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Apple III Troubleshooting and Repair

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rittwage

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I acquired a non-working Apple III over the weekend from VCF SE. It is surprisingly clean (I think the just-previous owner really cleaned it up) but as advertised, it does not work.

Power is good- I get 5v, 12v, everything there tests fine. It has a 12v 128k RAM board in it. I have removed all the chips and reinstalled them, checking for proper positioning and bent legs. I didn't find any problems, and none of this affected the screen output. I've even moved around the RAM (within the same bank only, it has a mix of 8k and 4k chips).

When I turn it on with everything installed, I get the first screen.


If I remove the memory board, I get the second screen, but it's not static- the dots move around like a TV tuned to a dead channel.


Has anyone troubleshot a III before and know where to start? The service manual doesn't seem to be much help- it says replace memory board or main motherboard. Real helpful...

BTW, it does the exact same thing without the CPU installed, so I don't think that's even running. I think the screen is black if I remove the ROMs.
 

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So you got that Apple III. Very nice! I was tempted to get that, but I have enough projects right now that still need my attention.

Does the motherboard have its original sockets? From what I have read, these use awful low quality sockets. If it were me, I would start by using an ohm meeter and testing the continuity between each chip leg and the pins on the bottom of the motherboard. I had to do that to get an Apple II Plus motherboard working a little while back. (I think the II+ uses the same kind of sockets).

Is there any corrosion or visible damage inside? If not, then good deal. Even a pristine looking socket can have problems The Apple II Plus motherboard I mentioned visibly looked almost brand new, and just removing and cleaning chips did not help.

Long term you might want to save yourself some sanity and replace the sockets.
 
It looks pretty good- no corrosion that I can see. I will start checking the sockets... that will be a big job. The board is full of TTL like the Apple II/+. I expected it to have VLSI, but as far as I can tell there is little to none. I guess this is still essentially a late 70's design.
 
No luck. I removed, checked the legs, and re-seated every chip on the board, twice. I tested all the 4116 RAM, one was bad, replacing didn't change anything. I tested all the pins on the chips to the corresponding pin on the bottom of the socket (bottom of board) - this was tedious, but I didn't find any breaks.

The symptom is the same. I don't think the CPU is running at all, since removing it has no effect. I don't think there are schematics out there for the ///?


I also have no way to test the 8K RAMs. I don't suppose 4K RAMs can be swapped in like the second row is? (They leave one row of pins unused in the sockets).
 
Have you tried picking it up 4" and dropping it? (Please don't.)

You can test if the 6502 is running with a multimeter.
 
Have you tried looking at this repair guide?
ftp://ftp.apple.asimov.net/pub/appl...ple III Service Reference Manual-OCR-1982.pdf
It's pretty technically dense, but it has a ton of great information, schematics, etc.

I'm having a very similar issue with my III right now - my screen displays something very similar to the first picture you posted, except some of the characters on mine flash every half second or so. I've also checked my supply voltages, replaced the RAM with known good chips... nothing. Maybe we can work together to figure out what's going on.
 
fri0701 said:
Have you tried looking at this repair guide?
ftp://ftp.apple.asimov.net/pub/appl...ple III Service Reference Manual-OCR-1982.pdf
It's pretty technically dense, but it has a ton of great information, schematics, etc.

I'm having a very similar issue with my III right now - my screen displays something very similar to the first picture you posted, except some of the characters on mine flash every half second or so. I've also checked my supply voltages, replaced the RAM with known good chips... nothing. Maybe we can work together to figure out what's going on.
Click to expand...


Thanks, it belongs to KC9UDX now, so you guys could work together on it. I did get some flashing on power up sometimes. What is on the screen always followed a similar pattern, but did vary a little. I am pretty sure the CPU was not running.
 
Thanks for finding that PDF. I tried numerous web searches to find useful information and came up with nothing. Hopefully somewhere in that monstrous file are complete schematics.

The CPU is probably running, I haven't much of an opportunity to dig in too deep. But the reset circuit is not working so the CPU probably isn't doing anything useful. I ran out of patience trying to figure out the reset circuit so I've been trying to find schematics. I did run into some power problems, possibly due my disassembly, but I seem to have that sorted out. I also found a couple chips running far hotter than I think they should.

 
I have seen those schematics, and they are incomplete. Unless I missed it, I couldn't find the timer circuit for /RESET.

The reset circuit is partially working, with or without the keyboard. It goes high too quickly. I haven't had a chance yet to try to force it low for longer to see if the system does something different.
 
On the Mac II and IIx, one of the Sony audio chips is involved with timing /RESET. I'm not mentioning it because I expect it to be identical on the III, but in case it gives you a new idea of where to look.
 
Thank you. This thing probably doesn't have any hardware like that. But it is helpful to understand the thought process of the designers. This /// is an odd duck.
 
KC9UDX said:
I have seen those schematics, and they are incomplete. Unless I missed it, I couldn't find the timer circuit for /RESET.

The reset circuit is partially working, with or without the keyboard. It goes high too quickly. I haven't had a chance yet to try to force it low for longer to see if the system does something different.
Click to expand...

If you look closely at the keyboard schematic, reset is indeed generated on the keyboard circuitry schematic.
The IORESET line is buffered by a 74ls05 using (open collector) pull up resistor because it is designed to work with other reset sources (from cards).
RESET is different, it is buffered with a 74ls14 which is not open collector, so it must be the only output connected to the line.
The length of the reset pulse based on the components in the schematic calculates to about .25 seconds. To force a longer reset just use the reset key on the keyboard.

Bottom left
http://www.apple3.org/Documents/Schematics/Keyboard Logic.jpg
 
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Hi, i get also an apple iII with similar issue, is 5v ram, but get same kind of boot screen. Some news ?
 
Likewise (mine just powers up with garbage on the screen, though Ctrl-Open Apple-Reset does seem to do "something"). I haven't yet tried to reseat all the chips yet but this seems like a common failure mode.
 
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