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Just the @ Symbol

PostPosted: Aug Wed 25, 2010 1:46 pm
by littlenasboy
I just put together the replica I TE revision 3.

I only get the @ symbol when I turn it on. I made one mistake when I was soldering and put the ASCII keyboard socket in the place of the 74LS138. I tried to remove it but couldn't with the tools I have (only solder wick), so I left it in place because the IC fit in the socket anyway. PIN 1 on the 74LS138 looks bad after my attempted rework, so I jumped it to pin 2 on the 62256. It didn't help. I'm not sure if the logic chip is the problem.

I've check all the voltages listed in the manual and they look fine. I'm pretty sure the oscillator is in correctly. I don't see any open pins.

What else can I check? How can I wash flux off the board?

Please advise...

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 3:03 pm
by littlenasboy
I'm getting 1Mhz on the clock line to the 6502...

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 6:06 pm
by jsissom
What happens when you press the reset button?

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 6:12 pm
by littlenasboy
Nothing...I still get the blinking @ symbol...

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 6:14 pm
by littlenasboy
also, the reset button seems to providing the right signals...

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 6:28 pm
by jsissom
I believe the @ signal comes from the CPU that is driving the display so if reset doesn't cause the \ to appear, this would indicate to me that the 6502 might not be running. I'm not sure the best way to test this without an oscilloscope. With one, you can monitor the address lines to see if they are changing.

Re: Just the @ Symbol

PostPosted: Aug Wed 25, 2010 7:18 pm
by littlenasboy
I wish there was a simple way to see if the chips were working. I just ordered some chips (6502,6821,62256,74xx) from jameco to replace the ones I have.

Re: Just the @ Symbol

PostPosted: Aug Thu 26, 2010 9:27 am
by vbriel
Here's some homework to help. To see if the CPU is working, look at the READY and the SYNC signal pins on the 6502. Go find the 6502 datasheet and read the descriptions on those signals. There are clues in those signals if the CPU is working. The 6502 and the 6821 are the most suspect as they are most susceptible to static charge damage. I almost never run into a bad 74LSxx chip. Can you scan the bottom of the board to take a look at the solder? I'd like to see the area where the 74LS138 socket was desoldered.

Also, if you don't have a way to measure frequency (I have this option on my multimeter) then measure the voltage on the clock signals pins 37 and 39. A 1mhz clock signal will produce around 2.5V since it is pulsing.

Make sure you are pressing RESET to start the CPU. I know it sounds dumb but some people don't know that the apple 1 (and the replica 1) have no auto-reset circuit on it. When you press reset, do you see any change in the address bus signals on the CPU or the DATA bus. What are the video out pins registering on the 6821 pins 10-16 as well as 17-19 status signals.

The circuit will still reset without the max232, 74LS245, propeller and 8 pin EEPROM. Without the RAM chip, the reset will display a "\" but will not move the cursor down to the next line and you can't type anything in. This means in order to get the circuit working to a reset startup stage, you need the 6502, 6821, 28C64 EEPROM (or EPROM if 27C64 is supplied), 74LS138, 74LS00, 74LS04 as well as the 1mhz oscillator.

Vince

Re: Just the @ Symbol

PostPosted: Aug Mon 30, 2010 9:28 pm
by littlenasboy
I've replaced the 6502, 6821, 74LS138, 74LS00, 74LS04 chips and it still won't reset. (I guess that leaves the EEPROM.) All roads point to a problem with my soldering, but I still can't find it.

I do see 1mhz on pins 37 and 39. The address lines are all switching at various rates in the 100 kHz range. Pressing reset makes them all go low for a second, but then they do exactly what they were doing before.

I'll work on figuring out what the video out pins registering on the 6821 pins 10-16 as well as 17-19 status signals.

As soon as I find a scanner I'll let you let you see the bottom. Maybe I'll take a picture of it with my digital camera in the mean time.

Re: Just the @ Symbol

PostPosted: Aug Mon 30, 2010 9:44 pm
by littlenasboy
Would you be willing to send me another 27C64 just to rule it out? I could pay for it if necessary.

Re: Just the @ Symbol

PostPosted: Aug Tue 31, 2010 6:21 am
by vbriel
Yes, email me at vbriel@yahoo.com I'll get one out to you. Also, you don't have to worry about the video section as you are seeing a @ flashing cursor on the screen. Pressing reset does not reset the CPU or, if it does, it is the EPROM that is defective. When you inspect the bottom of the board, no bridges or anything like that?

Vince

Re: Just the @ Symbol

PostPosted: Sep Thu 02, 2010 7:18 pm
by littlenasboy
Let me know if you see anything in the scan.

Do you need my address to send the EEPROM or do you have that on file?