Apple IIc jumps to the monitor

[YannAros]

Hi,

 

I just got hold of an Apple IIc that has strange symptoms.

I do not have any floppy to test, I wanted to create some with ADTpro but I can't reach this point.

 

I can boot and reach the BASIC prompt using Ctrl+Reset.

Everytime I hit enter after having entered a command the Apple either jumps to the monitor or freezes.

When in the monitor I can read adresses and exit monitor.

I've read adress 64447 (0xFBBF)  to confirm that the ROM is version 255 (too bad I can't do a RAM Check)

 

Someone shall send me another ROM with a newer version to be able to launch the RAM test.

That would rule out 2 possible causes, the ROM and the RAM.

 

Do you have any other suggestions?

Did anyone see this before, I didn't find anything related by googling.

 

Thanks

[Baldrick]

My guess is that there is a bad RAM chip in there somewhere.

It's a common problem, and I'd very likely change all of the RAM, at least the lower 64K and start there if you don't want to upgrade the ROM at this point.

 

[YannAros]

Hi,

 

Thanks for the input, I think I will have to do this and start to desolder all RAM chips and put in sockets

 

[Keatah]

Apple was using rather unreliable MOSTEK 4164 for a long time in the //e. Wonder if that's the case here?

 

[YannAros]

I’ll have to check further but the chips have the apple logo on them

[Keatah]

Apple would buy from multiple vendors and stamp their logo on them. It didn't indicate extra reliability or anything.

 

[YannAros]

Yes these chips are just rebranded by Apple. I’ll start my de soldering job soon and will have a closer look at the chips.

[YannAros]

Some good news (at least for me ?). 
I’ve received some disks and an eeprom with ROM v4 on it.

First tested the disks and the games booted and ran fine…

Basic was still not ok, but after ROM replacement it was all fine. The now working self test showed no error.

 

So I have certainly a ROM that starts to fail and some part of basic is corrupted.

 

It might help someone in future having similar symptoms.

Edited June 25, 2021 by YannAros

[bluejay]

On 6/19/2021 at 3:46 AM, Keatah said:

Apple was using rather unreliable MOSTEK 4164 for a long time in the //e. Wonder if that's the case here?

 

Thought they used MT4264s? From what I understand they can be replaced with a 4164 but are not exactly the same.

 

I wonder how the ROM went bad. Perhaps it was just bad contacts between the socket and the chip (assuming it was socketed at all in the //c)?

Edited June 25, 2021 by bluejay

[YannAros]

I've cleaned up the pins and the socket and re-socketed the ROM several times without success.

The new one worked at first start.