Voltage issues and 1 LED not lighting [resolved]

[captaincarl777]

Hi all,

I'm hoping for a helping hand with my Crack build.  Used to do some electronics/electric work decades ago, but have become very rusty.  Relatively new to the headphone scene.  North America, getting 117.9VAC out of the wall.  I completed the build over 3 evenings (glow test was good, resistance checks were good), but blew the fuse immediately when it was time for the Page 49 Voltage Check.  Discovered an unsoldered joint and a shorted diode (banded end of 18L to 21L was touching 20L), oops!  "Fixed" both issues.

Resistance checks were still good.  Voltage Check time again, both tubes glow, but only the LED attached to A3 lights up, A8 is dark.  Both have the banded end to the center post.  Terminal voltages measured as follows:

1   146.7    (high!)
2   163.5    ok?
3   0      ok
4   163.5   ok?
5   76.8           ok

6    0      ok
7   146.2   (high!)
8   0      ok
9   102.1   ok
10   0      ok

Visual inspection looks good to me.  I don't think I have any cold solder joints or things connected to the wrong places, though I could have done a better/cleaner job at almost everything. 

Can someone point me in the right direction?  I'm wondering if the short killed a diode, a cap, or <gasp> the transformer.  Any help anyone can offer will be much appreciated, I'm really looking forward to hearing tubes through headphones for the first time!

[Paul Birkeland]

The high voltage at terminal 1 is consistent with the LED being dark at A8.

If you can measure the DC voltage at A8, that can be helpful.

Often times the center post side of the LED won't be soldered all the way, and that can cause some problems like this.  The LED can also be destroyed by excessive heating. 

-PB

(The voltage at terminal 7 will mellow out when we get terminal 1 where it's supposed to be)

[captaincarl777]

Thank you for the help!  DC voltage at A8 is 11.8, it took several seconds to stabilize there, it was up around 16v for a second or two.

I'm almost certain that I overheated the LED.  I remember seeing a link around here somewhere for a 5 pack, going to go hunt that down now.

[Paul Birkeland]

Yes, you'll want to just replace it.  You can also wire A3 and A8 together to get yourself up and running.

[captaincarl777]

I received the LEDs, and replaced the one that appeared bad.  All resistance and voltage checks were ok, so I plugged in some headphones and ran a few tunes through it.  Woo woo, this amp sounds really nice, I'm looking forward to getting to know it better!

Thanks again for your help, Paul, much appreciated!!

[adydula]

Congrats on your build and fixing your issue!!

Enjoy the music, this amp is really a wonderful amp!

Alex