A little Smoke Anyone!

[roadkingbluzs]

Ok got your attention ,I hope. Working on a 68 Pro Reverb, cap job, blackface bias etc. Rocking out for a while and then a little smoke!. Has anyone experienced an output transformer being slightly compromised (arching and a littlle smoke) and still work? I have metered xfmr and checks ok. Your thoughts, experiences. BTW I have seen an output xfmr go up in smoke big time. P.S I will be get back to Pro Reverb later this week. Thanks for any comments.

[Firestorm]

You're sure it's the OT that smoked?

[roadkingbluzs]

Yes. Bringing voltage up with variac and watched a bit of arching and smoke.

[Firestorm]

Then the only thing to do is assume the OT has reached the doorstop phase of its lifecycle.

[Phil_S]

Then the only thing to do is assume the OT has reached the doorstop phase of its lifecycle.

Agreed. It may pass out of circuit "testing" but smoke tells you it's been compromised. When you put it under load, it will continue to short. Worse, it can take other stuff with it when the ship sinks. Cut your losses.

[roadkingbluzs]

Point taken. Now to trouble shoot cause I guess at this point will consider tranny upgrade. Still, any success stories of compromised xfmr continue to operate?

[Reeltarded]

As a door stop, or a chock. Do you have an airplane? Filler for a cement sculpture. Projectile for a trannypult.

Is it at all rusted that you can see? Magnifying glass.. delaminating? Someone previously ran it on a short load for a long time. Manufacturing booboo shows up 40 years later, ect, ect, and ect..

Watch out for upgrades. Expensive isn't usually better in my experience, if fact almost exactly opposite.

[roadkingbluzs]

I like the trannypult. I here you on the upgrades, something generic will do for this one.

[Zippy]

Has anyone experienced an output transformer being slightly compromised (arching and a littlle smoke) and still work?

As a former USN electrician, I've seen a number of smoked windings. The reason that reduced load doesn't work is that a smoked winding is smoked from voltage-induced breakdown - reduced load will have much the same voltages, hence the arcing will continue.

Since it is already committed to death, if you are up for an adventure, you might take it apart to see where it failed. If it is in an outermost winding - which is often the case, due to an outer winding rubbing against the case - you might try stripping back a few layers to remove the offending wire with burnt insulation to revive it.

Regardless, if you've never peeked under the hood of an OT, it is insightful.

[Structo]

Does your variac have a current meter on it?

Do you know at what voltage it failed?

That is one of the drawbacks of a variac, things happen quickly and you usually don't know what happened.

I like using a light bulb current limiter.

You can start out with a 25 watt bulb which limits the current to less than 250ma.
If when you turn the power on your amp the light bulbs stays a steady bright, you know you have a short or other wiring fault that is drawing too much current.
And at this low current level it is unlikely that you have damaged anything if you turn it off quickly.

[img:800:618]https://lh3.googleusercontent.com/-ufJU ... imiter.jpg[/img]

[Gibsonman63]

For a semi-collectable amplifier, would it be worth looking at a rewind on the transformer? I have never done it and I am not sure what the cost would be.

[Phil_S]

A rewind will be expensive. I've taken apart a few transformers. As someone else said, it's an eyeful and worth doing if you have the time. It involves breaking down the varnish on the edge of the lams, splitting them apart one by one, and removing them. As you remove them, stack in exact order of removal in case you are able to re-assemble. You don't want to mix up the grain of the metal.

To start, sand the edge of the lams to remove varnish as best you can. A wire brush is good too. Then, take a razor knife and slice through between lam 1 and lam 2. When separated, carefully remove the first lam. Sometimes there is a wedge between the inside of the bobbin and the first lam that needs to be driven out using some sort of metal shim and a hammer.

This takes patience. Be careful and be gentle so you don't bend the lams. Plan on spending 60-90 minutes to get the first lam split and removed. There is a great deal of tension/pressure on it. Removing the 1st lam relieves a lot of the tension. Lam #2 will take another 25-30 minutes. After that, they will go relatively quickly.

Once the bobbin is free, you will be able to tell if the windings are interleaved or not. If they are interleaved, then you only hope of amateur level repair is if the problem exists in the outermost layer.

I had one that simply lost it's end lead from the outermost winding. I had dug around and broken a few turns before I took it apart. I removed about 10 turns, attached a lead and put it back together. Then I used the appropriate type of high temp spray varnish. Before spraying, I put it in the oven at about 225F, removed it, and quickly sprayed it while it was still hot. I was told this will cause the lams to suck some of the varnish into the very small voids and seal it up nicely. I haven't had a chance to use it, but it tests very well with low voltage input. the loss of 10 turns was apparently insignificant.

[Reeltarded]

'68 Pro Reverb... In a Dumble World..

I bet someone here is hiding the transformer you need, from his wife. These guys are very afraid of their wives.

[dcribbs1412]

'68 Pro Reverb... In a Dumble World..

I bet someone here is hiding the transformer you need, from his wife. These guys are very afraid of their wives.

That is too funny