Ampeg SVT-3 Pro -- Smoking Toroidal

[chinrest]

Hi all,

This head powers up, no fuse blowing, but the PT (toroidal) heats up REALLY fast and the wrapping starts to crackle and smoke. It appears that much of the wrapping has melted away.

Any tips on troubleshooting/repairs? I've never really tested or worked with toroidal PTs.

[txstrat]

Do the fuses have the right value? (not too high a value)

[Enzo]

The fact it is toroidal is irrelevant, it is still just a transformer. Disconnect all the secondary connections, so mains voltage ONLY is fed to the transformer. if it heats up that way, then it is bad.

[Steve Conner]

Even if the transformer wasn't bad to start with, it's now cooked and so needs replaced anyway.

What can sometimes happen is that one of the low-current auxiliary windings on the transformer develops a short. Because the wire is thin, it just smokes instead of blowing the main fuse. The short is almost certainly inside the transformer in this case, because electrical safety codes require these aux windings to have fuses of their own, so that a fault on the amp board won't cook up the transformer, causing a fire hazard.

Or of course, some part of the amp circuit could be shorted, and the owner applied the usual noobie "fix" of replacing the relevant fuse with a nail.

[chinrest]

Thanks for your replies.

I had already checked fuse values and they were correct. Upon further inspection, it seems that the internal cooling fan and several power transistors have been replaced. Obvious diagnosis right? It's still not clear to me why the PT continues to cook without poppin a fuse.

[Enzo]

COnsider the mains fuse, and if the transformer is bad, all the secondary fuses in the world won't help it. Let us say the mains fuse is 10 amps. That means at 120v that 1200 watts have to be drawn before that fuse is stresed, let alone blown. So that means that the transformer can cook itself to the tune of 1200 watts before stressing the fuse. 1200 watts will get a transformer pretty darn hot. Many space heaters for your home have lower power than that. if the short in the tranny draws less than the 1200 watts, no fuse blows.

You can adjust this for whatever the mains fuse happens to be, but even if it were only a 4A fuse, that is still 480 watts. Try cupping a 480 watt light bulb in your hands - it is pretty darn hot too.

[zero cool]

If anyone finds a source for these, please let me know as I have one here on the shelf with a dead winding. In my case it was the HV winding that smoked. the user replaced the HV fuse with a bigger one and smoked the transformer. the main cause for the fuse failure was most likely a shorted tube.

[Enzo]

Did you contact Loud technologies to ask if they had any in stock? They are Ampeg after all.

[zero cool]

I contacted them a couple years ago and a new trasformer was something like $200 plus shipping if i remember right or maybe it was $200 shipped, something like that. the $200 sticks in my head. I shelved the unit as it was just too expensive to repair.


***BUT***

I _MAY_ have a solution for all of us!!!! I have been talking to Antek about building a custom transformer for this. They have a stock model that has the correct High current winding but no High Voltage winding. They can build a special version of this for a very very reasonable price! a rough guestimate of price is about $70ea but they would need to make 50 of them for it to work out. I might be able to convince them to do it if i can get enough interest to get close to half right off the bat. I would probably buy 2 so that leaves 23 more I would need to find buyers for. If anyone would be interested let me know!! speak now!

I have used Antek transformers for several projects now and have been very happy with them. they are really doing a lot for the DIY community so I like helping them out too.

Zc

[Steve Conner]

What's stopping you from using the existing Antek part plus an extra little transformer for the HT?

It may even be a better solution. Doing it all on one transformer will always leave it vulnerable to shorts in the fine wire of the HT winding. But if you have two transformers, you can fuse the primary of the big one with (say) a 10 amp fuse and the little one with a 250mA fuse.

[soundguruman]

Hi all,

This head powers up, no fuse blowing, but the PT (toroidal) heats up REALLY fast and the wrapping starts to crackle and smoke. It appears that much of the wrapping has melted away.

Any tips on troubleshooting/repairs? I've never really tested or worked with toroidal PTs.

Fire Extinguisher?

[zero cool]

What's stopping you from using the existing Antek part plus an extra little transformer for the HT?

It may even be a better solution. Doing it all on one transformer will always leave it vulnerable to shorts in the fine wire of the HT winding. But if you have two transformers, you can fuse the primary of the big one with (say) a 10 amp fuse and the little one with a 250mA fuse.

There is not much room inside the case for 2 transformers. it can be done. but I was trying to find a better solution for everyone!.

The unit is already fused for the HV and the mains. But in my case, the owner blew the 125ma HV fuse and replaced it with a 10 amp fuse and melted the transformer LOL!

Zxc

[soundguruman]

There is not much room inside the case for 2 transformers. it can be done. but I was trying to find a better solution for everyone!.

The unit is already fused for the HV and the mains. But in my case, the owner blew the 125ma HV fuse and replaced it with a 10 amp fuse and melted the transformer LOL!

Zxc

Oh, I get it. It's not "smoking toroidal"
It's "smoking tutorial."
I mean, you spend hundreds and hundreds of dollars on these amplifiers, and the manufacturer has no critical parts support.
(they could not even supply me with an input jack)
Clue: IMHO The transformers are made for 100 VAC. No wonder they roast.
Next time, invest in something that has parts support. The last thing you need to buy is a disposable amplifier.

[zero cool]

Oh, I get it. It's not "smoking toroidal"
It's "smoking tutorial."
I mean, you spend hundreds and hundreds of dollars on these amplifiers, and the manufacturer has no critical parts support.
(they could not even supply me with an input jack)
Clue: IMHO The transformers are made for 100 VAC. No wonder they roast.
Next time, invest in something that has parts support. The last thing you need to buy is a disposable amplifier.

Ok I understand your frustration and agree but this isn't helping this thread. These amps have a known problem.I am trying to find a positive solution to fix them.

[soundguruman]

Ok I understand your frustration and agree but this isn't helping this thread. These amps have a known problem.I am trying to find a positive solution to fix them.

"These amps have a known problem."

I would say it's more than just one...