Nothing happens when I press reset

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Nothing happens when I press reset

Postby vbriel » Feb Thu 05, 2009 9:42 am

This is the most common issue with builders so I have a general set of things that I ask builders that I wanted to put as a sticky on this page to help out.

This is the most common problem replica 1 builders have. Getting the flashing cursor at the top of the screen is important but it is the easiest part to get working since the video section is only two chips (Propeller and 8 pin EEPROM) and a couple resistors. The replica 1 and the apple 1 are divided into two sections, the terminal section and the CPU section. On the replica 1, the terminal section is the 2 chips and the CPU section is the remaining chips on the board.

When there is a problem with the CPU section the reset button will not get any results on the screen. In other words, you should see a \ character followed by the cursor moving to the next line. When this doesn't happen there are many reasons, and the CPU is only on possibility.

While it is common for people to want to replace the 6502 right away, it may not be the problem. Your problem could be with the 6821.

Gotta ask, how good is your soldering? Is there any flux left on the board? I have seen this actually cause a problem. Here is the biggest causes of the CPU not resetting:

1. Incomplete soldering - Pins on a chip or 2 not soldered
2. Cold soldering joints - If you solder points on the chips look like triangles, you are good, if they look like basket balls there is cold solder spot
3. Bad 6821 PIA - I have started testing every one of these again as it appears that Jameco is supplying bad batches again
4. Bad 6502 - mos chips are subject to static damage and can fail
5. EPROM - While I do test these after programming them, they can fail

I have not had anything else cause the CPU circuit to fail. There are other chips onboard but they would not keep you from getting the \ upon pressing reset.

If the RAM chip was removed, you still would get a backslash on reset but the cursor would stay on the same line, not do a carriage return.
If the MAX232 was removed, the serial port would not work but you still would function with keyboard and video just fine.
If the 74LS245 was removed, the keyboards would not work but you should still get a \ upon pressing reset

The other 3 TTL logic chips are important to the memory decoding and other functions and would cause the CPU to not output a backslach "\" character but I have never seen these fail.

Read the other forum topics on how other builders resolved their issue. I will add a series of things to check when your board doesn't work.

Please keep in mind the 6502 and 6821 are NOS chips and are subject to static damage more than the other chips, so handle them with proper static precautions.

Vince
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