[darkfury18]

The charging circuit is pretty simple, so the only things I could think of that could go wrong are that U1 is dead or something may be wrong with the battery.

Just as a sanity check, I'd do a continuity check between the pins of U1, D1 and R8 to where ever they're suppose to be connected make sure they are properly connected.

With the charger connected, what's the voltage between pins of D1 and U1 to the negative battery terminal?

 

[kilgoretrout]

The resistance between the two leads of U1 I've marked in the image have resistance of ~80ohms. Is this too low, indicating a bridge? The rest of U1, D1, and R8 and what they connect to seem fine (less than 1 ohm).

I don't think this problem is with the battery because the voltage and current measurements for the battery terminals are much lower than expected and probably not capable of charging the battery.

The voltage between B- and the pins of U1 and D1 as I have labeled them in the image below are. These were all measured with the charger plugged in, but the volume was switched to off.

B- and U1-pin1: .01V
B- and U1-pin2: 15.85V
B- and U1-pin2: 1.26V

B- and D1 pin1: 16.23V
B- and D1 pin2: 16.90V

 

[kilgoretrout]

I forgot to include the image
here it is:
http://img183.imageshack.us/img183/9599/back2hm3.jpg

 

[darkfury18]

The 80R is fine since those two pins are connected by R8 (75R).

Is this with or without the battery in place? If it's without the battery, then U1 is dead.

 

[kilgoretrout]

This is without the battery in place. Is it the U1 that is definitely at fault or should I give another shot at reflowing the solder on U1. Also a question about placement of U1, the part had 3 pins in a straight line, so I bent the middle pin to fit the configuration on the board. Did I do this correctly?

 

[holland]

U1 seems to work, from my perspective. It would serve you best if you read the datasheet on U1.

http://www.national.com/ds/LM/LM117.pdf

U1 will always maintain a 1.25V differential between 1 and 2. Knowing the 1.25V (aka Vref in the diagrams), the 75ohm resistor there is used to generate current for the charging circuit.

Something is shorting the lower part of the battery circuit.

For sanity's sake, what is the output of U2.

Voltage across D4? If D4 is installed backwards, there will be a drop of 0.7V. If not, it should read about 12V (give or take approx 0.7V). When zener diodes fail, I believe they fail open (broken fuse).

Check resistance between B+ and B-.

 

[kilgoretrout]

Voltage across D4 is 0.1V , but it is installed properly with the white line on the board matching the black line on the diode.

What you mean by the output of U2?

I'm measuring 1.1 ohms between B+ and B-, which seems like a short circuit, but I can't locate a bridge anywhere.

 

[holland]

The 7812 is U2. I want to make absolutely sure it's only the charging circuit you are having problems with.

Snip D4 out and pull out the pins with pliers as you heat it. You should have bought extras, they are cheap. Clean up the pads with a braid. You can try to desolder it with a braid if you like, but don't over do it and damage the pads.

After D4 is out, do what MisterX suggested back on page 3.

 

[kilgoretrout]

I do not have extra parts for D4. Is there suitable replacement that I can buy from radioshack?

I assume you want me to measure the voltage across the input and output voltage of U2. I measured 3.57V.

 

[darkfury18]

Since the amp does work when it's plugged in, there shouldn't be any problems with U2.

holland brings up a good point, it could also be D4.

Radioshack has these guys, 1N4742A 1-Watt Zener Diode.

 

[holland]

Output relative to ground. It should be about 12V. However, it sounds like it's working. 15.85 - 3.57 is 12.28 and within tolerance.

Radio shack has 12V 1 W zener diodes (1N4742A). The legs of the 1W may be too thick for the opening on the board. Try to desolder it. You just need to lift one leg out. It's tight. Try to slide a long resistor leg underneath one of the diode legs. Put it out the other side, and twist it around (like you would with bread, for example). This will give you something to clamp on to with needle nose pliers and pull.

One thing you can do is just heat the pad on one side and pull on the other side and leave the solder blob there after it out. Clean up with a solder braid. I like Radio Shack braids, they work quite well.

The other is to actually try and suck up the solder before hand. This is trickier and you can't always get all of it and may still need the heat and pull and clean up stages.

I hope you have a set of helping hands. Watch out for the teeth biting into the board, don't lay it over pads.

 

[kilgoretrout]

Ok I managed to remove the diode and place in a new one, but I inadvertently removed the solder pad that is not in the path of the traces on the back. It seems secure, but how can I test it to be sure that it will work?

 

[Pars]

Ohm it out to R8 on one side and B- on the other. If you have continuity, you're good. If not, look it over carefully, figure out why, and fix it.

 

[holland]

when you had the diode out, did you try to power it with the battery? did it work?

 

[kilgoretrout]

Yes, I tried this. The only 9V battery I have here right now is the one I am going to use for the amplifier, and it is very low on charge. The amp powered on, but the power LED was dim. I didn't try plugging in headphones.