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