| avidrissman | Posted: 1 Jan 2025, 06:03 PM |
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 Member Posts: 3 Joined: 7-February 20 |
I built my clock kit in January 2020 when I got it and it initially worked great. I was banished to WFH for a little while and when I returned I found that it was all black, all LEDs unlit. I left it plugged in anyway, and somehow it came back to life. It’s been working for months just fine, but last night I took it home for NYE, and it blacked out again. The GPS module has power, and its LEDs seem to indicate that it’s functioning, so I’m assuming for now it’s OK. Also, I have just a multimeter (F117), and was hoping to be able to work out what’s up with just that. Diagnostic steps: - Diode mode on the 10016AS while the clock is unplugged. Using the black probe on pins 3/8 and the red probe on the other pins, I can get the segments to light, and the meter shows ~1.7V. What’s a bit odd is that testing with the probes the other way around doesn’t yield light but does show also about ~1.6V. I don’t know if that’s normal. In any case, I wasn’t thinking the LEDs were the issue. - Voltage probing on the 10016AS while the clock is plugged in. I’m expecting dashes on the leftmost display when starting, which is segment G, pin 10. If I put my black probe on pin 8, and the red one on other pins, I see -4.4V on most pins and -0.8V on others. Seeing negative voltages is weird, and the voltage levels is weird. The data sheet says 1.8V typical, so that seems high. - The MAX7219, the ATTiny, etc are all getting power. I’m seeing intermediate voltage levels on the other pins which I guess implies that something is going on, but this is well beyond the ability of my multimeter to handle. Thoughts? I’m feeling like I desperately need to find myself a oscilloscope, but I’m still feeling weird about the LED voltages. Thanks. Last edit by avidrissman at 1 Jan 2025, 06:19 PM ------------- |
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| mit | Posted: 2 Jan 2025, 07:55 PM |
 yeah whatever Admin Posts: 574 Joined: 4-May 16 |
The display is matrixed so it will be hard to interpret much when probing the digits while it's on. I wouldn't worry too much about seeing negative voltages, that just means pin 8 is intermittently going to 5V, so other pins are negative voltages relative to it. There are a few things that can cause the display to go blank, one is if there's a problem with the brightness circuit. The current sink via the LDR sets the reference current for the display segments, and if that current is zero, the display won't light up. If the display stopped working, then worked again for a bit, it's unclear whether this is likely to be the culprit. Some clocks got damaged by electrical surges, in those cases the MAX7219 chips got damaged but the GPS and ATtiny were mostly OK. But again, if it came back to life then it might not be that anything is damaged. It could be a loose connection somewhere. Another reason the display could be blank is that the signals between the ATtiny and the MAX chips are interrupted. The first thing the ATtiny does is send an enable signal to the MAX chips, if that signal is missed then they will remain off until the ATtiny is reset. I think if the power supply was dodgy and had very slowly rising power during power-on, it could cause something like this to happen. You could try powering the clock from a different USB source, or try resetting the ATtiny while it's on. You can do that using a short bit of wire to connect the reset pin to ground. I think an important thing to try is to use the multimeter in continuity mode and test as many of the connections as you can. It's possible there's a problem connection somewhere that's making intermittent contact. Take a look at the interactive PCB diagram here and see if you can spot any signals that haven't got good continuity. I would be most suspicious of the pins of the socket for the ATtiny. ------------- |
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| avidrissman | Posted: 4 Jan 2025, 12:33 AM |
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Member Posts: 3 Joined: 7-February 20 |
Thank you for your reply. I traced a bunch of the nets for continuity, and they all came up fine. Then I then read your comment about the brightness circuit, and that turned out to be the culprit. When I measured the brightness potentiometer, I was confused to find that it read open. So I turned the pot, and the display flickered on and off. Sometimes the display stayed on after turning it but sometimes not. That pot is definitely bad, and I guess I get to practice my desoldering skills now. I also noted that a corner of the pot looked melted a bit, and I instantly remembered accidentally touching the corner of the pot with a hot iron during assembly. The clock worked when I finished assembling it, so I had dismissed the damage as cosmetic. I guess not. This was my first soldering project, so I appreciate your advice. Thank you so much for this cool clock, and thanks again for your advice on troubleshooting it. I hope you have a great New Year! ------------- |
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| mit | Posted: 6 Jan 2025, 10:19 AM |
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yeah whatever Admin Posts: 574 Joined: 4-May 16 |
Good to hear you found the problem, replacing that potentiometer shouldn't be too hard, or at least, it should be doable without pulling off the digit in front. Good luck! If you don't feel up to it and want a quick fix, you could add another resistor, say 20K, and solder it across the legs of the pot, it wouldn't be adjustable but at least there would always be current flowing. ------------- |
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