Completed kit barely works due to low vcc

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Completed kit barely works due to low vcc

Postby Tim » Tue Dec 31, 2013 9:03 pm

I completed this kit recently. With a fresh battery installed, the kit (and all LEDs) apprear to work correctly when pushing the button, but the 3.4 volts vcc is being pulled down to 2 volts. The LEDs are so dim, they can hardly be seen even in a dark room. It's amazing the processor works at such a low voltage.

I measure 81 ohms across C1 and C2. Roughly a 400 ma load on the battery! I've checked my work four complete times, and re-heated all connections once. Looking at the kit on two different days and I still can't seem to find a culprit. This isn't a complicated kit, so I'm baffled why I don't have infinity resistance across both capacitors?

Not sure how many watch this forum, but if any others have run into something similar, I'd sure love to hear from you. One possible clue I note - 81 ohms is very close to what three of the 27 ohm resistors in series would be. I don't see how that would help based on the schematic, but who knows?

Tim, WK4U
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Re: Completed kit barely works due to low vcc

Postby AE9RB » Thu Jan 02, 2014 7:29 pm

I checked mine and what you see isn't normal. Try removing the caps and see if one of them is defective.
73 David AE9RB
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Re: Completed kit barely works due to low vcc

Postby Tim » Sat Jan 04, 2014 8:06 pm

Thanks Dave.

Well this reply is going to be rather academic. Let me explain...

I already did what you suggested before checking this post just now. I removed the bigger C2 capacitor first. There are NO markings on that part at all. With the part out of the circuit, I measure on that part 58.5 ohms one way, and 60.5 ohms the other way. Not sure why the difference, but thinking it was possibly an electrolytic caused me to check both ways. Now measuring across the pads where C2 used to be, I see 2.36 Mohms one way and 9.16 Mohms the other way. Again, not sure why the difference.

Now the kit works better. At least all the leds light up brighter. The kit works as it should. Then I took C1 out of the circuit (what to loose?) and the part was destroyed in the process. I noticed no change in measurements or performance with C1 out of circuit. Now vcc is pulling the battery down about 0.2 volts, so still the resistance isn't high enough. I dunno?

Anyway, it's time to switch gears and start the Peaberry V2 kit. I thought the start would be soldering, but instead I'm in torroid winding land. Never done that before, so I'm giving it a good go on my own. This is fun.

I am gratefull to experience SMT soldering on this dice kit before starting the radio. It's been about 10 years since working on stuff this small. Back then, I didn't need any glasses or magnification. Today... well... you know :)

Tim, WK4U
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Re: Completed kit barely works due to low vcc

Postby AE9RB » Sat Jan 11, 2014 7:30 pm

Looks like you had a faulty cap. I sent you new ones today. Theoretically, the battery will last longer with the caps installed. I would have sent them sooner but the spares from the guy I get the kits from turned out to be wrong so I had to order some.
73 David AE9RB
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Re: Completed kit barely works due to low vcc

Postby Tim » Sun Jan 19, 2014 10:42 pm

David.

First, thank you so much for sending me the two capacitors! What a nice surprise. You are a first class ham, and I'm glad to be dealing with you.

I finally got time to spend on my work bench. Measured the resistance of the larger capacitor (C2) before mounting, and measured a few Meg ohms, so it was certainly better than the original part. Installed both capacitors and really didn't notice any change. It still operates as before. I did not measure vcc, but if the LED's tell a story, the voltage (or voltage drop) hasn't changed any.

When I first put a battery in the kit, the LED's light up in a sequence almost twice as bright as they do after the boot up sequence. This only lasts about a half second. Not sure what that means, if anything?

Either way, I'm very happy with the kit and the experience building it. Again I appreciate your efforts to send me replacement parts without asking. At first I really didn't know what it was I got in the mail - he he.

Please keep doing what you're doing. It's all right by my standards!

Tim, WK4U
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