LED out troubleshooting help

[aphays]

Full disclosure - this is my first amp build, so I have no doubt this is my mistake somewhere. I have doubled checked all the directions, and I will go back through and triple check them as well. However, the amp passed all resistance checks and voltage checks during/following the build. Worked great at first for a few minutes, then I started getting popping in the right channel. Found one loose solder, fixed it, that didn't solve the problem. I discovered that the LED closest to the middle of the amp (attached to A8) lights up ten seconds after amp is turned on, but consistently goes off again after the amp is on for three minutes. I re-soldered this LED and tried to make sure there is some play in it. It was pretty tight before, but now has some room to move around. Didn't solve the problem. Just to clarify, the other LED (attached to A3) stays lit without issue. Here are the voltages I'm getting once the LED turns off as well as the initial voltage checks following the build.

Terminal      (initial voltage check)    (with LED out)
1.                 83.6                               151
2.                 178                                168
3.                  0                                   0
4.                 178                                167
5.                  83                                 80

6.                  0                                    0
7.                  107                               151
8.                  0                                    0
9.                  109                               103
10.                 0                                  100

Drew H.

[Paul Birkeland]

Hello Drew,

Can you post some photos of your build? 

The LED will go out for a large number of reasons, but the LED itself shouldn't really be your focus. 

With no headphones plugged into the amp, can you power it up, wait for the LED to go off, then gently poke around with a wooden chopstick to see if you can make the LED flash?  That tends to be a good way to figure out what might not be all the way connected.

-PB

[aphays]

Here is a pic of the final build before I re-soldered said LED. And here is a closer one of the LED while off. I did poke around with a wooden stick, and once the LED is off it stays off. But I may not have poked the right connection to get it to flash. I'll give this another shot in a bit. I appreciate the quick response!

[Paul Birkeland]

Can you carefully measure the DC voltage at A8 with the LED out?

Terminals 2L and 5L sure look like they could use some extra heat.  3U looks like it could use some extra solder and some extra heat.

When the LED is out, do both halves of the 12AU7 still glow?

[aphays]

I'm getting 1 when it's on, and 0 when it's off. Both halves of 12AU7 do glow regardless.

5L looks pretty weak in person - I'll hit that one again with a bit more solder.

[Paul Birkeland]

If the LED has been damaged, then I would expect you to see about 12-13V at A8 when it went out.  Since you have the proper voltage during operation, then you don't, we can conclude that the LED is OK and leave that alone.

If one half of the 12AU7 stopped glowing after a few minutes, I would be scrutinizing the green twisted pair of wires that go from the B socket to the A socket, as those are responsible for heating the tube.  A tube with one half unheated would create your exact issue.

With these problems determined not to be the issue, we are most likely left with a solder joint issue.  You could also try pulling the 6080 and running the amp with the 12AU7, just to take some kind of unusual 6080 behavior out of the equation (though this is exceptionally unlikely).  Also exceptionally unlikely would be a 12AU7 that has an intermittent half, especially if you don't see half of the glowing dots on the tube itself extinguish at some point. 

[aphays]

Thanks so much. I'm going to go back through it with a fine tooth comb tonight. And I will especially take a look at the green wiring - even if that doesn't seem to be the cause. I'll report back what I find. Thanks for your help!

[aphays]

Consistently the A8 LED winks on after a few seconds of the amp being turned on. Each time it stays on for about 3 minutes and then begins to flicker on and off, eventually going off completely. I cannot find a single spot that I can tap with a wooden dowel, including on the LED itself, that repeatedly makes it come on or off. The flicker seems to be independent of my tapping/pushing. And eventually it goes off for good. I rechecked the voltages during the first three minutes when it was on. All the numbers were normal except for Terminal 10, which was 100. This remains 100 when the LED flickers off. This one should be 0. I retouched any solder connection that seemed like to could need it, and all the numbers remained the same, and the LED continued to flicker and turn off after 3 minutes.

I tried running the amp without the 6080 tube, and the situation was the same. Also, I was able to confirm definitively that the 12AU7 tube stays lit on both sides even once the LED goes out for good. What should I try next?

[Deke609]

100 volts at terminal 10 suggests to me that terminals 9 and 10 are shorted. Based on your first photo, my guess is that the body of the 100 uF cap that connects 9U and 10U is touching the tops of those terminals.  The manual calls for the body of this cap (and of the cap connecting 6U and 7U) to be spaced a 1/2" from the terminals.


Otherwise, I would look for another short connection between 9 and 10.


cheers,


Derek

[Paul Birkeland]

100 volts at terminal 10 suggests to me that terminals 9 and 10 are shorted.

Good catch, I didn't even notice that in the original post.

The 2.49K resistor on the headphone jack that connects to the white wire and the outside of the headphone jack isn't connected.  Do remedy this 100% before you attempt to use the amp.  If you use the amp in its current condition, you will damage your headphones.

[Paul Birkeland]

It is also totally possible that the 3K resistor lead that isn't really soldered at 3U is causing some problems as well.  If this resistor isn't well connected, then the 6080 will warm up and screw up the operation of the 12AU7 in a rather unusual manner.

[aphays]

I will check out all of these things this evening and report back. Thanks!

[aphays]

I'm was going to cautiously call this solved. I checked all the spots mentioned above. Clearence at 6U & 7U as well as 9U and 10U is greater than it appears in the picture, and seems to be in spec. The pic is a bad angle for sure - sorry about that. I heated or added solder to all the headphone jack connections, as well as 3U and 7L and 9L just to be safe. However, I noticed the problem still occured, and I had the multimeter attached at 12U to check the voltage at 10. Voltage there is now 0 as it should be. So the voltage problem is solved. However, when I removed the black alligator clip, the LED immediately went out. I started tapping around 12 and 13 and found a lose connection at 13. Just to be safe I heated up 12 as well. Turned the amp back on and it ran for several minutes with no issues. Just enough to give me hope. Then the LED started flashing off again. When it does, I get a bunch of terrible static through the headphones so it's very easy to detect. Don't worry - using an old, cheap pair of iem's, so no worries about damage. I started tapping around again, and each tap brought the LED back on no matter where on the chassis I tapped until it eventually clicked off for good after ten or so seconds. No tapping of any kind, anywhere revived it.

I let the amp cool down and plugged it back in. It worked again as normal for about 3.5 minutes, then I got the static again. This time, I think I have tracked it down to one of the following: B5, B2, or B3, with most of my suspicion on B5 and B2. Pressure in between these seems to eleviate the problem, so I need to check these solders.

It's getting late and I have to leave on a business trip early in the morning. Probably have to let this drop till I get back on Sunday. I'll definitely post and update and some new pics Sunday evening. Again, I really appreciate your help. Definitely making progress - hopefully close to solving it! Sorry my soldering skills are so poor.

[Paul Birkeland]

The break will probably be helpful since it will bring fresh eyes when you return.  The tapping changing the functionality of the amp is a good indication of a connection that isn't all the way soldered or perhaps a broken wire, though this is much less likely. 

[Deluk]

Although it doesn't affect anything electrically, the red wire coming onto A7 should route into the terminal from the outside of the socket. That way it doesn't interfere with the LED. LED is then in free air and much easier to check and replace if needed.