Blinking "@", No Reset

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Blinking "@", No Reset

Postby minimicrite » Jun Wed 06, 2007 8:24 am

I've gotten everything soldered up, and all the initial tests went fine (power to proper pins, etc...). The first test with the Atmega8, 7HC74, and 74HC166 resulted in a row and a half of 0000's, followed by periods, until I powered down.

When I get to the final step of inserting the remaining IC's to complete the Replica 1, I get a blinking "@" on the screen, and no response from the keyboard (I've tried two). Additionally, pressing the Reset button does nothing.

I tried reseating the large IC's this morning before I left for work, but it did not solve anything. I've been staring at my soldering job hoping to spot a problem, but I haven't noticed anything yet. I'll do a high-res scan tonight and see if I can spot anything.

I've got a bit of debugging ahead, I know, but, based on the symptoms, should I be concentrating my efforts somewhere in particular?

FWIW-Keyboard is PS/2, power is ATX, and jumpers are set properly for both.
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Postby vbriel » Jun Wed 06, 2007 3:51 pm

Pressing reset to start is required since the Apple 1 didn't have an auto-reset circuit, neither does the replica 1.

It isn't the keyboard controller, or RAM because you would still get a reset "\" response anyways. It isn't the max233 because that isn't required for a reset to work either. Same thing as with the ATMEGA8515, even if it was removed, it would still display a \ when you hit reset.

Check the orientation of your oscillator. Pin 1 is the pointed corner and faces the max233 chip.

It could be, CPU, 6821, oscillator, EPROM (I tested all shipped) as well as any of the 3 glue logic chips 74LS00, 04 or 138.

Check your main clock signal first, pin 37 is the pin on the 6502 that recieves the 1MHz clock from the oscillator. Check to make sure that has a frequency of 1MHz. If you don't have a way to measure frequency that is ok, just check the voltage. It should be around 2.3 to 2.6V because it is oscillating at 1 million times a second.

Next, check the RESET line. When you hit reset, it should drop down to 0V until you release the button, then it should go back up to 5V.

Then check the Sync line on the 6502 (pin 7) and read what that is registering.

You can go through all the Address lines and see what address it is pointing to and see if they change with a reset. record your addresses.

I would focus on the oscillator (make sure your boards heart-beat is working), then check the sync signal and see if that is working.

That should give you some homework for a while. Remember, don't get frustrated, my first replica didn't work but I knew I could get it going. After some work, it did and here we are today :)

I also offer a free up and running program where I will get your replica running but I like people to try first. I have the luxury of having chips available to swap out so it's easier for me to work on. Plus I know how the replica 1 should react.

I've had a few people send theirs in and 50% of the time it is a missed solder pin on a socket. The other 50% was a defective chip. Unfortunately I can't test all the chips, only the programmed ones, so like most kits, there is a risk of a defective chip.

Let's see what you find first.

Vince
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Progress report

Postby minimicrite » Jun Sun 10, 2007 1:15 pm

I've been a bit busy since I put the Replica 1 together, but I have checked a couple of things and managed to get a scan of my solders.

* The oscillator is installed correctly.

* Reset functions properly. Voltage drop from 4.6v to 0v when switch is depressed.

Hopefully, I will have some time tonight to check the voltage on pin 37 of the 6502.

Unfortunately, I am limited to a simple multimeter.

Scan of the board (click for full size: 2550x1531):
Image

Thanks again, and progress report later,
-matthew surles
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Postby minimicrite » Jun Sun 17, 2007 12:06 pm

Update:

Clock signal, pin 37, on the 6502 is registering 2.8v. The sync line on the 6502 is registering 5v.

I think I've about exhausted my abilities with my voltmeter. I guess I'll be in contact about the up-and-running service.

Thanks,
-matt
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Postby vbriel » Jun Fri 29, 2007 3:37 pm

Just to update readers, the problem was a bad 6502.

Vince
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Postby Ckelly » Feb Wed 25, 2009 8:18 pm

After assembling a new Replica 1 TE, I am also getting this problem, i.e. I'm getting the blinking cursor on startup, but reset has no effect.

I have replaced the 6502 and 6821 with spares, but this makes no difference.

I have double-checked the oscillator orientation; it matches the PCB and the picture in the manual.

All power checks listed in manual are confirmed.

On the 6502:
---------------
all address lines read +3.6V
all data lines are ~0V
RDY (pin 2) is +5V
SYNC (pin 7) is 0V -- is this a problem? a previous poster said that his was +5V
Clock on pin 37 is +2.5V

The Reset button causes pin 40 to go from +5V to 0 for the duration of the press, then reverts to +5V.

Anything else I can check?

Thanks,
Christiaan.
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Postby Ckelly » Mar Sun 01, 2009 1:25 pm

After discussion with Vince, the solution turned out to be with the 27C64 EPROM. The solution is to tie pin 27 to +5v.
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