[Solved] A8 LED Doesn't Light

[baby-alligators]

Hello everybody, thanks for taking a look.

I have successfully completed all of the resistance checks on my build of the Crack, but when I power the amp up to do the voltage tests, the LED going from the center terminal of nine-pin socket A to A8 does not light.

The other LED does light up, and both of the tubes light. Here (http://imgur.com/a/XNymh) are some pictures of the build.

Any ideas on what the problem could be? I'd appreciate any help. Thanks for reading.

[fullheadofnothing]

Voltages...?

[baby-alligators]

Voltages...?

Sorry, not sure how I forgot that. Here they are:

(https://i.imgur.com/Jf65HIt.png)

[Paul Birkeland]

Several causes will produce what you're describing.  I will list them off:

1.  A solder joint on either side of the 22.1K resistor between T1 and T2 is cold or not connecting well.
2.  A solder joint on the wire going from T1 to A6 is cold or not connecting well.
3.  The solder joints and connections on A4/A5 are not conducting properly, and only half of the 12AU7 is conducting.  This will be visible from the top side by only one half of the 12AU7 having glowing dots at the top and/or bottom of the plate structure.
4.  The HLMP-6000 diode is damaged.  This can be confirmed by measuring the DC voltage at pin A8.  If it's about 12V DC instead of 1.57V DC, the LED has been overheated or broken.  This can be fixed by wiring in a wire jumper from A3 to A8.
5.  The ground jumper that connects the upper and lower ground of the volume pot was not installed, or A7 is not seeing a solid ground reference.  This can be confirmed by measuring the DC voltage at A7, which should be 0V.  Be careful not to hit A6 when measuring this terminal, and for some measure of protection, you can turn the volume pot all the way up when taking this measurement.

Let us know what you find!

-PB

[baby-alligators]

Re-flowed/attached according to 1, 2, 3, and 5, but the issue was the diode as described in point #4. I replaced the diode and it lit up after turning on the amp.

I performed the voltage tests again and got the following results:

(https://i.imgur.com/TJAyUm1.png)

Am I safe to proceed?

Thank you so much for your help.

Also, where do I measure the "Tip or Ring terminal" voltage from? I understand it's part of the headphone jack, but I don't really understand where. Is there a picture of this available? Sorry for the dumb question.

[Paul Birkeland]

That all looks good. 

When you look at your 1/4" plug, the tip is the tip of the jack, the ring is the middle contact, and the sleeve is adjacent to the body of the plug.

On the headphone jack, the tip is where the white wire connects to the jack, the ring is where the red wire connects, and the sleeve is where the black wires connect.

-PB

[baby-alligators]

Thanks for the help on the headphone jack!

Just checked the voltages. There is an initial jump as it starts up to ~9.7 V (probably okay?), but it does not seem to be bottoming out at zero as it is supposed to. It is jumping around from anywhere from 0-50 mV. Any idea what the issue is? Or is this okay?

Thanks for all of your help.

[Paul Birkeland]

it does not seem to be bottoming out at zero as it is supposed to. It is jumping around from anywhere from 0-50 mV. Any idea what the issue is?

50mV is 0.050V, which is sufficiently close to 0.  If you have a solder joint issue with one of the 2.49K resistors, you will see that DC voltage rise, but never fall, and that's not something that's desirable at the output of a headphone amp!

-PB

[baby-alligators]

Finally ready to use it then! Thanks so much for all the help. I really appreciate it!