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Tube amp produces loud hum in one channel please help

post #1 of 27
12/22/10 at 8:09am
Thread Starter 

Hi all. Please kindly advise me about the following problem.

 

I recently bought a balanced Bijou for my 600 ohm Beyer T1. The combo was lovely until I did some thing stupid.


I took off the transformer cover and tighten (I think no excessive tightening) some screws inside the amp. Then I have big hum (hmm kind of sound) consistently in one channel via the Beyer T1. Putting back the transformer cover and un-tightened the screws did not reduce the hum. My seller is very kind and nice and helping me to trouble shoot with photos. I am putting a post here just to get more help, as I think the problem may not be solved easily.

So I tried the following things with no luck:
1) burnt-in, warmed up the amp
2) disconnected any input to the amp, just listened to the amp with headphone w/o input
2) switched all tubes between the two sets of amp and power boards (as this is a balanced tube amp), still had same hum from same side (I am using the single end output btw).
4) both of my headphones got the same hum from the same channel (600 ohm T1 and 80 ohm DT770).
5) The hum does not change with volume adjusting.
6) used a different outlet.
7) used a two prong "cheater" outlet
8) turned all the light dimmer to off all over my office (it was an old residential house, some dimmer are not working).
9) Used chopsticks to push/tap tubes slightly when the amp is running. And slight pushed PCB boards. I did not hear much changes when I did these procedures.
10)checked about wires and connections (note that I have no DIY or EE skills to do this extensively though).

Big photos will high zoom from inside of the amp can be found in my photo album here:
http://www.head-fi.org/gallery/image/view/id/81198/user_id/126562

 

Please kindly advise me. keep in mind that I have multi-meter with me but can not solder and have almost no electrical or DIY skills.


Since I swapped tubes between the 2 same sets of amp and power boards (this is a balanced tube amp), whatever noise from tubes should shift as well, correct?
Unless  the join between PCB and certain tube is "cold"? I think I did not tighten the screw excessively to cause this........but I do not know.....
 

Thanks a lot for reading.

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post #2 of 27
12/22/10 at 8:10am
Thread Starter 

This is a balanced Bijou BTW.

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post #3 of 27
12/22/10 at 8:31am

your pictures are too small.  need high res.

 

your chances of fixing this are low, FWIW.

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post #4 of 27
12/22/10 at 8:36am
Which screws did you tighten? Can you provide zoomed in shots of those areas?

Odds are you overtightened a screw to the point of breaking through the wire to cause a disconnect, most likely at a grounding point.

Does the signal still come through? Can you hear sound in both channels?
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post #5 of 27
12/22/10 at 9:09am
Thread Starter 
Quote:
Originally Posted by holland View Post

your pictures are too small.  need high res.

 

your chances of fixing this are low, FWIW.



Please click on "URL to inbed", once the photo loads, please click on the displayed photo once more, that will give you big photo with high res. Thanks

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post #6 of 27
12/22/10 at 9:11am
Thread Starter 
Quote:
Originally Posted by jdkJake View Post

AWhich screws did you tighten? Can you provide zoomed in shots of those areas? Odds are you overtightened a screw to the point of breaking through the wire to cause a disconnect, most likely at a grounding point. Does the signal still come through? Can you hear sound in both channels?


Excellent point, Sir. I am getting sound ONLY in the Channel that does not hum. The channel that hums produces no sound.

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post #7 of 27
12/22/10 at 9:14am
No, I mean really, significantly zoomed in shots around all of the screw terminations you tightened.

You are looking for a needle in a haystack, so, the overhead shots are next to useless regardless of resolution.
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post #8 of 27
12/22/10 at 9:16am
Quote:
Originally Posted by zzffnn View Post





Excellent point, Sir. I am getting sound ONLY in the Channel that does not hum. The channel that hums produces no sound.



 



So, I suspect you tightened one or more of the Green terminal blocks. Is that true?
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post #9 of 27
12/22/10 at 9:22am
Thread Starter 

Slide1.tif

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post #10 of 27
12/22/10 at 9:26am
Thread Starter 

I tightened probably all screws accessible around green PCB boards. Sorry, what is a "green terminal block"? Do you mean the PCB printed boards or the green wires coming in and out of the transformer? I did not touch the green wires. The red circles above indicates the screws that were tightened. As you can see, there are too many to zoom in. Please advise me what one may see in the zoom in, I can certainly shoot some zoom ins.


Edited by zzffnn - 12/22/10 at 9:29am
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post #11 of 27
12/22/10 at 9:29am

No, each of the PCB's have wires running to light green colored terminal block like this:

 

 

 

Did you tighten any of these connections?

 

EDIT: Guess that image did not post right, here is what I am talking about:

 

   http://www.tycoelectronics.com/catalog/pn/en/282836-3


Edited by jdkJake - 12/22/10 at 9:31am
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post #12 of 27
12/22/10 at 9:33am
Thread Starter 

Got it. I did not tighten screws on the green blocks. They are too small for my screw driver at the time. I did not touch them.

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post #13 of 27
12/22/10 at 9:43am

Okay, well, I suspect that one of the wires that goes into one of these terminal blocks has come loose and is no longer properly seated in the block. It may even be laying outside of the block. More than likely, it is one of the signal wires rather than one of the ground wires.

 

I would look carefully at each and every terminal block to be sure the wires are still actually seated within each of the blocks

 

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post #14 of 27
12/22/10 at 9:58am
Thread Starter 

Thanks JK. I will check

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post #15 of 27
12/22/10 at 10:19am
Thread Starter 

I have not checked all terminal blocks but can rule out problem with the single end headphone output. Because the balanced headphone output sounds exactly the same as the SE: music in one channel and hum in the other channel (no music).

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