Power LED is not steady

Power LED comes on but flickers from time to time.

The board is sensitive to supply voltages lower than 5V, and some power supplies don’t quite deliver that. Try some other supplies. Or check your supply as we show in this video:

Duration: 3m14s

Alternatively, you can try if a different USB power cable helps in combination with your supply.

Low voltage from clock generator

At the end of assembly step 3, the clock voltage is low and unstable.

This is the symptom for a cable break in the probe cable. The voltage on the USB power input will appear correct, and that can make it confusing. Try a different multimeter.

No test pattern after build step 7

After installing the RAM and EPROM, the LEDs don’t run in a pattern.

Although you haven’t installed the rest of the components yet, at this stage the CPU is complete and should be able to run programs. This must become evident from the scanning LED pattern, as Walter shows in this video.

If the “Power OK” LED stays off, first use a multimeter to verify there’s no short circuit, for example in the same manner as at step 1.

If the “Power OK” LED is off or flickering you can also check the power supply as explained above.

If the “Power OK” LED is lit and stable, double-check that all components are placed in the correct orientation: the ICs, resistor arrays, LEDs/diodes and electrolytic capacitor have a polarity that matters. Start by checking the ICs and diodes. Don’t forget to check the orientation of the resistor arrays: R4R5 and R8.

If you find out you’ve misplaced a component, you need to desolder it. Be careful not to use excessive force and don’t overheat! Desoldering requires a lot of patience and restraint. We find it safest to cut loose the offending component, and then desolder the individual pins one by one. It’s better to sacrifice the part than to damage traces in the board. We’re happy to send you a new part if you need one. When cutting, try to leave a long pin on the board for easier handling when desoldering. Ground pins can be more difficult to remove. Remember not to rush it or use too much force and heat, otherwise traces will break. Don’t despair if it takes you half an hour or more to desolder one part. But if you know somebody with a desoldering gun, then it’s just a 5 minute job.

After soldering the new component in place, use a multimeter to verify that all traces are connected. Check this by measuring from the pin (not from the pad) to another component on the board that it should be connected to. Use the schematics to find good test points. This way you may find that you still have to run a patch wire on the backside to restore a connection.

If all components are placed correctly, and the test pattern still doesn’t show, continue with the next section.

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.

Look for these joints with a loupe or by taking pictures with a smartphone.

Reference card courtesy AdaFruit

Gigatron example

Not every fault can be easily spotted by eye. If everything looks fine to you, continue to troubleshoot with a multimeter. This Excel document gives reference voltages for every IC pin: Troubleshooting.xlsx.

There are two worksheets in this document, each with instructions at the bottom. (If you have a small screen you may miss those.)

In general, pay closest attention to the ground pins. Those are always a bit more difficult to solder, especially when using a lower wattage soldering iron. The reason is that these pins are connected to a larger body of copper and that draws away more heat compared to other pins.

You should also check if the EPROM and RAM chips are seated firmly, and none of their pins are bent or misplaced.

If the LEDs light up in some arbitrary combination and then briefly change, that is evidence of program code trying to execute. After all, only program code can change the lights! If all four LEDs stay off, it is also possible that the RAM is not seated.

If none of this helps, you might want to check that there is no short between pins. For example, some leads on the back side might be touching.

As a final resort, backside cleaning might be an option. Sometimes excessive leftovers of soldering flux can cause problems. The reason is that, depending on its composition, flux may be conductive. Removing flux is a tedious job, and advice on the Internet on it varies widely. Don’t follow Internet advice, but get advice from the store where you bought your soldering materials. The potential need for cleaning and the required method depends on those.

For your reference, consider this: 1. We have seen boards with copious amounts of flux on them that worked perfectly fine. 2. We haven’ t heard back of anyone yet who had to do this to make his Gigatron work.

In summary:

If you can’t find the bad soldering joint or short, assume you’ve missed it and look for it again and again and again!

Slow performance

Pictures runs slow, or
Racer screen updates slowly, or
The Mandelbrot clock drifts several minutes per day.

Check if you have not accidentally disabled the retro scan lines without realising it. Most applications need the retro lines enabled to work well. Toggle between modes with the [Select] button.

Anything else

We have a user forum for support where we hang out with other Gigatron builders to discuss new ideas. The forum has a dedicated section for support on kit assembly. If none of the above solves your issue, you can ask for advice in there. Maybe somebody has already experienced the same thing and can help you right away. Of course, there are also the contact methods that are listed in the Assembly Manual, but you might get a quicker response in the forum.