Gigatron trying to start but doesn't

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swperk
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Joined: 13 Jul 2019, 20:37

Gigatron trying to start but doesn't

Post by swperk » 13 Jul 2019, 21:13

I just finished assembling my Gigatron kit. I followed the manual, verified that the power supply was working, and then verified that the clock circuit was active. However, after I had installed most of the ICs and got to the part where I apply power and see the "blinkenlights," I didn't see them.

What happened instead was that the power LED lit up and stayed on, then the leftmost blinkenlights LED lit up, then the remaining three lit up a fraction of a second later. All five LEDs then remained on steady.

I confirmed that all ICs were installed in the proper place and in the proper orientation, along with the diodes, electrolytic cap, and the resistor arrays. I checked every solder joint and ruled out any shorts or bad joints.

Then I used the troubleshooting spreadsheet and confirmed the values on the sheet for all the ICs. Incidentally, I found what I believe to be an error in the spreadsheet. I believe that the values for U17 Pin 14 and U21 Pin 14 should be swapped.

Vcc is 5.19 volts, CLK1 and CLK2 are 6.25 MHz. Looking at the A15 test point, I see activity sometimes. In other words, sometimes when I power up the Gigatron, A15 is steady at 0 volts. Other times when i power it up I see a TTL waveform. I don't see any correlation with A15 activity (or lack thereof) and the behavior of the LEDs. The voltage across each of the four blinkenlights LEDs is steady DC. Is that the way the 74HCT273 is supposed to drive them, or should there be some sort of multiplexing waveform even when they are on steady (not blinking)?

Finally, my VGA monitor seems to be trying to recognize something coming out of the VGA port every so often, but nothing ever displays and it goes back to standby.

I'm stumped, so any and all help is very much appreciated!

Thanks,
Stan

swperk
Posts: 11
Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 13 Jul 2019, 23:50

Replying to my own post: I just resoldered every IC pin, diode, and bypass capacitor on the board, and the behavior is unchanged. Of course, I also checked for solder bridges, unsoldered or unseated pins, etc. and found none. I'm running out of ideas...

Stan

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 14 Jul 2019, 00:36

swperk wrote:
13 Jul 2019, 23:50
Replying to my own post: I just resoldered every IC pin, diode, and bypass capacitor on the board, and the behavior is unchanged. Of course, I also checked for solder bridges, unsoldered or unseated pins, etc. and found none. I'm running out of ideas...
Still this is exactly the right approach to solve this symptom. I've seen exactly this several times before now. Your system is running code and gets pretty far doing so!

As xopr has remarked earlier: a connection can be 'ok' at low frequencies, such as tested with a multimeter. But still fail on higher frequencies.

To answer your other observations:

1. A TTL signal on A15 means it's repeating a full-memory scan, but it doesn't come out of it. In the ROM code it does such a scan exactly once: when you observe the left LED momentarily lit all by itself. If it's repeating this signal, it doesn't jump out of this loop (or it keeps resetting). That can be a soldering joint in the ALU, MAU or PC section. That's where I would start looking, but in reality it can be anywhere. It can also be a bent pin, like one of the issues we found in this thread.

2. The four LEDs on the left side get a steady signal from the XOUT register. There's no high-frequency modulation.

3. Thanks for the note about the spreadsheet. I'll check it later and update if needed.

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 14 Jul 2019, 13:37

4. Your Vcc is beautiful. I don't suspect resetting is an issue. (In that case you'll see the "Power OK" led flicker anyway.)

About the reference sheet: you're right of course: U17-U20 are all the same, and U21-U24 are all the same. Thanks for spotting, I updated the document.

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 19 Jul 2019, 19:58

So I did a little more investigating, and I see nice looking pulse waveforms all around the circuit board. Then I checked the cathodes of all of the signal diodes around the instruction decoder (U14), and I found pulse waveforms on all of them except D32 and D33 (cathodes are wired together to U14 Pin 10) which are stuck high. I have not delved into the logic to see if this is expected behavior. Is it?

Thanks,
Stan

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 19 Jul 2019, 23:31

That means it's stuck in a software loop that exercises all instructions except subtract. If it has 7 different instruction types in it, that certainly isn't a simple small one! It can't be the memory scan mentioned earlier (ROM address 0020), because that one doesn't have the logical AND and OR instructions in it. I don't spot any clear candidate in the boot code: it doesn't have the logical OR instruction anywhere. It also can't be the video and virtual CPU already, because those have subtracts all over the place.

In this case to satisfy my curiosity I typically trace the program counter to get the addresses, and then use the ROM disassembly to figure out which loop it doesn't get out of. Although it's a rewarding exercise in understanding the guts of the system, in my experience, it can be quicker to find the joint with visual inspection, a loupe and good lighting.

My hunch: ALU is a suspect. All of its 10 chips must work together for the "isZero"-detection required for escaping from many of the software loops. But it can be any connection, including diodes or one of the memories.

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 20 Jul 2019, 00:14

Okay, I'll definitely have another look! What are the chances that there is a defective IC?

If I don't have a logic analyzer handy, what are my other options for testing?

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 20 Jul 2019, 00:43

swperk wrote:
20 Jul 2019, 00:14
What are the chances that there is a defective IC?
That would be a first in hundreds of kits, while the symptoms you describe are common.
swperk wrote:
20 Jul 2019, 00:14
If I don't have a logic analyzer handy, what are my other options for testing?
I don't have one either, I just use my scope. But with a bit of care, even a volt meter can figure out where it's looping.

swperk
Posts: 11
Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 20 Jul 2019, 01:01

I've got a scope and a logic probe, and so far here's what I've determined:

Looking at the program counter (U3-U6) I find that PC0-PC7 are counting as expected, PC8 is stuck low, PC9 is stuck high, and PC10-PC15 are stuck low.

Where should I start looking next?

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 20 Jul 2019, 01:10

And something else new: Now when I power up the Gigatron, I get the usual startup display of the 5 LEDs lighting up momentarily, but after a second or so, the pattern on the 4 blinkenlights changes. Sometimes none are lit, sometimes the leftmost one is lit, sometimes the leftmost three are lit, and sometimes all 4 are lit.

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