128k logic board problem

[josefl710]

I'll have to see if an SE or a SE/30 donor board will have compatible chips ( i have two of those from leaked battery units), id hate to pull them from my working plus

 

[gooddi]

I'll have to see if an SE or a SE/30 donor board will have compatible chips ( i have two of those from leaked battery units), id hate to pull them from my working plus

you may need 512k board than plus.

when i try to replace LS166 with plus's ic, it does not works.

i hope you get the result finally.

thanks for reading.

 

[techknight]

you forgot the 74F253s. Those are the muxes for the row/column address strobes for the RAM, if those are bad, youll get this screen as well as sad macs, depending on which ones bad. 

Also, Keep in mind you may need to start looking at the PALs. These machines are old enough that if they were stored in a moisture laden environment for 30 years, they will seep through the epoxy and start attacking the silicon die, and you get this.... 

 

[gooddi]

you forgot the 74F253s. Those are the muxes for the row/column address strobes for the RAM, if those are bad, youll get this screen as well as sad macs, depending on which ones bad. 

Also, Keep in mind you may need to start looking at the PALs. These machines are old enough that if they were stored in a moisture laden environment for 30 years, they will seep through the epoxy and start attacking the silicon die, and you get this.... 

thans for your very useful information.

i will investigate 74F253S.

and which one is PAL ? Please tell me more about PAL.

did you mean ASG? i can't find PAL on my logic board.

WIKIPEDIA say that about PAL

 - BMU0/1, TSM, TSG,LAG,.....

RIGHT?

Thanks again, techknight.

 

[josefl710]

Found an old article from byte magazine that lists a logic diagram, perhaps it can be of some use here

 

[gooddi]

Found an old article from byte magazine that lists a logic diagram, perhaps it can be of some use here

wow... great!!

thanks ^^

i will try this at weekend and report the result.

have a good day

 

[techknight]

Even though I got a million and one things on my plate right now, I need to take on one of these boards with issues like this just to see if I cant figure it out... 

 

[Elfen]

thans for your very useful information.

i will investigate 74F253S.

and which one is PAL ? Please tell me more about PAL.

did you mean ASG? i can't find PAL on my logic board.

WIKIPEDIA say that about PAL

 - BMU0/1, TSM, TSG,LAG,.....

RIGHT?

Thanks again, techknight.

PAL - Programmable Arith-Logic Unit, similar to an EPROM/ROM. They only contain a few bytes of code (less than 500 bytes, but some can be 2 - 4 kb in size) usually for the I/O Chips and not the main CPU. They also behave like tiny cpus - 4 bit jobs like in a calculator, doing repetitive loops with the I/O Chips.

Fun Stuff they are.

Side-Rant-o-Fact - the Apple II Disk Drive Card has 2 PALs for the Disk Read/Writes. (I forget which ones) If one blows, it becomes a killer card that kills a couple of chips inside the Disk Drive itself. If the other one blows, any disk you put to either drive, that disk gets erased.

 

[josefl710]

I did a good check with magnifying glass and even traced a good chunk of connections using schematics found at this forum but everything checked out. So I see why everyone says cold solder joints and traces are rarely a problem. I have two working 512ks somewhere in my pile. When I get some free time il have to try and use one of them as a donor board.

Teknight, I can't think of anyone else more qualified to get to the bottom of this. But I'm curious what your troubleshooting process is. Do you get a jig extension and power the board up while probing every chip for expected logic output? Is an oscilloscope your best bet, or perhaps a logic analyser? Or desolder one chip at a time and put it on a breadboard to test it's full functionality against its data sheet/logic truth table. If only that equipment was not so expensive I would think it fun to get to the bottom of the issue.

Some IC somewhere in these boards has gotten nasty bit rot I say!

 

[techknight]

I look up the function of each and every IC in the datasheet, and follow the truth table. 

Then, i hook up an o-scope, 1 channel on an input, and 1 channel on an output. I basically figure out if the chip is following its truth table. If not, then its bad and has to be replaced. 

And this is the hard way. The better way is have a working board side-by-side and compare scope shots, its really easy to figure it out at that point. 

 

[josefl710]

Thanks for that technight, I just got me a cheap oscilloscope this weekend (birthday present from wife ) now looking forward to tackling this board some more. Even got me a cheap USB based logic analyzer! But that's still in its way. I'll update this thread if anything new develops!