Any electronics experts out there?

"Full of the sound of the Gran Fury, signifying nothing." (granfury) 04/14/2020 at 17:40 • Filed to: None

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I’m trying to revive a cheap Lepai LP-2020A+ amplifier that I’m sure I picked up at the Goodwill Outlet. It won’t power on, but if I jump a couple of connections on the relay it will start up. A bit of research has shown that capacitor C15 may be installed backwards, but I can’t find any polarity indications on the board (unless I just don’t know what I’m looking at):

The other thing that concerns me is the soldering on the amplifier chi p itself:

Wildly inconsistent, and apparently missing solder on one of the connections. There were some sloppy bits of solder debris on the board that popped right off as well, although they did not appear to be shorting anything out.

Also of concern is the condition of capacitor C1. It’s not popped, and voltage flows across it, but it looks beat to hell. I’m assuming that it was probably too much heat during assembly:

I’m used to capacitors like this being mounted flush to the board, but that obviously isn’t the case here, not that that’s really an issue, just poor workmanship. The apparent heat damage is a little bit of a concern, but that’s probably just poor workmanship again.

I can get the unit to power on if I jump a couple of pins on the relay, but that’s not a permanent fix, just a troubleshooting step. I haven’t checked for audio output when I do that, but figured that it wouldn’t matter since the real problem lies elsewhere.

So, how do I know if C15 is installed backwards? Should I complete the soldering on the amplifier chip? Anything else I should be looking at? Thanks in advance

facw > Full of the sound of the Gran Fury, signifying nothing. 04/14/2020 at 18:10

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It’s been a while since I got poor grades in my electrical engineering classes, but IIRC capacitors shouldn’t be directional?

Full of the sound of the Gran Fury, signifying nothing. > facw 04/14/2020 at 18:13

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Resistors aren’t directional, but diodes and capacitors are. At least that’s what I remember from my classes several decades ago...

facw > Full of the sound of the Gran Fury, signifying nothing. 04/14/2020 at 18:20

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Diodes definitely are, but capacitors shouldn’t be, since they are just two plates (rolled up in this case), it shouldn’t matter which plate is positive or negative. Though there may be implementation details that matter that I’m forgetting.

In any event, have an xkcd about diodes:

MiniGTI - now with XJ6 > Full of the sound of the Gran Fury, signifying nothing. 04/14/2020 at 18:51

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Electrolytic capacitors are definitely polarized. See if one leg of it goes to a ground trace.  Some boards have a big ground trace around the perimeter. 

Full of the sound of the Gran Fury, signifying nothing. > MiniGTI - now with XJ6 04/14/2020 at 19:04

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Thanks for that bit of info. Yeah, the striped side of the cap is attached to ground on the board , so it’s installed correctly. One thing that I did notice was that, when I jumped the relay to get the unit to power on, I was only getting around .55V on that cap, unlike the full 12.35v when I checked the voltage on C1. It was a challenge to test that; I could have used an extra hand or two...

Full of the sound of the Gran Fury, signifying nothing. > facw 04/14/2020 at 19:05

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Perfect. How did they make a cartoon about me and my ex???

TorqueToYield > facw 04/14/2020 at 19:40

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electrolytic caps are all polarized. they’re not constructed symmetrically.

https://en.wikipedia.org/wiki/Electrolytic_capacitor

“Electrolytic capacitors are polarized components due to their asymmetrical construction and must be operated with a higher voltage (ie, more positive) on the anode than on the cathode at all times. For this reason the anode terminal is marked with a plus sign and the cathode with a minus sign. Applying a reverse polarity voltage, or a voltage exceeding the maximum rated working voltage of as little as 1 or 1.5 volts, can destroy the dielectric and thus the capacitor.”

Ceramic caps on the other hand are not polarized.

TorqueToYield > Full of the sound of the Gran Fury, signifying nothing. 04/14/2020 at 19:44

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The solder on that one pin is extremely concerning.

It looks like at one point there was a whisper of a hair of solder connecting the pin which is why it probably passed a functional QC check, but it’s unlikely it survived long. It also may be making intermittent or high resistance contact.

facw > TorqueToYield 04/14/2020 at 20:00

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Thanks!

Full of the sound of the Gran Fury, signifying nothing. > TorqueToYield 04/14/2020 at 20:12

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Which image? I’m assuming that we’re talking about the 5th one.

TorqueToYield > Full of the sound of the Gran Fury, signifying nothing. 04/14/2020 at 20:30

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Yea the 5th one. 

Full of the sound of the Gran Fury, signifying nothing. > TorqueToYield 04/14/2020 at 20:35

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Solder? What solder?

I’ll redo that one and see if it makes a difference.