Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: [email protected] on September 28, 2024, 12:36:31 PM
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Hi All,
I built several kits back around 2010 and they've been working great for me all this time. Recently, the SEX kit (not sure which version I have, has C4S upgrade, headphone plug update that disables speakers when plugged in, maybe something else?) has started having a noisy right channel. I hear it both in the right speaker or in the headphones, with and without any source plugged in, and the noise level is constant regardless of volume set.
It sorta sounds likes "sshhhhhhhhhhhhhhh." As the SEX powers on, I briefly hear it first in the left channel, which then after a few seconds the left noise tappers off to a being almost perfectly quiet. However, as the left gets quiet, the right gets slightly louder and then stays at a consistent noise level that can be heard above the music being played. I've flipped the tubes, switching left/right, and I still hear the sound in the right channel. I looked at the following post (https://forum.bottlehead.com/index.php?topic=14635.msg132983#msg132983) and reflowed the solder on everything in the right side of the unit and the noise still remains.
I've been running this for about 14+ years now, with the original tubes, and it's worked beautifully all this time. Is there a part that I might need to replace to get this working properly again?
Thanks,
Pavel
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If at all possible, I'd post some photos of your build as it sits now, as that will give us an idea of which kit you have, and there may be a part that's visibly upset. It would also be a good idea to do a voltage check to see if anything is obviously awry.
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Here are some photos. I don't know where my original manuals are, so not sure where I can find the version needed for the SEX kit I have to check voltages, etc.
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That's a SEX 2.0 with the C4S upgrade.
Measuring the DC voltages at pin 3 on each octal socket and the DC voltage on the metal tab of Q2 on each C4S board will give a reasonable amount of information about how the amp is operating.
It's very likely that you'll need to hook the amp up to some cheap speakers and run it inverted in its base so you can poke around with a wooden chopstick, listening for an area that makes noise when you poke it, which is useful for directing resoldering efforts.
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I pocked around with a chopstick on all connections and heard no audible changes while poking around.
I'm not sure what you mean by pin 3 on octal socket, as that is ground? From octal pin 3 to ground on plug, it is 0V
Right Channel (A)
Q2 on C4S - 310.1 V
Left Channel (B)
Q2 on C4S - 315.9
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Oh, and I found my old CD with manual, but no longer have a CD player to access the files. The label on the CD says the Rev 6-29-06, is it possible to get a digital copy of this PDF?
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From octal pin 3 to ground on plug, it is 0V
Right Channel (A)
Q2 on C4S - 310.1 V
Left Channel (B)
Q2 on C4S - 315.9
Yeah I'm forgetting that this kit has the autoformer output and the back bias arrangement. Can you check the DC voltages at pin 4 on each tube as well?
The 300V on the metal tab of Q1 would not be great, just to be clear this is the bigger transistor that's an MJE5731A, not the little metal encapsulated 2N2907. 300V on the metal can of each 2N2907 isn't abnormal, but 300V on the metal tab of both MJE5731As would mean the amp couldn't really make much in the way of sound.
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Ah, I forgot that Q ment transistor, I measured the voltage at the resistor lead near the Q2 marker (the 300V measurement). Let's try this again.
Right Channel (A)
Q2 Tab on C4S - 92.3 V
Octal Pin 3 - 0 V
Octal Pin 4 - 0 V
Left Channel (B)
Q2 Tab on C4S - 75.9 V
Octal Pin 3 - 0 V
Octal Pin 4 - 0 V
When measuring Octal Pin 4, I can "hear" the measurement through the headphone output, depending on setting of volume knob.
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The third time is the charm, let's see pin 1 on each tube.
Your metal tab measurements are mostly good, could you let me know what your voltages are on pin 6 of each socket as well?
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Right Channel (A)
Octal Pin 1: -17.47 V
Octal Pin 6: 2.674 V
Left Channel (B)
Octal Pin 1: -16.94 V
Octal Pin 6: 2.284 V
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Can you swap the tubes to see if that 2.28V switches to the A side?
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The voltages I am reading now (with tubes switched) are:
Right Channel (A)
Octal Pin 6: ~2.654 V
Left Channel (B)
Octal Pin 6: 2.283 V
On channel A (right), the voltage started around 2.7 and was slowly going down by 1 mV every few seconds or so, down to 2.654 after a few minutes. The left channel (B), was much more stable, staying within a few mV or the original value.
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On the little C4S boards, there's an I, O, and G terminal. Could you let me know what the I terminal is on each side after the voltages are stabilized?
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A/Right, C4S I terminal: 315 V
B/Left, C4S I terminal: 318.5 V
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Can you also verify that the high Q2 voltage stays put when switching tubes?
-PB
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Yep, channel A reads ~90V with either tube, and channel B reads ~78V with either tube.
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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!!!
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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.
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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.