Gigatron trying to start but doesn't

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marcelk
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Joined: 13 May 2018, 08:26

Re: Gigatron trying to start but doesn't

Post by marcelk » 20 Jul 2019, 01:36

swperk wrote:
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?
That's interesting, the system makes it's way though page 0 (boot sequence) and page 1 (vertical blank) into page 2 (visible scanlines). Normally it gets stuck on page 0 already. In page 2 we use more addressing modes. So don't forget to check the soldering connections of the resistor arrays and the resistor in the "Wired AND". The voltage check sheet doesn't have these components, only the chips.

You can also try to apply a small force on some areas of the board while powering on. Then observe if the behaviour changes. This can help to locate the approximate area of the weak link.
swperk wrote:
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.
That's fully consistent with the hypothesis (and also mentioned on the troubleshooting page):
https://gigatron.io/?page_id=880 wrote: System doesn’t start

Blinkenlights don’t come on, or
Blinkenlights come on in random state and stay there, or
Blinkenlights change briefly but then nothing happens, or
The video looks completely garbled.

If the “Power OK” LED is lit and stable, but still the computer doesn’t start properly and there is no video (or only garbled video), the most likely cause is a poor soldering connection for one of the pins. If this is the case, the system may work perfectly fine on one day, and be completely broken on another.

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 21 Jul 2019, 23:40

After resoldering everything (again), examining every solder joint with a loupe, and heating and cooling the board to see if the behavior changes, nothing is different. I've captured waveforms from several points, and although most look reasonable, I have found some questionable ones (to me, anyway).

I captured a couple of different waveforms from U17 pin 14, and they look very glitchy to me.
U17P14a.png
U17P14a.png (23.55 KiB) Viewed 552 times
U17P14b.png
U17P14b.png (25.59 KiB) Viewed 552 times
The general shape (and glitches) are also on pin 14 on U18, U19, and U20. Conversely, probing pin 14 on U21-U24 all show a very clean waveform.

I probed the anode of D3 (part of the AND gate)
D3Anode.png
D3Anode.png (23.1 KiB) Viewed 552 times
and it looks very different from the waveform on its cathode,
D3Cathode.png
D3Cathode.png (23.06 KiB) Viewed 552 times
like there's a capacitive load. I'm not sure if this is significant.

As before, any and all suggestions are appreciated!

Thanks,
Stan

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 22 Jul 2019, 06:47

The glitchy signal at pins 14 on the right-hand side of the ALU are incoming data bus lines. There's all kinds of different activity on there before they settle, depending on the instruction being executed. Here's one snapshot on one of my boards:

DS1Z_QuickPrint19.png
DS1Z_QuickPrint19.png (40.74 KiB) Viewed 545 times
DS1Z_QuickPrint22.png
DS1Z_QuickPrint22.png (41.6 KiB) Viewed 545 times

On the left-hand side of the ALU the equivalent pins are pins 2, because AC inputs and BUS inputs swap sides for easier routing.

The pull up curve at the wired-AND is beautiful.

I would try to measure the resistance to Vcc of the pins >1 on arrays R4 and R5. Preferably through a trace leading to a pin on a connected component (so not directly on the array pins themselves).

If those all read 2.2K, I'm gravitating towards a short between two adjacent pins somewhere. The diode-mode voltage check method is blind to most of those.

P.S: I sent you a private message you may have missed.

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

Re: Gigatron trying to start but doesn't

Post by swperk » 27 Jul 2019, 14:35

Thanks for the additional info. I've spent the last few hours going over the board with high-power loupes looking for solder bridges, unsoldered or poorly soldered joints, and bent pins, and found nothing wrong. I confirmed (again) that all of the ICs were in the proper places and correctly oriented. I checked that all of the ground pins on each IC are, in fact, a low resistance to ground.

I'm at a loss as to what to do next. Any other suggestions?

Thanks,
Stan

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 27 Jul 2019, 16:19

swperk wrote:
27 Jul 2019, 14:35
I'm at a loss as to what to do next. Any other suggestions?
I'm sorry to hear. But don't worry, we've always managed to get them working. The components are surely in the correct orientation. All tips and experience we have are collected in the assembly manual with additions on the troubleshooting page. You may also take inspiration from the tips in other threads here, such as this thread. Have you measured the resistances? And tried switching it on and off while applying gentle pressure to different parts of the board? There is an additional idea I sent in a private message on July 20th. May I also suggest to take high resolution images of the contacts?

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 12 Aug 2019, 02:29

Good news! After a very educational session with Marcel, we determined the problem to be with the ROM. Marcel sent me a new one, and the Gigatron started right up! Thanks again for all the help, Marcel!

swperk
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Joined: 13 Jul 2019, 20:37

Re: Gigatron trying to start but doesn't

Post by swperk » 16 Aug 2019, 21:18

I'm trying to post a photo of my now happy Gigatron, but I keep getting "HTTP Error.' Is there a filesize limit? Is JPG not allowed?

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marcelk
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Re: Gigatron trying to start but doesn't

Post by marcelk » 17 Aug 2019, 06:01

Indeed, there is a file size limit, supposedly of 5 MB per attachment. "HTTP error" is phpBB's poor way of saying "too big" (or "wrong extension"). jpeg and jpg is fine of course. The forum doesn't scale down images by itself. I normally have to make pictures a bit smaller before uploading.

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

Re: Gigatron trying to start but doesn't

Post by swperk » 17 Aug 2019, 13:03

Here's a photo of my Gigatron now happily flashing its Blinkenlights! Thanks, Marcel!
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Gigatron1.jpg
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