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Electronics => Repair => Topic started by: jaysgr on July 28, 2015, 11:48:38 am

Title: Fender Stage 100 transformer question
Post by: jaysgr on July 28, 2015, 11:48:38 am
Hello,
 I asked for help in another thread but it was suggested to me to make a new thread.

So, I have a fender Stage 100 which burned out two transistors. I changed them and now it works, but I noticed something concerning the transformer and I am not sure if it is a problem, or it is normal. The thing  is that when I move the secondary cables toward the transformer I get audible noise on the output. Please check the video from the link below to see what i mean. Also if I separate the yellow cable from the two reds, the noise is there even without moving the cables.

Also, the problem at first (before the transistors gone bad), was that the sound was decreasing at a random point and then it came back again, and that's why I am suspecting the transformer.

https://www.youtube.com/watch?v=1JoUNXjaNXE&feature=youtu.be (https://www.youtube.com/watch?v=1JoUNXjaNXE&feature=youtu.be)

I am quite new in electronics, but I am willing to learn, that's why I ask.
And I am very careful!
Thanks!

Title: Re: Fender Stage 100 transformer question
Post by: oldway on July 28, 2015, 12:03:03 pm
When you move the output cables of the transformer and you approaches them from inputs, it is normal that the magnetic fields produced by the 50 or 60 hz current causes hum induced voltage.

There is nothing wrong with the transformer.

It just seems that the inputs are not sufficiently shielded, but this rather seems to be an amplifier design problem.
Title: Re: Fender Stage 100 transformer question
Post by: Shock on July 28, 2015, 12:58:19 pm
Was there more twists in those wires previously? If they became untwisted twist the back again to something like this.

(http://thumbs.dreamstime.com/t/twisted-wire-image-hi-res-rendered-artwork-could-be-used-any-graphic-design-45219394.jpg)
Title: Re: Fender Stage 100 transformer question
Post by: jaysgr on July 29, 2015, 09:34:47 am
Thank you very much for your answers! Very helpful!
Title: Re: Fender Stage 100 transformer question
Post by: jaysgr on July 29, 2015, 10:08:13 am
Well, I guess I could use more of your advice..

I twisted the cables as Shock mentioned. The thing is now that when I put the head back to its cabinet the noise begins again ( the cabinet has a metallic paper shield inside).
I took the head out again and I placed a metallic plate over it, near but without touching anything. And the noise came again!

Well, I guess that, as oldway said, the inputs of the transistor are not sufficiently shielded? So the transformer has gone bad and needs replacement?
And also, is it possible that the transformer, because of the burnt transistors, was got very hot and the shield gone bad? Because this wasn't a problem before.
Thanks again!
Title: Re: Fender Stage 100 transformer question
Post by: David_AVD on July 29, 2015, 09:24:14 pm
I'm guessing that there's nothing wrong with the transformer.

In the video the hum seems to come on quite sharply when you get to a certain cable position.  This may be because the output stage is breaking into supersonic oscillation and what you're hearing is the 100Hz ripple from an overloaded power rail, not 50Hz hum.

This may also be why the original output transistors burnt up.  You could have another fault that is leaving it teetering on instability and moving the transformer wiring or metal plate is tipping it over the edge.