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October 17, 2021, 11:38:00 AM

Author Topic: Please help: weak output from new Crack build [resolved]  (Read 1196 times)

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Offline beatcomber

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Please help: weak output from new Crack build [resolved]
« on: July 31, 2021, 12:08:19 PM »
Hello all -

I just today completed my Crack build. All of the voltage tests checked out, the tubes and LEDs all lit up, etc. etc., so I was expecting glorious sound when I plugged in my headphones. What I got instead was barely audible, weak, thin sound from both channels with the volume pot maxxed out.  :-[

These are the troubleshooting steps I've taken so far, all to no avail:

• Made a visual inspection and touched up a few questionable solder joints.
• Swapped in a known good 12AU7 tube.
• Compared my build to the image of the completed chassis in the instruction manual - everything looks correctly assembled.
• Went step-by-step through the entire manual to make sure I installed everything correctly - no mistakes found.
• Finally, as a last resort, I reflowed nearly every solder joint I could access, just to be on the safe side.

I'm at a loss here... What else I can check to try and narrow this down? Could it be something as simple as a bad power tube?

Thank you in advance...
« Last Edit: August 21, 2021, 05:17:53 AM by Paul Birkeland »

Offline Paul Birkeland

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Re: Please help: weak output from new Crack build
« Reply #1 on: July 31, 2021, 02:36:19 PM »
Could you post some build photos?

No, if your voltages are ok it's very unlikely that you have a dead tube.

What headphones are you using to test the amp?
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #2 on: July 31, 2021, 03:16:36 PM »
Could you post some build photos?

No, if your voltages are ok it's very unlikely that you have a dead tube.

What headphones are you using to test the amp?

I appreciate the response! Headphones are Sennheiser HD6xx (aka HD650).

I can't figure out how to post images here, it says I am not allowed to post external links.
« Last Edit: July 31, 2021, 03:25:42 PM by beatcomber »

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #3 on: July 31, 2021, 03:21:32 PM »
Let's try this: https: //postimg.cc/mhmZTjNN. (Copy/paste and remove the space after the colon.)

Offline Paul Birkeland

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Re: Please help: weak output from new Crack build
« Reply #4 on: August 01, 2021, 05:35:52 AM »
At terminal 19, the lead poking through the inside of the terminal strip looks like it's touching the other diode leaving terminal 18.  That needs to be trimmed.

The wire poking through B2 is somewhat close to B3 and should be bent around B2.

With the amp off, can you plug in your headphones and then give me a resistance reading between 12 and 6, as well as 12 and 10?
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #5 on: August 01, 2021, 06:37:50 AM »
At terminal 19, the lead poking through the inside of the terminal strip looks like it's touching the other diode leaving terminal 18.  That needs to be trimmed.

The wire poking through B2 is somewhat close to B3 and should be bent around B2.

With the amp off, can you plug in your headphones and then give me a resistance reading between 12 and 6, as well as 12 and 10?

Thank you for taking the time to review my build.

Here are better views of the areas of concern:

https: //i.postimg.cc/pdtSkswk/IMG-7380.jpg
https: //i.postimg.cc/SK7vPWq4/IMG-7381.jpg

The resistance readings, with the headphones plugged in and the power off, are:
12 & 6 = .26
12 & 10 = 0

I went through all of the resistance readings, and all were within spec, except for the RCA jack center pins. Instead of getting 90k-115k, I couldn't get a reading at all. Maybe I've somehow shorted them? Here's a closer look: https: //i.postimg.cc/7PFYwk0L/IMG-7382.jpg

Offline Paul Birkeland

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Re: Please help: weak output from new Crack build
« Reply #6 on: August 01, 2021, 06:55:00 AM »
Those resistance readings indicate that you've likely overheated the lugs on the headphone jack where the red and white wires land, and they have moved a bit under the spring tension of the contacts on the jack and are no longer working properly.  We also see this sometimes from cheap 1/4" to 1/8" adapters that don't quite fit properly. 

If you're not using an adapter, you'll need to replace the headphone jack.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #7 on: August 01, 2021, 07:19:30 AM »
Those resistance readings indicate that you've likely overheated the lugs on the headphone jack where the red and white wires land, and they have moved a bit under the spring tension of the contacts on the jack and are no longer working properly.  We also see this sometimes from cheap 1/4" to 1/8" adapters that don't quite fit properly. 

If you're not using an adapter, you'll need to replace the headphone jack.

Thank you! There's an electronics store near me, so getting a replacement should be easy and quick.

Just to confirm... if the jack is indeed toast, it can still pass a little signal, correct? I'm asking because I can hear a little music playing.

Offline Paul Birkeland

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Re: Please help: weak output from new Crack build
« Reply #8 on: August 01, 2021, 10:35:38 AM »
Maybe just a little.  It's very, very, very unlikely that a general electronics store will carry the appropriate jack, so if you don't find one that completely identical, I would reach out for a replacement from us.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #9 on: August 01, 2021, 10:44:09 AM »
Maybe just a little.  It's very, very, very unlikely that a general electronics store will carry the appropriate jack, so if you don't find one that completely identical, I would reach out for a replacement from us.

Terrific, thanks so much.

Here in the Boston area, we are lucky to have a well-stocked electronics parts supply store (You-Do-It Electronics), and they have just about everything that online vendors like Mouser and Parts Express carry. I'm pretty sure I can get a proper replacement jack.  :)

I will follow up tomorrow!



Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #12 on: August 03, 2021, 03:59:28 PM »
I just wanted to pop in quickly to say that I haven't had the chance to do any more troubleshooting - very busy at work this week - but will probably pick up a new headphone jack and fire up the soldering station on Thursday or Friday.

Offline beatcomber

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Re: Please help: weak output from new Crack build
« Reply #13 on: August 10, 2021, 05:06:10 AM »
Update...

As Paul correctly predicted, my local electronics supply store did not have the correct headphone jack, so I ordered a couple from Mouser, which cost about $10 including shipping. Kind of frustrating for a part that cost less than a buck.

I received the replacement and began the process of desoldering the old one, and ended up busting at least one of the 2.49k ohm 1/4 watt resistors. ($^@$%!!!) I went back to You-Do-It (You-Blew-It) Electronics, my local store, and they stock every resistor under the sun except for the one I need. The closest I saw in stock was (IIRC) 2.75k ohm or maybe 3k, I can't remember for sure.

If I order from Mouser, it will cost me another $10, which is outrageous. I plan to call Bottlehead HQ later today, and see if they will be willing to help me out.

Very frustrating, but there's not much else I can do...

Offline Paul Birkeland

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Re: Please help: weak output from new Crack build
« Reply #14 on: August 10, 2021, 05:38:36 AM »
You could use a different resistor value there without ill consequences.

We do also provide replacement parts for a very reasonable cost. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man