hello,new question about apple II

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Joined: Jul 13 2007
Posts: 179

recently,my apple II clone has a little problem,I plug tv's av connector.original it will display color screen.but last week,it show gray screen,especially games.only black and white,do you have this experence???how to find miss ic chip.thx

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speedyG's picture
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Joined: Nov 16 2011
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Re: hello,new question about apple II

hello,
the big problem is starting with the word "clone" - you never know how close/similar the "copycat" is to the original...
For example the clones with 2 CPUs on Board ( 6502 and Z80 ) have been altered so much that they are nearly not comparable to the original apple II-series.... on the other hand and under the condition that the clone is very similar to the original the following statements might be valid:

there are several components responsible for the Generation of the videosignal:

In Position A9 there is an 74LS151 that is responsible for mixing the video data that comes from the Character generation an from Hires... but in the main for the color responsible are the foloeing components:
position B13 an 74LS02 and thereafter at the position B12 an 74LS11 with the pattern running up to the resistor R5 ( 1kOhm Value or in rings brown, black and then red followed by silver or gold ) and then the line continues to a coil L1 with the value of 27 µH then to the capacitor C2 with the value of 47 pF and then passing a capacitortrim C3 with the value 5 to 50 pF ending up at the resistor R6 with the value 2,7 kOhm ( again in rings : red, violett,red and the silver or gold ) and then the Transistor Q3 which should be 2N3904 . The Collector of that Transistor leeds up to the video levelregulator ( resistortrimmer R11 with a value of 200 Ohms and is located close by the Videojack ( a Chinchfemale ) on the back where you plug in the videocable. [b]This entire line from location B12 ( the 74LS11 ) up to the Transistor is responsible for the generation of the so called "color burst" signal that contains the colorinformation of the videopicture.[/b] The first guess will be to open the case and to start the computer very carefully and thereafter try to adjust the colorburst signal at the capacitortrimmer C3 between row F and row H at the position outsde of Row 14 ( it looks like a small capsulated staple of metal circled plates with a very small kind of "screw" in the middle of the "Tower". Use a isolated nonmagnetic very small screwdriver preferrable with a plastic TIP !If that does not help then you might try to replace th ICs at position B10 ( an 74LS74 ) ; at position B13 ( the 74LS02 ) and the next IC at position B12 ( the 74LS11 ). Try to use Chips manufactured from Texas Instruments if possible because their timing is closest to specifications and best possible quality....
Probably it might be usefull to change the Transistor Q3 ( a 2N3904 ) at the position behind the rows G and 14 close to the outside of the board and close to the resistortrimmer R11 that is responsible for the level of the videosignal ( the Transistor might have been aging faster if it got too hot ). NOTE: Exchange of the components or the Transistor of course are ONLY to be EXECUTED at the computer SHUT OFF ! DON´T CHANGE COMPONENTS ON A RUNNING COMPUTER WITH POWER ON ! This will most likely kill the Computer-Mainboard and the powersupply ! In Case of missing experience with a solderiron ask an experienced friend to support you in that task ! Check out all solderpoints in this described path of the "color burst" signal and probably resolder them. The problem might result from an aged solderpoint or a so called "cold solder point" which means that it was not soldered correctly and started corroding.
If this stuff is to complicated - I urge you to consult an experienced friend or technician !

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gsmcten's picture
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Re: hello,new question about apple II

Adore,

send me a pm with your email address. I have a schematic JPEG for you.

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Steven (gsmcten)

"Apple ][.....It's ALIVE!!!!!"

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Joined: Jul 13 2007
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Re: hello,new question about apple II

hello

you can send this mail kankuochin at gmail.com

thx

speedyG's picture
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Joined: Nov 16 2011
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Re: hello,new question about apple II

hello, checkout your email in something like an hour... I´ll scan the relevant parts and mail them as PDF-Files
and you might give me a short feedback that the PDFs have been successfully recieved...

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

speedyG's picture
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Re: hello,new question about apple II

the PDFs have been sent and should be in your email. just give pm that mail was recieved OK....

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....