Preamp Safety Measures (Incorrect Transformer Input Wiring)

[Greenback25]

I have a guitar preamp with a toroidal transformer that was wired incorrectly for the European wall outlets. As I fired it up, the sound was there for mere seconds, and then smoke came out of the preamp. Parts of the transformer insulation melted:

https://i.imgur.com/xCVmR8d.jpeg

Would you say that this transformer is damaged beyond repair or could I make it work by connecting the right cables to the mains input? Also, could any components downstream be damaged by this false input wiring of the transformer?

The preamp in question is a Langner DCP-1 clone, I attached the schematic to it below. I also attached the transfomer spec sheet.

Langner-DCP-1_Shematic(2).pdf

AS-05T200.pdf

I would need to remove the two current input wires of the transformers and reattach the other two wires that are currently in a heat shrink, but I'm not sure how safe it is to operate inside a preamp such as this.

Would I need to actively drain the filter caps or is it safe to work inside this preamp if it was left unplugged for a few hours? Any other regions of the preamp that might hold lethal voltages?

[Reeltarded]

The insulator is melted. It's dead.

[Greenback25]

The insulator is melted. It's dead.

This transfomer needs to be replaced 100%? What would happen if I would rewire the input wires correctly and then turn it on again? Would the transformer melt again or worse?

What about the safety hazards inside these preamps: Are there any lethal voltages left after the preamp is unplugged for a few hours or are the filter caps inside these preamps big enough to still hold lethal voltages for months and years?

[romberg]

What about the safety hazards inside these preamps: Are there any lethal voltages left after the preamp is unplugged for a few hours or are the filter caps inside these preamps big enough to still hold lethal voltages for months and years?

If you do not know how to safely measure the voltages and discharge capacitors involved with tube electronics, you should not be messing around inside the chassis. 270V DC is enough to kill you with just a touch. It is not worth risking your life to save a transformer that is already dead. Your best bet is to take it to someone who does know how to safely work on such equipment.

Mike

[pdf64]

I wonder why the mains fuse didn’t blow immediately? What value fuse was in there?

…
I would need to remove the two current input wires of the transformers and reattach the other two wires that are currently in a heat shrink …

Mains electricity is dangerous, please don’t mess with dangerous stuff that you clearly don’t understand.
FYI the action you propose will just cause the exact same thing to recur.

The thing needs a competent tech to sort it out, connecting 240V to 120V equipment will probably cause a lot of damage.

[Stevem]

It's not just the transformer that's bad now, so are all the power supply filters from having twice the voltage applied to them as they should have had and the filaments in the output tubes may have gone open also.

I am rather shocked that the fuse did not blow before the transformer started to smoke!

Judging by what your asking in your post I think that repairing this at least for right now is above your knowledge level so I would take it to a tech who is well versed in tube amps.

[romberg]

I wonder why the mains fuse didn’t blow immediately? What value fuse was in there?

If you peek at the schematic, it has a 1A which seems like a whole lotta current (at 230VAC) to support just 5 12ax7s.

Mike

[T Wilcox]

This is a mistake I made by following the cut sheet for the Antek transformers primary
There are 2 reds and 2 blacks that can be wired series or parallel for 120 or 230V
What the cut sheet doesnt show is that you have to wire a specific red and specific black together which I did not do

Here is the proper spec sheet instructions for reference
https://www.antekinc.com/tech-faq/

I failed to identify wire 2 and 3 as I assumed both reds and both blacks would be electrically equivalent. Not sure why they are not colored differently but after speaking to Antek rep it sounds like this is not the first time this happened.

The preamp will be sent back to have transformer replaced and bench tested

Todd

[pdf64]

It’s a series circuit; provided the windings are connected in like polarity, how can that make any difference?

I wonder why the mains fuse didn’t blow immediately? What value fuse was in there?

If you peek at the schematic, it has a 1A which seems like a whole lotta current (at 230VAC) to support just 5 12ax7s.

Mike

That’s with a 230V PT, whereas I get the impression that it was wired for 115V.

Don’t toroids draw high current at initial power up? So might require a higher than expected fuse value.

[LOUDthud]

The way I read this, the transformer has two primary windings, each has a red wire and a black wire. Call them R1, B1, R2, B2. For 115VAC you connect R1 to R2 for one side of the Mains and B1 to B2 for the other side. Normally, for 230VAC you connect B1 to R2 and apply Mains to R1 and B2. What can happen if you don't check continuity is you connect the 230VAC Mains to R1 and B1 and connect R2 to B2. This short on the R2 B2 winding should have blown the fuse. At any rate, now it's toast.

[T Wilcox]

The way I read this, the transformer has two primary windings, each has a red wire and a black wire. Call them R1, B1, R2, B2. For 115VAC you connect R1 to R2 for one side of the Mains and B1 to B2 for the other side. Normally, for 230VAC you connect B1 to R2 and apply Mains to R1 and B2. What can happen if you don't check continuity is you connect the 230VAC Mains to R1 and B1 and connect R2 to B2. This short on the R2 B2 winding should have blown the fuse. At any rate, now it's toast.

Close! the short on R2 to B2 was not in the circuit since it was jumpered. The 230V was across only one of the 115V windings.
I'll own it but I still think this crap could be avoided if they just striped or changed wire colors on one of the primaries...

[martin manning]

Close! the short on R2 to B2 was not in the circuit since it was jumpered. The 230V was across only one of the 115V windings.

The R2-B2 jumper created a shorted (120V) secondary. All other secondaries would have been delivering 2x rated voltage.

[R.G.]

Let me reinforce the good advice already here.

This transfomer needs to be replaced 100%? What would happen if I would rewire the input wires correctly and then turn it on again? Would the transformer melt again or worse?

The transformer is either 100% dead right now, or even if it does work on being reconnected, it's so damaged that it might... fail unpredictably. It might slowly overheat and melt down. It might fail totally in a few minutes. The very worst thing that could happen is that it would appear to be OK. That would fool you into thinking that it's OK and then in the future it could fail in a way that makes the chassis and/or signal ground hot to the AC line. That is - it could easily become an electrocution hazard. You have to make your own choices, but with a damaged power transformer, you're taking chances on being electrocuted - that is, becoming dead. Dead is more expensive than a new, not-damaged transformer, any way you look at it.

What about the safety hazards inside these preamps: Are there any lethal voltages left after the preamp is unplugged for a few hours or are the filter caps inside these preamps big enough to still hold lethal voltages for months and years?

It's good that you thought to ask. In any tube equipment, the voltages can be lethal any time it's turned on. The question then becomes how long do you need to wait for it to be safe, as you've guessed. That depends on whether the original designer thought about that question and designed in stuff to make it safer. In my own amps, there is always a "bleeder" setup, so that the places with hazardous voltages are drained down within a specified time. This is an unusual approach. It is NOT common.
The real answer is that you can't tell. In your case, it's ... possibly, probably, maybe OK after a few hours. Most tube amps are. But my friend the professional amp tech tells ugly stories about getting shocked from amps off his stack of incoming repairs. He uses a ground wire with a power resistor in series to probe all the filter caps before he will start tinkering. As to months and years, the longer the safer. One ugly possibility is that high voltage capacitors have a rebound effect. When takes off power, and discharged down to zero, then open circuited again, they have the tendency to re-grow voltage. This is BAD for things like high voltage glass dielectric caps. CRTs (remember those?) are never safe unless freshly shorted.
High voltage electros do some of this, but they also have higher internal leakage that sucks it down, and the two effects compete.
You got good advice again: your question indicates that you're still learning, so if you go tinkering the circuit, expect that it's still deadly. Measure every place you intend to touch until you know how the circuits work well enough to measure fewer places. We really do want to keep you alive and tinkering,