Replica I rev. C used to work

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Replica I rev. C used to work

Postby sandro » Sep Tue 03, 2013 12:51 am

I have a Replica 1, rev. C from 2004-2005. I haven't used it in several years, but I just cracked it out to prepare for a campus retro-computer fest. Saddly, it's not responding to keyboard/reset. I just have a solid block cursor in the upper right corner.

I assume that either the 6502 or 6821 is fried, but I wanted to get some help troubleshooting before i just started pulling chips. My old manual really didn't have too much in the way of debugging info, and I'm not entirely sure how much the TE is different from mine.

Is there a standard practice for determining if either the 6502 or 6821 is good?

Any debugging tips would be greatly appreciated.
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Re: Replica I rev. C used to work

Postby PaulB » Sep Thu 05, 2013 7:07 am

Hi,
If you haven't done this already, try pulling the chips and re-inserting them, it may just be bad connection.
If that doesn't work you may have to ask Vince for some new chips. :roll:

Also check for shorts across your board, make sure no dust or foreign stuff is causing problems.

Good luck with it and the fest, sounds like great fun. :)
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Re: Replica I rev. C used to work

Postby vbriel » Sep Fri 06, 2013 5:32 pm

The 6502 doesn't require RAM to start the Woz monitor. So, even without RAM, if you press RESET the Woz monitor will still display the backslash. However, it will not advance to the next line. So, RAM is not the issue. If you have a way to test signals, you want to test if you have a 1mhz signal on phi2 pin 39. If you only have a volt meter, it should register about 2.5V if oscillating at 1mhz. If you have this, then the 1mhz oscillator is functioning.

Next, test the sync pin 7 and see what is on there. If the 6502 is operating, the sync will pulsate at different various rates depending on instructions. Last, check the ready pin to make sure it isn't being pulled low at pin 2.

An obvious would be to check voltages at each chip. It is hard to test the 6821 but you could test the output pins that go to the video chip ATmega8 in your case for changes in the output.

Usually what fails in the 6502 or the 6821 but in rare cases, it is the glue logic 74LSxx chips. I've not seen a 28C64 go bad yet, but they haven't been 8 years old yet so you can't rule that out. Just some things to work on.

Vince
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