Bottlehead Forum
Bottlehead Kits => S.E.X. Kit => Topic started by: soundfel on May 18, 2020, 03:52:11 PM
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I finished my SEX 3.0 but had a dead right channel so decided to resolder everything on the A side and changed the pot to an Alps 100k. Now, every resistance on the B side is wrong, everything reads OL. I didn't touch anything on this side and the soldering seems OK. Any help is appreciated!
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B15 bolts to the chassis, is that 0 ohms?
For the rest of the terminals that should read 0 ohms on the b side, there's a buss of black wires that bounces between the terminal strips and terminates back at the high voltage power supply. One of those wires has to be loose or missing for you to be betting OL on terminals that should otherwise be 0.
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Thanks for the reply! Ok, I rewired the pin a1/a12 cable and fixed a solder on 12L. I’m now getting 46k ohm at terminals 4 and 18 (should be 249k) but the rest are OK now. Any ideas?
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Why did you rewire the A1/A12 cable? What did you replace it with?
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I just thought it was in a bad shape after the first re-soldering and that maybe that was cause of the bad readings. I used the same kind of cable and the same connection (red A12 to B1, black A1 to B12 and drain to 15L).
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In case it helps, here's a picture of my build.
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What's your pot wiring like?
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It is wired the same way as the stock pot was
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Maybe its just the angle of the photo but I do not see anything connected to the wiper of the alps pot (pin 2)
This is what feeds the voltage amplifier grid of the 6fj7 pin 10 on that tube.
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Without a grid load your going to have big problems. this load comes from the pot in this instance.
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Hello and thanks for your reply. I think it is the angle of the photo. Both wipers are connected to pin 8 on each side. Do you think I should go back to the stock pot?
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I'm sure you can use the Alps pot but I feel that the circuit may be miss wired. please check that the wiper of the pot is soldered to pin 8
of the 6fj7 socket and the grid stop resistor. then make sure the grid stopper resistor is connected to pin 10 of that socket.
pin 8 is an unused pin for that tube that makes it a convenient mounting place to mount the grid stopper resistor. Having said that I still feel that perhaps your volume pot may be miss wired so perhaps a few more photos of the pot showing the connections to the pins could be useful.
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Thank for answering, here are some more photos
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Now that the resistances are OK, how are the voltages?
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Hello, Paul. Not looking good... I´m getting around 485v on terminals 2, 6, 7, 10, 13, 16, 20, 21, 24 and 27. The rest of the terminals on that section are 0v. On terminal 20 I get 20v, 487v on 33, 243v on 34 and 35 and 0v on 29, 31,32 and 36. It marks around 22v on H2, H3, H5 and H6, and 0v on the rest.
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Did you pass the glow test?
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Well, if you don't pass the glow test, none of those DC voltages will be correct. So neither tube glows? When you replaced that STP cabling, you disabled the heaters. This is the wiring that feeds pins 1 and 12 on each 12 pin socket. Follow that backwards and you'll be at the 10,000uF cap and 0.1 ohm resistor, then backwards from there you get to the H terminal strip with the big rectifier diodes that mount to the chassis.
Something is not connected or broken along that path. You can forget about doing any other debugging of any kind until your amp passes the glow test again.
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Both tubes are off. All the wiring looks fine. I replaced the wires that run 31/32 and resoldered but still no glow. Since I'm getting no voltage at H4, could it be a problem with the power transformer? I ran the transformer secondary test on the manual again and it's OK, the AC voltage are also normal. Do you think I should replace the capacitor or the 0.1 resistor?
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I think you should stop replacing parts without actually knowing they are bad. You had glowing tubes, now you don't. All that you've changed is the STP wiring that heats the tubes. If the capacitor is bad, it will pop its top and blow out its goo, and you would be telling me about that. The primary way that happens is if you install it backwards. You wouldn't have ever passed the glow test if you had put that cap in backwards. Likewise I couldn't figure out a way that you could kill the 0.1 ohm resistor. If you start pulling these parts out and replacing them without any reason to do so, you will continue to make new problems that you will have to track down.
With one probe on H2 and the other probe on H4, what DC voltage do you get? How about with one probe on H5 and the other probe on H7?
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You are right Paul, I get impatient but that only makes more trouble. Voltages are -18 on both those readings. Thank for your help!
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Voltages are -18 on both those readings. Thank for your help!
No, they are not. Maybe it says -18mV? Or Maybe -0.018 or -0.18? If in doubt, you can post a photo of what your meter is showing for one of those readings.
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When I turned the amp to test these voltages again, tubes were glowing. I tested the voltages and they were OK, but afterI flipped the amp to test it with my headphones, tubes went back off and the voltages are wrong again. This is what I get between H2 and H4. It´s the same on H5-H7.
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It could just be an effect of the photo, but in the first pic you posted in this thread it looks like one of the ends of the thick solid green wiring isn't connected to a terminal of the power transformer (or is poorly connected).
You may also want to take a look at the condition of at least one of the 1.5uF capacitors (big black ones). It looks like it got pretty toasted. From the first pic you posted it looks like the internal metalized film is exposed and possibly damaged.
cheers and good luck, Derek
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What is your incoming AC line voltage? Having 20V there makes me think you are outside of the US.
When you flip the amp over and that changes the glowing behavior, that means you have a loose connection. This could be in the green twisted pair of wires going from the power transformer to the H terminal strip, on that terminal strip itself, or back up front where the 10,000uF cap and 0.1 ohm resistor are. This is not a bad component.
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Thanks foryour reply Derek. Those capacitors have suffered my clumsy soldering indeed, but I’m guessing that wouldn’t have to do with my tubes not glowing, so I could deal with them later. Am I right? And Paul, I am outside of the US but we have 120v. The AC on the IEC measured 123. I’m going to check those connections and will report back. Thank you.
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I would be inclined to tell you to change the batteries in your meter too.
The power transformer has a 6.3V winding feeding those terminals. You shouldn't see anything above 10V there.
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Hello there, I just wanted to report back. I checked the soldering and now all voltages are normal. I’m glad to inform that sound is awesome on both channels so I think I’ll try to install the C4S upgrade. BTW, I checked those H voltages before resoldering with brand new batteries on my meter and I got the same 20v. Now, I get 6v.
Thanks for all your help!
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The issue is "academic" now that the amp is working, but I find it really curious that there was 20V DC on the heater wiring. How could that happen? I thought that maybe B+ and the heater wiring were accidentally connected, which at the high current draw of the heaters might produce serious voltage sag on B+ and result in an additional 10 or more DC volts on the heaters. But if that were the case, I would expect B+ to be tiny (as a result of the very same voltage sag), but that doesn't appear to have been the case. Weird.
@PB or anyone else: any guess as to how this happened? Just curious.
cheers, Derek