Noisy SEX, Right Channel [resolved]

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Yep, channel A reads ~90V with either tube, and channel B reads ~78V with either tube.

[Paul Birkeland]

I'd reflow the solder joints on the C4S board on the A side, though don't reflow the ones for the LEDs.  You could also verify that the cathode resistor value is the same between halves, which would be the DC resistance between ground and pin 3. 

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I reflowed all the joints (minus LEDs) on the A C4S. Since I lifted the C4S off the post to do this, I also reflowed all the pins on the octal plug. No difference in voltage measurements and the right channel continues to be noisy.

Do you mean pin 4, as three is ground? For these measurements, the GND probe was at GND bolt near plug.

A/Right:
R Pin3-GND: 0.2 ohm
R Pin4-GND: 7.3 k ohm

B/Left:
R Pin3-GND: 0.2 ohm
R Pin4-GND: 7.6 k ohm

I measured the other pins as well, and the resistance is approximately the same between A/B for all pins.

[Paul Birkeland]

Sorry, should be pin 6.  The arrangement of the big half and the small half in the 6DN7 is hard to keep straight in my head.

[[email protected]]

For resistance measurements, should the amp be plugged in and on or unplugged and off?

If off, the values are:

A:
R Pin 6 - 1.27 k ohm
R Pin 4 - 1.3k ohm

B:
R Pin 6 - 1.27 k ohm
R Pin 4 - 2..05k ohm

[Paul Birkeland]

It is probably worth swapping the C4S boards between channels since it's just 3 wires per board.  Beyond that, you'd need a clip lead or two to continue with the debugging. 

[[email protected]]

Okay, this seems like progress. I just switched the C4S boards and the noise is now on the left channel! Could one of the transistors be going bad?

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Oh, and I went and reflowed every connection on the problematic C4S (labeled A on board, now left channel) again, it still remains noisy.

I did check the resistance value of two resistors on each A/B boards and they were nearly identical between the two boards. Could it be the LEDs? I've refrained from reflowing those. They light at similar intensities between the two boards.

[Paul Birkeland]

There are a lot of possibilities for what that means.  It could be a transistor issue, solder issue, LED issue, trace issue, etc. 

We don't sell that board anymore, but the SEX 3.0 C4S upgrade is the same thing on one board.  You can cut that board in half with tin snip and it will otherwise function the same.

[[email protected]]

As a Hail Mary, I did go ahead and reflow the LEDs, carefully. No change, noise continues.

Thanks for all the assistance, I've sent you a PM on getting a new C4S board and if I need anything else.

I'll post here once I get the new parts and if issue is resolved.

[[email protected]]

I received the SEX 3.0 C4S upgrade this morning and worked this afternoon on getting it installed, using the old wiring as a guide. The amp is now very silent, the noise only appearing briefly in both channels as the tubes warm up and disappearing after a few seconds. Voltages on OA and OB are 75/79V. Not sure which part failed on the old board, but I am glad the amp is back to a working condition!!!

While I am in the mood for tinkering, I was curious about the volume pot. I've noticed that at volumes above 1/2, I begin to hear noise in both channels (though slightly more heavy in left) and perhaps a slight buzzing sound. Is this normal? Is the volume pot the cause of the noise (I hear a little static as the volume is increased/decreased that stops once moving the pot) or some other issue in the amp I haven't diagnosed yet? If it is something to do with the volume pot, is it possible to just remove it? I have the old Submissive for selecting sources and regulating volume from the Eros, and can use my computer output for computer volume.

Meanwhile, I am just so glad to have the amp be noise free on typical volumes I listen to!!!

[Paul Birkeland]

If you have nothing plugged into the SEX amp and you turn the pot all the way up, it would be normal to hear some noise.  Even with a source connected, turning the pot way up may just bring up the noise floor of what's in front of the SEX amp, so I wouldn't necessarily recommend parts swapping to attempt to solve what may just be normal operation.

[[email protected]]

Thanks, I didn't think about that. I tested my tape deck and there was zero detectable noise from that source even at max volume. At this point I think the SEX amp is back to full functionality! Thanks again for all the help.