is on RFI boards." (I made that mistake too, comparing a troubleshooting board to a unbuilt kit RFI board.)
The Rev specific schematics you asked about are there as well.