Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: mcwhart on March 15, 2014, 04:40:55 AM
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Hello,
Just installed upgrade kit on my S.E.X 2.1 kit. Powered up, and there is low level hum with only a very faint signal heard through headphones with volume turned all the way up. All four LEDs light up, and both tubes are working. Any ideas?
After looking again, I realize I did something very stupid: connecting wires to A6 and B6 instead of A5 and B5. Ugh. Anyway, might anyone know what damage I could have done, or what may need to be replaced? I reconnected correctly, and I still get low level hum with only the faintest signal through headphones from the left channel only. Tubes and LEDs working...
Thank you for your help!
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Hello there,
How are the voltage measurements?
I do not anticipate that any harm has come to the PCB.
-PB
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Hello,
I checked all the voltages. Here are the ones that did not measure according to or near spec.:
On the 2.1 C4s,
Terminal 0A voltage is 0, should be 75
On the S.E.X 2.1,
Terminal 15 voltage is 9, should be 19;
Terminal A2 voltage is 310, should be 375;
Terminal A3 voltage is 9, should be 19;
Terminal A5 voltage is 55, should be 75;
Terminal B5 voltage is 65, should be 75;
Terminals A6 and B6 voltages are 2, should be 2.5;
Terminals A7 and B7 voltages are -3.8, should be -3.15;
Terminals A8 and B8 voltages are 2.25, should be 3.15;
Terminal C1 voltage is -3.8, should be -3.15;
Terminal C2 voltage is 2.2, should be 3.15;
Terminal C4 voltage is -3.8, should be -3.15;
Terminal C5 voltage is 2.5, should be 3.15.
Thanks for your help. Please let me know if there is something else I should be checking.
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0V at OA means that you have a cold solder joint on the middle leg of Q2 on the "A" side on the PCB.
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Thank you. I removed and switched the q2 transistors, resoldered, and I am getting a satisfactory reading of 70v at 0B but only about 7v at 0A.
Just powered up and I am getting an audible hum through the right channel at zero volume, nothing from the left. I get sound in the right channel when I increase volume, but again, nothing from the left. Any thoughts? Thank you
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I just did another voltage check after getting low level hum at zero volume and sound with increased volume on right channel only. No sound from left channel. Tubes and LEDs all lit.
Terminal 15: voltage is 9, should be 19
Terminal A5: voltage is 60.7, should be 75
Terminal B5: voltage is 70.4, should be 75
Terminals A7 and B7 both measure -3.89, should be -3.15
Terminals A8 and B8 both measure 2.26, should be 3.15
Terminal C1: voltage is -3.89, should be -3.15
Terminal C2: voltage is 2.3, should be 3.15
Terminal C4: voltage is -3.89, should be -3.15
Terminal C5: voltage is 2.49, should be 3.15
ON THE C4S:
0A voltage is 61.2
0B voltage is 70.6
Any suggestions would be much appreciated. Not sure where to go from here.
Thanks!
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Any advice? Anyone? Thank you. I just don't know what I should be looking for.
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Check the connection of the 0.1uF cap at A5. Sounds like a lead might be shorted.
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I checked the connection as suggested, resoldered it, and still, I am getting no sound from the left channel, and low level hum from right channel with volume down and sound in right channel with volume up. I rechecked resistances. The only discrepancy I found was with 14 and 24, which are supposed to be variable. 14 was 1.8, and 24 was 0.5 with the volume turned all the way down. Measurements at A7, B7, A8, B8, C1, C2, C4, and C5 are 4.10-4.18.
Voltage discrepancies are the same as in my last post.
Really at a loss here. Unit was working fine until I tried unsuccessfully to install the upgrade. Any suggestions much appreciated.
Thanks
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Can you post a couple photos of the channel that isn't working?
The 9V issue is independent from the C4S upgrade, but would cause some serious issues.
I would be triple checking that the 249K resistor on that side is still properly connected.
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Triple checked point you recommended and all appears good. Here are some photos. Measurements and symptoms remain the same as in recent posts. Thank you. Oh....forgot to mention that I pulled upgrade board out of the way for photos. Thanks
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The left channel output cap looks pretty scorched. Did that happen during the C4S install?
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What's the voltage at A1?
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Yes, I nicked that cap with soldering iron while installing upgrade--would that impact sound? Will check voltage at A1 and reply with that when I get home from work.
Thank you for your help. I really appreciate it.
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I switched the output caps and now I get low level hum in both channels. Level of hum is constant, does not change with volume adjust, but it definitely more than it should be, more than it was before I installed upgrade. Still getting sound from right channel only. I checked voltage at A1 and it is as it should be, near 0, .008. Any other ideas?
Thank you
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Is the 680 Ohm resistor on the A side touching anything other than the terminals it's connected to?
It may be hitting the pin that has the 220 Ohm carbon comp attached to it, which would short the input to that channel and throw the bias way off on the output section.
-PB
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I'm having a bit of trouble following this thread. The funky voltages posted seem to be on the A side, but it's the left channel that is being listed as not working. I know I often suffer from the brainrot caused by flipping a chassis and flopping channels, but the B side would be the left channel. Could you please post a full list of voltages for all terminals called out in the manual? Also, were any of the standard tests to confirm channels performed, i.e. swapping tubes, switching cables, et cetera?
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Checked the 680 ohm transistor to be sure it was not touching anything other than terminals.
Switched tubes, wires, etc., no change: low level hum in both channels, not affected by volume, sound from right channel only.
Voltages are as follows:
Terminal Voltage
1 204
2 0
3 0
4 409
5 204
6, 16 389, 390
7, 17 both 0
8, 18 both 0
9, 19 .016, .050
10, 20 both 410
11, 21 354, 355
12, 22 both 0
13, 23 both 0
14, 24 both 0
15, 25 18.2, 17.36
A1, B1 .016, .049
A2, B2 367, 369
A3, B3 18.2, 17.4
A4, B4 both 0
A5, B5 70.7, 60.5
A6, B6 2.29, 2.31
A7, B7 -3.73, -3.71
A8,B8 2.23, 2.22
C1 -3.84
C2 3.28
C3 alternates between 0 and .046
C4 -3.84
C5 2.46
C4S MEASUREMENTS:
0A 70.1
0B 59.3
10 412
20 412
Tubes still light up, as do all 4 LEDs.
Is there any other information I can provide? I am at a loss for what to do at this point.
Thank You
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Hello again, wondering if anyone has other ideas about this situation with my amp? I am not sure whether to keep trying to fix it or to send it in for repair. I have tried all that has been suggested and still can not get it to work properly. Thank you
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Your voltages all look perfect (now), so there is a next step we can work on. What is the lowest AC voltage range that your meter has?
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I believe it is 200 mV. The meter is a Radio Shack #2200820 22-Range Digital Multimeter.
Thanks
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Alright,
What you want to do is download or obtain a 60Hz test tone. This will let us play AC through your amp and you can use your meter to measure it as it passes through each stage.
The first step is to play the tone into your SEX amp, then measure the AC voltage between ground and each lug on the volume control (turn it all the way up).
After getting these numbers, we can move on with testing other points in the circuit. To perform the measurements on the pot, the amp can be off, but for the other points we may look at, it will need to be on.
-PB
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OK. Thanks.
Here are my readings, volume all the way up, amp off, 60 Hz test tone playing through inputs:
Going from left to right side of chassis, or from A side to B side:
Bottom lugs: .4 mV, 347 mV, 348 mV
Top lugs: .6 mV, 434 mV, 434 mV.
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Alright, this means that you have sound passing through your RCA jacks and out your level control, so we can rule those out.
The next spot to measure this AC voltage will be A1/B1.
-PB
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I am assuming with power on, right?
With power on, and no 60 Hz signal, I get around 19 mV at A1 and around 50 mV at B1.
Thanks
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You need the power on and the 60Hz signal. PB is having you trace the signal through the circuit step by step to find out where it is dropping off.
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Oh, OK, thanks.
With 60Hz tone I am getting about 15mV on A1 and about 29mV on B1.
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Alright, well, the signal is dead there...
What do you get at 19 and 9?
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At 19 I get -12.3 V, at 9 I get -4.2 V.
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If you get a negative voltage you are measuring DC. The signal is AC.
Check the setting on your meter and make all the measurements again.
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Very sorry about that. Meter was on auto setting.
Here we go again:
Volume Pot all the way up with power off and 60 Hz tone:
top lugs, from A side to B side: .5 mV, 1.377 V, 1.377 V
bottom lugs, A side to B side: .5 mV, 1.039 V, 1.039 V
Volume pot all the way up, Power on, 60Hz tone:
A1: 17.36 V B1: 23.1 V
9: 17.38 V 19: 23.1 V
Thanks for your help and patience
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Ok, the signal is getting through to these points. A lot of signal!
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Is a lot of signal good or bad at this point?
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Let me make sure I am following you. The first measurements should be from the three lugs on each side of the pot to ground or the chassis. The second measurements are to pins 1 of each tube (grid of the second stage).
If you are measuring this, you are getting amplification from the volume pot (about 1V) to the grid of the second stage of the tube (17V AC/23V AC). That is what is supposed to happen.
You should also get more AC voltage on pin 2 of each tube.
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Yes, this is correct. By lugs on the pot I am assuming you mean the upper and lower row of three tabs, or lugs(?) on the volume pot. Those are where I took the first six AC voltage measurements, followed by A1 and B1, then 9 and 19.
Thanks
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Yes, we are moving in the right direction. I'm guessing it will be something silly like I did 2 weeks ago. So far you are doing all right.
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Great. Thanks!
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Any suggestions as to where to go next with this? Thank you
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The output section of the tube is pin 2. You should have DC and AC amplified voltage there. You should be able to turn it on, re-verify the AC voltage on tube pin 1 then you should read more on pin 2 of each channel.
If the DC there messes with the reading measure instead to terminals 20 and 40. That is after the Parafeed cap and the input to the output transformers. There should be a transformer wire on terminals 20 and 40 as well as the Parafeed cap.
If you get amplified voltage on these terminals the signal is dead somewhere in the transformer wiring to the speaker outputs.
Or, maybe, the headphone jack has failed and permanently opened the circuit to the speaker outputs.
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Your C4S upgrade is working perfectly, we can now move completely away from that as a source of not having sound.
Grainger is correct, pin 2 on each tube socket is the next place to measure voltage.
Then terminals 15 and 25.
Then where each red wire connects on the binding posts.
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Here are the AC voltage readings I get for points you indicated:
A1: 16.5v
A2: 218v
B1: 23.2v
B2: 239v
Terminal 20: blinks back and forth between about 240v and about 405v
Not sure where terminal 40 is?
Terminal 15: 222v
Terminal 25: 246v
Binding Post red wire on A side: 8.15v
Binding Post red wire on B side: 9.03v
Thank you
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Binding Post red wire on A side: 8.15v
Binding Post red wire on B side: 9.03v
You have sound coming out of your SEX amp...
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Ah, yes I do. Thanks for your help.
Still a low level hum that was not there before attempted upgrade install, and the right channel is definitely louder than the left. Tried switching tubes and connection wires, no change. Any ideas about what might be causing that? The amp was much quieter and balanced before the upgrade than it is now.
Thanks
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Any advice as to where I should be looking to resolve low-level hum issue and stronger signal from right channel?
Thank you
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Still a low level hum that was not there before attempted upgrade install, and the right channel is definitely louder than the left. Tried switching tubes and connection wires, no change. Any ideas about what might be causing that? The amp was much quieter and balanced before the upgrade than it is now.
Your tests revealed that the voltage coming into the SEX is out of balance.
Low level hum could be from a loose wire or a capacitor that only has one lead well soldered.
If you have this hum in both channels, look at the 220uF/250V caps first. I generally will poke at them with the amp off and be sure that when they move, they move with the terminal strips, not independently from them.
-PB
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In this post I will just address the channel imbalance.
The measurements at A1 and B1 (the grid of the power section of the tube) are 2.6dB different, indicating that the gain of the driver section is different. A difference is not unusual, but yours are at the normal limits of plus and minus. It's possible that is the entire problem, but let's make sure. As a first test, swap the tubes from side to side to see if the level difference follows the tubes. If the level difference switches channels and retains the same magnitude of difference, then a replacement set of tubes is the answer.
The above test assumes the power section gains are both equally matched. However, the AC voltage measurements were made at such a high input voltage level that the power sections are strongly overdriven, so the measurements at A2 and B2 (the power section plates) don't mean much. To get better information (hey, I'm an engineer, I LIKE more and better information!), provide the 60Hz tone but turn the volume control down until the middle terminal of the controls is at about 100mV (0.10 volt) AC. You should see around 2.0 volts AC at A1/B1 then, and around 30 volts AC at A2/B2, and around 1.0 volts at the speaker terminals. This will be a complete test of the gain structure of the amp in its linear range - specifically, the driver stage gain, the output stage gain, and the transformer ratio.
If the hum persists after following PB's suggestions, we'll address that next.
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Thank you PB and PJ. I will try what you suggest when I get home from work.
I have already tried switching the tubes, and there was no difference in hum or difference between right and left channel output. The low-level hum remained, independent of volume level, and the right channel remained louder than left, regardless of placement of tubes.
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Since this has rolled to four pages I'm not going to read all the posts.
We have seen some builders get an AC wire too close to the signal wire and pick up hum this way. Just check that your wires are dressed like the manual. Bottlehead is very careful that they layout the two such that they won't interfere.
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OK. This is in response to PJ's instructions for addressing channel imbalance.
Something is not right here.
With the 60 Hz tone, and the volume adjusted so that the top center lug on the volume pot measures 100mV (bottom center lug is more like 60 mV) I get the following readings:
A1: 10.9 VAC
A2: Blinking between 250 and 340 VAC
B1: 1.75 VAC
B2: 22.7 VAC
Speaker terminals on A side, red wire: 5.6 VAC
Speaker terminals on B side, red wire: .88 VAC
So the B side readings seem somewhat close to what PJ indicated they should be, while the A side readings are 10 times what they should be.
I will power down and recheck all connections.
Thanks
In this post I will just address the channel imbalance.
The measurements at A1 and B1 (the grid of the power section of the tube) are 2.6dB different, indicating that the gain of the driver section is different. A difference is not unusual, but yours are at the normal limits of plus and minus. It's possible that is the entire problem, but let's make sure. As a first test, swap the tubes from side to side to see if the level difference follows the tubes. If the level difference switches channels and retains the same magnitude of difference, then a replacement set of tubes is the answer.
The above test assumes the power section gains are both equally matched. However, the AC voltage measurements were made at such a high input voltage level that the power sections are strongly overdriven, so the measurements at A2 and B2 (the power section plates) don't mean much. To get better information (hey, I'm an engineer, I LIKE more and better information!), provide the 60Hz tone but turn the volume control down until the middle terminal of the controls is at about 100mV (0.10 volt) AC. You should see around 2.0 volts AC at A1/B1 then, and around 30 volts AC at A2/B2, and around 1.0 volts at the speaker terminals. This will be a complete test of the gain structure of the amp in its linear range - specifically, the driver stage gain, the output stage gain, and the transformer ratio.
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Swap the tubes, if you haven't already.
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With tubes swapped:
With the 60 Hz tone, and the volume adjusted so that the top center lug on the volume pot measures 100mV (bottom center lug is more like 60 mV) I get the following readings:
A1: 13.06 VAC
A2: Blinking between 182 and 326 VAC
B1: 2.8 VAC
B2: 38.5 VAC
Speaker terminals on A side, red wire: 6.56 VAC
Speaker terminals on B side, red wire: 1.54 VAC
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Thanks, that helps!
This is a puzzler for sure! Here are three things I see:
1) the A-side driver cannot have enough gain to turn 100mV into 10-13 volts, so I suspect some wiring error in or around the volume control - specifically, the middle lug of the control on that side should go to A4 but perhaps the hot lug does instead.
2) The high and unsteady output at A2 suggests to me that the tube is oscillating. First check is the integrity of the 220 ohm resistor at A1 - they are kind of fragile mechanically. Perhaps a picture of the A socket would help, if you can get a good close-up.
3) The volume controls are not well matched given the 60mV/100mV difference. A mismatch at very low settings is not uncommon but that seems a little more than usual, and at a higher setting than usual. I'd suggest getting the wiring sorted out before going for a replacement; spareparts will send you another control if you email.
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I checked wiring around the volume control and it appears to be correct. Also checked the wiring and connections throughout and reflowed a few connections. Plugged it in and turned it on and now I am getting low level hum on both channels, with no signal from the right channel and a very weak signal from the left channel. Ugh. Do I go back to resistance and voltage checks from here? Frustrating.
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Man, I'm sorry - I hate when that happens, that point where nothing makes any sense and every thing you try makes it worse. It's no comfort, but we've all been there, or will eventually!
Yeah, there's something wrong and it's invisible. Either it's hidden - like a broken wire that is covered by insulation - or you can't see it because you've passed it over too many times. Don't discount the latter, it's a real thing that happens to us humans, I've been there more times than I care to admit. Whatever, you can't see the problem and it's not for a lack of looking.
So anyhow - start with the resistance checks again. Do them carefully and slowly. Every time you look at a wire, check to see if the wire got damaged when the insulation was stripped off - we're looking for hidden problems here. Get a bright light!
Then, I'll still say it looks like there's a problem in the input connections. So trace the signal path. Set the volume control about halfway, and measure each volume control resistance to ground. The highest resistance should be about 100K, that's the input to the volume control so make sure the resistance from that point to the RCA jack center pin is zero ohms. Then measure resistance from T14 and T24 to ground. Then A4 and B4. These should all be the same. Here I'm just trying to check all the input wiring and volume control action. I don't know what is wrong, so I'm just trying to determine that some things are right so we can cross them off the list.
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I can't believe I am writing this, but it works! Before running resistance and voltage tests I went through everything one more time, this time looking at it from a different angle, and I discovered a loose red wire on one of the bottom lugs on the volume pot. I also noticed a red wire in the headphone jack assembly that was possibly touching two terminals. Clipped off the end of that, flipped the chassis over, held my breath and powered on. It sounds superb as a headphone amp for my Grado 325is phones. For speakers I am using new Blumenstein Orcas with only about 150 hours on them, and as good as it all sounds now I am sure it will improve with time.
Thank you so much, PJ, PB, Grainger, and Doc B for all of your help and PATIENCE. This has been a rewarding project, if challenging at times. I learned a lot, and I have a hell of a nice amp. I really appreciate all you have done.
Best regards,
Steve
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Congrats!! So glad it's working correctly now.
Yeah, it usually turns out to be something really simple. Not always of course, but pretty darn often. We'd all make fun of you, but every single one of us has had the same experience, more than once! Real life has this way of keeping us humble ...