Apple III Troubleshooting and Repair

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Apple III Troubleshooting and Repair

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 this screen.

https://www.applefritter.com/files/20180423_192943.jpg

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

https://www.applefritter.com/files/20180423_193013.jpg

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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I am having trouble also with

I am having trouble also with my A///'s But I'm a bit further along. I pass the mem test and am stuck on RETRY
I might suggest reseating the IC's on the motherboard. I think all teh IC's under the mem board is related to driving memory.

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No luck. I removed, checked

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).

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Do you have the service

Do you have the service reference manual?
It might help

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Schematics, plenty Manuals

resurection of an Apple /// should always start at:

Schematics, plenty Manuals and Documents as well as Software are availiable for download at:
http://www.apple3.org/index.html

sincerely
SpeedyG

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I've already spent too much

I've already spent too much time on a machine I don't really care about. Does anyone want this machine? I'll take some photos- it's in great physical condition.

$250+shipping.

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I'm interested.

PM me photos of what you have and I will make a decision in short order. Thank you.

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macnoyd wrote:PM me photos of

macnoyd wrote:
PM me photos of what you have and I will make a decision in short order. Thank you.

Will do this evening.

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Apple III Troubleshooting and Repair

I'd buy your machine in a flash, but as I'm in AU, then the shipping would be a killer.
Below is what I'd try before giving up....
CPU does not seem to be working?
Is it held in a permanent reset state? Check that 6502 pin 40 changes state when reset pushed.

Is the CPU getting a 1 or 2MHz signal on pin 39 or pin 37?
The Apple III can be switched in software (? environment register $FFDF bit 7) from 1 to 2 MHz. Is this system working or broken. That is: a clock signal of 1 or 2MHz should be on 6502 pin 39 and pin 37.

Check the clock generation circuitry.
There are 4 x 74LS161 counters used as dividers for the 14MHz signal. Are these chips all working? If you don't have a chip tester, try them in an Apple II as it has a similar clock divider circuitry.

Dead bus transcievers.
A common fault is to remove a card whilst the computer is active, this might blow one or more bus buffer chips. Check the tri-state bus transceivers. I suspect that they are the same as the Apple II, but without a III machine in front of me I can't be sure.

Hope that helps.
Laurie

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