A8 LED Issue [resolved]

[parm289]

Hi all...

I'm unfortunately having an issue with my crack amp, no speedball.  The amp had been working great for the last several weeks; I had gotten some noise upon the initial build and reflowed some suspicious joints which solved the problem.  This is my first soldering project and the quality of my joints likely reflects that.  Today, I went to swap in a new 12au7 tube that had just come in which led to a bit of a cascade of issues.

I turned on the amp with the new 12au7 installed and heard some static / crackling / popping.  I let the tube run for about 10 minutes, and the noise was still there - it sounded similar to the noise I got on my initial build, so I figured the jostling from tube rolling must have worked a joint or two loose on the 12au7 socket.  I put the original 12au7 back in the amp, and heard similar noise, which eliminated the new tube as the issue.

I reflowed several of the A joints, including A8, and went through the resistance checks which read fine.  When I went to perform the voltage checks, I noticed that the A8 LED was no longer lighting up.  I took the voltage readings and there are issues at T1 and T7, both of which are reading 150V.

After reading several other threads re: LED issues, I tested the LED directly with my DMM in diode mode and am not able to get the LED to light (the A3 LED lights fine in testing).  I also took a voltage reading at A8 and got 12V.  During the tube glow test, both sides of the 12au7 are lit.

Given the proximity of my A8 LED to the solder joint, I suspect that reflowing A8 overheated and burnt out the LED.  I also see what appears to be some residue on the surface of the LED.  But given the manual's clear warning that the LED is often a symptom, I wanted to post here for advice before replacing the LED.

I've attached pictures of the build and have included my full resistance and voltage read-outs here.

Thanks in advance for reading and troubleshooting.

Resistances:
T1: 1.91M ohms
T2: 1.84M ohms
T3: 0.2 ohms
T4: 13.5M ohms
T5: 14.5M ohms
T6: 0.2 ohms
T7: 2.93K ohms
T8: 0.3 ohms
T9: 2.95K ohms
T10: 0.3 ohms
T12: 0.3 ohms
T13: 4.5M ohms
T14: 0.3 ohms
T20: 0.3 ohms
T22: 0.3 ohms
B3: 2.93K ohms
B6: 2.95K ohms
RCA Center: 95.4K ohms (L) / 93.8K ohms (R)
RCA Ground: 0.3 ohms / 0.3 ohms

Voltages:
T1: 151 V
T2: 168 V
T3: 0.9 mV
T4: 168 V
T5: 76 V
T6: 0.7 mV
T7: 150 V
T8: 1.0 mV
T9: 99 V
T10: 0.6 mV
A8: 12 V

[Paul Birkeland]

Yes, you probably killed that LED.  You can add a jumper wire between A3 and A8 to get your amp back into operating condition.

[parm289]

Thanks, Paul.  I replaced both LEDs (I took two from the speedball kit & ordered some more HLMP-6000s from Mouser to backfill) and re-soldered the center pin.  Tried to give the new LEDs a slight bit more slack this time to hopefully avoid issues in the future.

Resistance and voltage checks passed, and the amp is performing ok so far.  Got one bout of cracking/popping a few minutes into listening, but going to let the amp run for a while and reflow if that persists.

Appreciate your help, and the timely response on a Saturday evening.

[parm289]

My question is still resolved, but I wanted to reply here to give an update on the ultimate problem with my amp in case any future bottleheads come across this thread upon a search.

It turns out that my amp still suffered from crackling/popping/static after replacing the LEDs.  I carefully prodded around the amp with a wooden chopstick while running and found that the problematic connection was in fact on the headphone jack, on the bottom row of joints closest to the interior of the chassis.  I re-soldered that connection and the amp has been running great, so it appears that joint had been weak and probably jostled loose while tube rolling.

I understand this joint is a common point of failure, so...make sure to check it's solid regardless of where you think the problem is.  At least I've now got some more soldering experience with two fiddly LEDs.