Lost sound in the right channel [resolved]

[TheRedMantra]

My amp is the stock circuit (no stepped attenuator). It’s been working flawlessly for many months. Today I rolled in a new 6AS7G tube and after about an hr the right channel started to buzz. Then it cut out completely. I put in a known good tube and the right channel was still silent. No buzz, no hum, no noise.

All LEDs glow. I checked resistances and voltages and all are to spec. I cleaned the contacts in the headphone jack. I use a 1/4” to 3.5mm adapter and checked it in another amp (it works). I tried a second set of headphones and the issue seems to be the amp.

I tugged on the solder joints and checked for cracked cables. Nothing. What else can I check?

 

[TheRedMantra]

After sleeping on the problem I warmed up the amp and tried something different. I rocked the knob for the Balance pot forward and backward and started hearing the sound in the right channel come and go. I found a “balance point” where it seems to be back to working but if I breathe on it wrong I have issues. I have a 2 Quiet upgrade waiting to be installed but I wanted to use the stock amp for 6 months and roll tubes to get a really good feel for it before upgrading.

Is it more likely that I have a wonky balance pot or maybe a bad solder joint? I’ve visually inspected them and they’re clean looking with no damaged wires.

Also, unrelated, I’ve noticed when changing to an unused input to test for noise floor in new tubes that sound can bleed through the selector switch sources; only at dangerously high, unlistenable volumes. Nothing I can perceive when listening, but I’m thinking of upgrading the switch down the road. Any recommendations?

 

[Paul Birkeland]

maybe a bad solder joint? I’ve visually inspected them and they’re clean looking with no damaged wires.

This is very likely the issue.

Also, unrelated, I’ve noticed when changing to an unused input to test for noise floor in new tubes that sound can bleed through the selector switch sources; only at dangerously high, unlistenable volumes. Nothing I can perceive when listening, but I’m thinking of upgrading the switch down the road. Any recommendations?

You can certainly replace the selector switch, but I would not expect improvement from doing this.  If you have no source plugged into a set of jacks, it will be very receptive to picking up the neighboring source.  If you have multiple sources plugged into your amp, having the source actually plugged into the set of input jacks is going to mitigate this effect substantially, so in essence you are only likely to experience this problem with unused inputs at unrealistic listening levels.  As soon as you hook up a source, that input isn't likely to pick up signal from its neighbor.

 

[TheRedMantra]

Okay. I’ll remove and re-solder all the connections to the balance pot. Unexpected bit of fun for today. The practice is making me better at this though. Thanks Paul.

 

[Paul Birkeland]

Okay. I’ll remove and re-solder all the connections to the balance pot.

I would not do that.  Plug a cheap pair of headphones into the C2A and run some music into it, then poke around the balance pot with a wooden chopstick to see if you can figure out what you need to poke to get the issue to occur.

Keep in mind that it might be a wire that connects to a balance pot that's the issue, but it could be the end of that wire that doesn't connect to the balance pot. 

If you desolder, remove, then reinstall all of these connections, you may end up making more issues than you have now.

 

[TheRedMantra]

 

[TheRedMantra]

I chopstick tested and it seemed that the issue moved all around from almost every connection out of the balance pot. I ended up cleaning and redoing all of the joints. Still the same issue. I invited a friend over this morning who does professional custom audio installations for a living. He examined it and is also convinced it is the balance pot. I’m no expert, but he is.

It’s worked perfectly for many months. I’m not sure what else I can explore beyond installing the 2 Quiet Attenuator that I have. I don’t want to complicate the circuit more if that isn’t the issue, but I don’t know what other options make sense.

 

[Paul Birkeland]

You could scrape off all the connections from the lugs of the balance pot into the balance pot itself on the right channel and the signal wouldn't drop out.  If solder has flowed into the pot, there's some chance of the signal shorting out.  Part of the problem is that if the issue is around the selector switch and you install the attenuator upgrade, you'll have to go back and debug the issue anyway. 

 

[TheRedMantra]

I chopstick tested again; and this time the noise migration/stabilization happened in the volume pot at two points. I redid the connections and did a resistance check. 5 dropped to 31.5ohm and 15 to 31.7ohm. Is it possible I damaged one of the 33kohm resistors? No clue how to get the resistance figures back in line.

 

[Paul Birkeland]

The 33.1K resistors could go totally open or dead shorted and you still wouldn't lose a channel. 

5 and 15 are the outputs of the C2A that feed the headphone jack, which is on the complete opposite end of the circuit from the volume and balance pots.  These resistance readings are the 2.49K resistors across the headphone jack.  If you are measuring 30 ohms at those points, you probably have headphones plugged in.

 

[TheRedMantra]

I totally left the tester headphones plugged in. Measurements are to spec! I was still having intermittent sound in the right channel. After redoing the connections to the pots I pressed the center lower terminal of the volume pot with a chopstick the sound was perfect. When I released it the sound cut out of the right channel. Same with the furthest lower terminal closest to the tubes; but when I pressed that one I also saw the center terminal flex. I bent them down very slightly toward the chassis plate and now the sound is clear in both channels. I can tap them and not lose sound or hear buzzing. But now the channel balance doesn’t do anything.

If bending those terminals helped what could have been or still might be wrong?

 

[Paul Birkeland]

I think this is a rare instance where you can put in the attenuator upgrade and roll the dice on that one.  If you want to be ultra sure, you can solder the middle lug of each level of the volume pot together with a jumper wire to put the amp in mono, and that will confirm that everything after the attenuator is working happily.

 

[TheRedMantra]

I installed the attenuator upgrade today and the amp is back to being happy. I wonder why all that happened but it was good motivation to upgrade. Paul, thank you for being so responsive and helpful.