Hum and low volume in right channel [resolved]

[mortenpedersen]

Hello, first time builder here.

Just finished a stock Crack 1.1 build. Everything measured out fine, leds and both sides of both tubes are glowing.

Left channel seems fine but right channel is humming, with a thin low volume sound that stays the same - even at 0% volume.

The same channel also makes a loud pop and fades out when the amp is turn off. Not so with the left channel. For whatever it's worth every other time that pop also seems to go through my USB-DAC and cause Foobar on my laptop to throw a playback error.

I painted both the bell and the panel - upper side only.

Hope some of you have a few suggestions on what to do.

Thanks, Morten.

[Doc B.]

Could be a bad connection. Might be worth rewetting your solder connections. Or it could be a flaky tube.

[mortenpedersen]

Thanks for the feedback Doc, I went through all the soldering - everything looks nice and shiny. Redid a few to be sure, especially related to grounding, input and volume control. But still exact same issue, left channel sounds perfect, right channel humming along with tinny sound and stays the same no matter the volume level.

I noticed that touching the input tube increases the same humming, its not related to the socket as I'm barely touching the glass with my fingers. Perhaps that's normal, but it doesn't happen if I touch the 6080 follower.

Could it be a bad tube?

[Doc B.]

Could be a bad tube. You could try cleaning the tube pins before giving up on the one you have.

[Paul Birkeland]

If your voltages all measured well, it's unlikely to be a tube issue (though it's still a remote possibility).

How does the solder look on the center pin of the right RCA jack?  If it's spilled over a bit and is touching the shell, even just a little bit, you'll have the problem you describe.  Also, missing the upper to lower jumper on the volume pot grounds can create some unique issues like this. 

If all else fails, sometimes we can spot issues in photos that aren't as obvious from a description of the problem.

-PB

[mortenpedersen]

Thanks guys, I've attached a pdf with images of the relevant parts. They seem fine to me.

Morten.

[Paul Birkeland]

Man, I think you may get the award for best photos posted in 2017.

I'd reheat and reflow the solder joint at A5, but otherwise everything looks ship-shape. 

If that doesn't help, you'll want to download a 60Hz test tone and play it into the amp.  Next, set your meter to AC volts (the 2V scale to start if your meter isn't autoranging).  Set the volume pot to about 50%, then measure the AC voltage between ground and:

Left RCA jack center pin
Right RCA jack center pin
Upper middle pot lug
Lower middle pot lug

Report those values, then we can keep working.  Oh, the amp won't need to be on to measure those AC voltages accurately either.

-PB

[mortenpedersen]

Thanks man

And when you say ground, do you mean ground buss at terminal 12U?
And I don't need to turn the amp on when testing?

[mortenpedersen]

Well, resoldered the A5 but it didn't change anything.

I set my meter at 200 AC, which is the lowest setting. Measuring from ground buss at terminal 12U I get the following readings with no headphones in the jack and the amp turned off:

Left RCA jack center pin - 2.1V
Right RCA jack center pin - 2.1V
Upper middle pot lug - 0V
Lower middle pot lug - 0V


Additional readings:
Upper left pot lug - 0V (volume pot grounds)
Lower left pot lug - 0V (volume pot grounds)
Upper right pot lug - 2.1V
Lower right pot lug - 2.1V

Thanks, Morten.

[Paul Birkeland]

Well, you have a good, strong source, so that will help.

If you got 0V at the middle pot lugs, then the pot is turned down all the way or just too far, so I'd turn it up a little more until you can get a pair of readings.

-PB

[mortenpedersen]

Thanks, the readings were done with the volume pot at 50%. Here are some more:

75%
Upper middle pot lug - 1.0V
Lower middle pot lug - 0V

100%
Upper middle pot lug - 2.1V
Lower middle pot lug - 0V

Hope that helps.

[Paul Birkeland]

Do you have the 60Hz signal playing into both channels? The lower deck doesn't seem to be getting any signal.

[mortenpedersen]

Absolutely, still measuring 2.1V on both RCA center pins.

But somehow there's no signal carried from lower middle pot lug to A7, so I was thinking that the pot (or input tube or both) is broke.

Thanks, Morten.

[Paul Birkeland]

What resistance do you get from ground to the lower middle lug of the pot at no volume, half volume, and full volume? 

Can you post a few photos of the wiring on the 9 pin socket and the pot?

[mortenpedersen]

Just went through resistance check according to manual, all readings were spot on.

All readings (0%, 50% 100% volume) for lower middle lug are over limit/infinity  - not so for upper middle lug. I've attached a pdf with pictures of the wiring.

Thanks, Morten.