Noise in left channel - modded SE 40 - diyAudio
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Old 25th February 2013, 02:00 AM   #1
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Default Noise in left channel - modded SE 40

Hello all. My rev 3 SE 40 has lately developed a noise in the left channel only. Jeff G did a full tilt mod to the board 5-6 yrs ago and I've since enjoyed the amp immensely. The last few months the amp began having a "crackling" sound in the left channel that disappeared after a few minutes of warm up. The static like noise now is worse and does not go away completely. It is not volume dependent, is not a cable or tube, and the amp plays fine through both channels, save for the noise in the left.

I have rolled quite a few 6SN7 tubes through the amp, but not for about the last year. When the crackling first began I found that slightly wiggling the rear 6SN7 sometimes stopped the noise... or aggravated it!

I'm no tech but I can solder and could probably work a test meter if told what to test for. Any ideas where to start oh wise ones?
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Old 26th February 2013, 06:02 AM   #2
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Hi Dan,

When you say it's not a cable, I'm assuming you've independently swapped the speaker and interconnects from left to right? With regard to the tubes, I'm assuming you've switched the output tubes from left to right and rebiased? Have you tried swapping out the 6SN7's?


Assuming that all of the aforementioned has been performed, I would first look to the tube sockets. Both the three output tube sockets on the left side, and V1-V2. It is not uncommon for these tube sockets to become loose. Especially when "rolling tubes." This would support what you have already noticed while wiggling V1.

Note: The factory identifies the rear 6SN7 as V2, but it is actually V1.

The last somewhat common possibility is a stressed resistor. This would most likely be a factory resistor that has not been replaced. Most of the factory resistors are carbon film with steel endcaps.

Servicing the pin sockets:

Generally, this what I do to tighten this type of tube socket. I use the straight probe from a cheapo pick set - the diameter is .055" and it works very well for the SE-40 boards. The more expensive pick sets, like Snap-On, tend to be heavier and won't fit between the ceramic and the pin contacts.

Simply put, you will thoughtfully but forcefully, drive the tool between the ceramic and the slots on each side of the pin hole. Apply light tension toward the center of the hole until the socket pin gives slightly. The socket pin for each tube pin is a fork, and this action will narrow the gap between the tongs of the fork. I need to point out though, that under no circumstances should more than half the socket pin extend into the circular hole on either side. At this point I do make some effort to center the fork in the hole as well.

Not only is it possible for the tube socket to become loose at the connection point with the tube pins, it is possible for the socket to become loose in the board. If the socket is loose in the board, you have a damaged via. You will notice that the tube socket is soldered into vias shaped like Mickey Mouse's head. The problem is the through connection, or barrel, if you will, becomes separated from the solder pad on both sides of the board. You will not be able to view the via from the socket side of the board. It will be obscured by the socket itself. However, on the other side of the board, you should be able to view the solder meniscus around the socket pin as it connects to the solder pad. If you look closely, you may notice a fracture all the way around this Mickey Mouse shaped via. Your first inclination may be to reflow the joint. However, it will always fracture. Let's start by having you inspect this connection on all eight pins on V1, V2, V3, V4, & V5. I would suspect the problem would reside with V1, given your symptoms. You will have to look very closely, and may require a magnifying glass and additional light.

Testing for stressed resistors:

You will flip the amplifier upside down with everything connected, except your interconnect. With the amplifier on, on the left side of the amp, you will go from resistor to resistor spraying them with component coolant. When you get to a distressed resistor, you will certainly hear either a reduction or an increase in static. There is no need to perform this on any of the resistors that have been replaced, IMO. You should make a visual inspection for discoloration on all resistors prior to starting this procedure. Remember that lethal voltage is present in many locations. I would never physically contact any component with the spray wand on the can of coolant, nor would I touch any part of the chassis while doing this. Further, you might elect to use a small piece of card stock to shield untested resistors from over spray while applying coolant.

If any or all of this is a bit more than you signed on for, feel free to send your board in. It should take less than five minutes to remove the board, and it is typically four pounds packed to ship. My fee would be a very small penance as I do this kind of thing every day.
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Old 27th February 2013, 03:06 AM   #3
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Thanks Shawn, good advise, I appreciate it. Swapping IC's the static stays left. Swapping speakers cables, the static goes with the cable connected to the left.

I have 2 sets of 6L6's, have swapped and biased, no help. I've got dozens of 6SN7's too...to no avail.

I will check out the socket pins and solder joints to the board. The solder joints from pins to board and a failing resistor were top of my list to check.

Thank you again Shawn. Now I just need some time to open it up... or I'll pull the board and send it your way. What's your current backlog?
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Old 27th February 2013, 07:42 PM   #4
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My pleasure, sir. I'm currently running about a week out.
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Old 3rd March 2013, 03:25 AM   #5
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Default You've got a board coming your way Shawn

I've looked at the board, and I quickly found the end of my knowledge

Confirm what is the correct shipping address and what info you'll need from me please.

Thank you
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Old 3rd March 2013, 09:04 AM   #6
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PM sent.
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Old 3rd March 2013, 08:58 PM   #7
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Thank you Shawn. I'll update once I've shipped the board your direction.
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Old 8th March 2013, 01:38 AM   #8
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The board is on it's way Shawn. PM sent.
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Old 9th March 2013, 09:14 PM   #9
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Dan,

Just wanted to let you know, your board arrived safely this afternoon. I'll let you know what I find.

Shawn
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Old 10th March 2013, 12:44 AM   #10
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Thanks Shawn
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