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Strange Problem and Solution
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Strange Problem and Solution

by uberdude on Sun Feb 23, 2014 9:37 pm

I just assembled the TV-B-Gone kit and had a strange problem, when powered on, the green light blinks for over 5 minutes and the IR LEDs just barely turn on. I was testing this using a benchtop power supply rated at 5A and set to 3.7Vdc to simulate a LiPo battery. The current draw according to the benchtop supply was 5mA. Testing the kit with a LiPo battery, two alkaline AA, or two Lithium AA all yielded the same result.

phone 163.jpg
LEDs per original kit - a very faint purple glow
phone 163.jpg (143.06 KiB) Viewed 1440 times


I wasn't sure what the problem was, but I suspected Q5. To cut to the chase, I solved the problem by installing a low ohm resistor (39 ohm is the first one I grabbed) and placing it between the collector of Q5 and the bases of Q1-Q4. This drastically increased the brightness of the IR LEDs, the green light blinks for approximately 55 seconds, and the current draw on the benchtop averages to about 100mA (though it is peaky). Also, I get good indications that it is working on the o-scope.

phone 160.jpg
LEDs with 39ohm resistor between Q5 collector and Q1-4 base
phone 160.jpg (192.5 KiB) Viewed 1440 times


I suspect that when Q5 turns on, it shorts the power supply, causing a power supply dip and this resets the micro. Thats why the green LED never stops blinking, the micro is constantly restarting. Any other thoughts? I'm really surprised this kit didn't work 'out of the box.' Have others experienced this issue or am I alone in this? I'm posting this in case others experienced this and they can try my solution.

Here is an o-scope shot that indicates the power supply dipout with the original kit.

tvbgone 018.JPG
o-scope with original kit, channel 1 at pin 5 of micro, channel 2 at Q5 collector. Indication of power supply dropout?
tvbgone 018.JPG (204.63 KiB) Viewed 1440 times

uberdude
 
Posts: 2
Joined: Sun Feb 23, 2014 9:04 pm

Re: Strange Problem and Solution

by uberdude on Sun Feb 23, 2014 9:43 pm

Here is an o-scope shot with the 39 ohm resistor in place between the collector of Q5 and the bases of Q1-4.

tvbgone 007.JPG
With 39ohm resistor installed, Channel 1 at pin 5 of micro and channel 2 at collector of Q5
tvbgone 007.JPG (172.49 KiB) Viewed 1438 times


Also, since it seems to be the thing to do on this board, here are shots of the front and back, it is very tricky to get a good shot of the back. Q5 was removed for troubleshooting. I realize it looks like the crystal isn't soldered well, but I get a rock solid 8mHz sine wave from it.

tvbgone 023.JPG
Front of board with Q5 removed for troubleshooting
tvbgone 023.JPG (242.36 KiB) Viewed 1438 times

tvbgone 027.JPG
Back of Board
tvbgone 027.JPG (202.7 KiB) Viewed 1438 times

uberdude
 
Posts: 2
Joined: Sun Feb 23, 2014 9:04 pm

Please be positive and constructive with your questions and comments.