[zzffnn]

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/81202/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.

 

[zzffnn]

This is a balanced Bijou BTW.

 

[RexAeterna]

i'm not totally sure but humming usually indicates something is up with the dc offset and the bias. see if you can check those with your multi-meter if you can.

 

[diego]

I would try moving the signal cables away from the heater wires first. The heater wires are the twisted red and yellow ones. Do it with a chopstick as you did ( as long as it´s made of wood or plastic) and be very careful because there are deadly voltages inside that amp.

 

[zzffnn]

Thank you guys very much for helping.
 
I got some advice from my seller and other DIYers and found a loose signal wire which was off from the volume pot. Also found a very loose (not completely off) terminal block in one of the amp board.
 
I soldered the loose signal wire back to the volume pot. However,I could not solder back the terminal block. Its base has three contact points, and I was not fast enough to fill all three contact points with solder to re-attach the terminal block.
 
Please advise me what to do: how do you solder a terminal block back to PCB?
 

 

[zzffnn]

Thanks everyone for helping here. We fixed the problem.
 
As holland and my seller pointed out, the hum and lost of signal was due to a terminal block in one of the amp board and a signal wire connected to the volume pot. We soldered back both and got sweet music from both channels with no hum. My seller, Shinyfalcon (my local builder), and I worked closely together and solved the problem. 
 
May you all have a joyful new year!