Transforming Smoking During Tube Glow Test - Need Help Troubleshooting

[yorkmichael]

I'm building a S.E.X. 4 Kit and ran into a problem during the tube glow test right after installing the tube heater circuit (page 74). When I connected the amp to power during the glow test, the tubes did not glow and after about 20 seconds I could smell smoke coming from the power transformer. I unpowered the amp immediately and waited for the B+/B++ voltages to drop before inspecting further. There is no sign of any kind of miswiring. No obvious location of a short circuit. I removed the tubes and tried to power the amp once again to test the transformer voltages. I'm not getting voltage readings off the transformer (terminal 7 and 9 or terminal 11 and 12), but I am still getting 470+ VDC off the B+/B++ voltages off the terminals on the PCB (with multimeter ground on Terminal 40). I'm not getting any voltage off the 6.3VDC terminals. The only thing I can think of is the dangling wires from the OT transformers might have shorted something during the tube test (but no sign of this with blackened wires or components). Of course, it's possible I also mis-wired the heater circuit, but it doesn't appear like I did as far as I can tell. If I mis-wired something and need to replace the power transformer out of pocket, then that's ok - expensive lesson learned (as long as I can figure out what I did wrong, so I actually learn something).

I'm attaching pics of my build thus far so someone can inspect the wiring and hopefully spot where I might have made a mistake. I'm 99% sure the transformer is blown - but is there a definitive way to verify? If there's a proven way to test the power supply circuit board assembly to validate this incident didn't damage any other components that would be helpful too.

Note for the pic of the heater wiring - it may at first appear like I wired it backwards. I accidentally clipped the black wire in the 7" three-conductor wire bundle a bit too short (page 72). Since the instructions on page 72 said "it does not matter which wire goes into which hole" I wired the black wire into the left hole on the 6.3vDC terminals but the other end of the black wire is attached to B1. This is consistent with the instructions (the left 6.3V terminal is connected to B1 and the right is connected to B12. I also noted in the instructions, B1 was connected to A12 and B12 was connected to A1. I connected the same way but swapped the red and black wires to be consistent. It should be electrically identical to the instructions - just with the red insulation swapped with the black insulation.

 

[Paul Birkeland]

Your power transformer hardware is not properly installed, but that's not what caused your issue. It could be where those two wires melted together in the shielded twisted pair wiring, is there exposed metal where they melted?

The low voltage heater circuit won't ever really draw enough power to blow the fuse, but it can draw enough power to fry the 6.3V winding, which is likely what happened here. It's probably a good idea to pop the power supply board and show us what the bottom looks like, but I'm guessing when you checked the DC voltages at the board as directed in the manual that everything was OK?

 

[yorkmichael]

Your power transformer hardware is not properly installed, but that's not what caused your issue. It could be where those two wires melted together in the shielded twisted pair wiring, is there exposed metal where they melted?

The low voltage heater circuit won't ever really draw enough power to blow the fuse, but it can draw enough power to fry the 6.3V winding, which is likely what happened here. It's probably a good idea to pop the power supply board and show us what the bottom looks like, but I'm guessing when you checked the DC voltages at the board as directed in the manual that everything was OK?

When you say the "power transformer hardware is not properly installed" can you be more specific please? What isn't installed correctly? Is it the #8 locking washers on the screw attaching the PCB to the standoffs? If so, there were not enough #8 washers in my kit to use them for this step, so I used the only other set of 4 locking washers included in the kit that hadn't yet been used. It's possible I was supposed to use these washers earlier in the assembly and accidentally used #8 locking washers at that time. If you were referring to something else, please clarify.

I took a photo of the bottom of the PCB. There is no obvious damage that I can see. It looks the same it did before I installed it. I'm not sure if the black and red heater wires melted together when there was power applied or possibly a soldering iron error. The red wire insulation is partially melted but it doesn't look like it melted through and caused a short. Perhaps there was damage to the heater wiring that was hidden and caused a short inside the wire?

What needs to be done to fix this? I'm thinking replace the transformer and also replace the heater wiring. Anything else? I'm still not sure what caused this. Even if the wires in the photo melted when they had power to them - it's not clear why. They do not appear to have been shorted together before the insulation melted.

 

[Paul Birkeland]

The star washers should be between the power transformer cover and the stack.

Before powering up with a new power transformer, you'll want to slip the STP out of the board and set your meter to beep for continuity, then put one probe on each of the free STP wires and check for low resistance. The tubes need to be out of the sockets for this.

It would probably be a good precaution to just replace the heater STP while you are performing this work.

-PB

 

[yorkmichael]

Is STP = Single Twisted Pair? Is this referring to the wiring for the heater circuit? If so, what is the purpose of this test for continuity/resistance? Is this to make sure there isn't a short in the wiring before powering on the new transformer or to get to the bottom of why the wiring shorted to begin with? Is there any reason to think something else other than the transformer and heater wiring needs to be replaced as well? Could this problem have damaged any components on the power supply PCB? Any idea what might have caused there to be a short in this wiring? Unless there is some internal short hidden by the twisted pair wiring insulation I can't see, I still don't understand what happened.

 

[Paul Birkeland]

Yes, this is the shielded twisted pair wiring, and the test is to be sure there isn't a short between the red and black wires.

I think the short in this wiring is the melted spot you captured in your pictures. If your iron is set to too low of a temperature, then it can take a long time for your solder to flow out, and those wires will heat up and that can cause problems.

It is indeed possible that the MBR745 diodes didn't enjoy the short circuit condition, so replacing them wouldn't be a bad idea. This isn't a super hard process if you have a spring loaded solder sucker to help you get the solder out, or some solder wick.

 

[Jameson]

@Paul Birkeland the SEX uses 4x 5820 and 2x 4007 on the board. Which are you suggesting he replace? We can add those to the replacement order.

 

[Paul Birkeland]

Oops, yes, still have the SEX 3 in my mind. The 1N5820 diodes should be swapped out just to be safe. As a bonus, they are also easier to change out!

 

[Jameson]

Great, thanks! @yorkmichael - We'll shoot you an email shortly.

 

[yorkmichael]

Yes, this is the shielded twisted pair wiring, and the test is to be sure there isn't a short between the red and black wires.

I think the short in this wiring is the melted spot you captured in your pictures. If your iron is set to too low of a temperature, then it can take a long time for your solder to flow out, and those wires will heat up and that can cause problems.

It is indeed possible that the MBR745 diodes didn't enjoy the short circuit condition, so replacing them wouldn't be a bad idea. This isn't a super hard process if you have a spring loaded solder sucker to help you get the solder out, or some solder wick.

Thanks Paul. I think what might have happened is I may have cut the outer insulation of the STP wire too deep when stripping 1" of insulation off the wire and cut into the insulation of the black and red wire to create the short. My soldering iron is set to 700 degrees, so I don't think the issue was holding the iron on the joint too long, anything is possible. It's also possible I inadvertently touched the wires that appear melted with the iron and didn't see it until it was too late. Fingers crossed the replacement parts do the trick. I will also test the heater wiring before connecting it to the board just to be sure there isn't a short.

 

[pevelg]

Hi All,

I am at this step as well but am taking a pause to make sure I am following and perhaps not making a similar error.

I prepared the two heater cables and made sure there was no continuity between the red/black/bare wires. After soldering the cables to 6.3V DC pads, I checked continuity between B1 & B12 (no tubes plugged in) and I got a positive beep. If I have the meter in resistance mode, it starts very low (hence the beep?) and then starts building slowly until I remove the leads. This happens even if I remove the heater cable and am just measuring between the two 6.3V DC pads. Is this expected behavior?

Thanks,
Pavel

 

[Paul Birkeland]

Yes, that's your meter charging up the capacitor.

It would also be a good idea to check for continuity on the 1N5820 diodes you pulled off the power supply board. They may beep with your probes testing them in one direction, but not the other. If they show very low resistance in both directions, that indicates they are fried.

 

[yorkmichael]

Yes, that's your meter charging up the capacitor.

It would also be a good idea to check for continuity on the 1N5820 diodes you pulled off the power supply board. They may beep with your probes testing them in one direction, but not the other. If they show very low resistance in both directions, that indicates they are fried.

Does this continuity/resistance test work when the 1N5820 diodes are still installed on the PCB? I'm not getting continuity in either direction, but I haven't removed them from the board yet. I think the replacement parts arrive today. The smaller diodes do test as if they are working while installed.