fired it up and smoke comes out of PT

[wsaraceni]

so im trying to figure out what i could have possibly done to screw this up. First fire up, and smoke comes out of the PT. The output transformer is a 269EX. I bought it as part of a ax84 kit and then tried to turn it into a matchless vibrobox. For the short time i fired it up, i was able to confirm a B+ voltage of 259v which is what the schematic says it should be.

so, im assuming two things.

1) i did something wrong
2) the PT is hosed.

is that correct?

here are some pictures of the build


[img:640:478]http://img.photobucket.com/albums/v217/ ... photo1.jpg[/img]


[img:640:478]http://img.photobucket.com/albums/v217/ ... photo5.jpg[/img]


[img:640:478]http://img.photobucket.com/albums/v217/ ... photo2.jpg[/img]


[img:640:478]http://img.photobucket.com/albums/v217/ ... photo3.jpg[/img]


[img:640:478]http://img.photobucket.com/albums/v217/ ... photo4.jpg[/img]

[Alexo]

Looks to me like you got some diodes backwards in your heater rectifier (second picture, one pair is right, the other is backwards, = short circuit)

[selloutrr]

that sucks!

Confirm you have the wires going to the correct places.

It's odd the fuse didn't trip first. I would suspect if a filter cap or wire was crossed the fuse should have blown at power up. Did you have the correct value of fuse in the holder?

Did you bring it up and form the caps? or was it a hard power on? a light bulb limiter or variac with amp meter should have forwarned of bad wiring by showing high amperage. If all the wiring is in the correct location. It would seem to point to a bad PT though that is rare. They usually only fail if they are abused, i.e. wired wrong.

[mcrracer]

ALWAYS USE A LIGHTBULB LIMITER FOR FIRST FIRE UP!
Saves parts, time and aggravation.

[es345]

Hi

on picture 3:

I assume that the green wire are coming from the PT.

on that picture going from top to down the upper 2 diode of the bridge are in the wrong direction.

[wsaraceni]

so i think i found it. i left off the ground in the rectifier circuit. i think the rest of the rectifier circuit should be fine. guess this means im ordering a new PT.

anyways, how do you fire up with a lightbulb limiter?

[wsaraceni]

so looking at the diodes, the schematic has a bridge rectifier (which i dont happen to have on hand. i thought i read that you can replicate with 4 diodes. if you look at the schematic. the four diodes make a square. how should it be? i guess ill just order a PT and a bridge rectifier.

[Alexo]

viola.

[Gibsonman63]

There is plenty of info on the internet on building a light bulb limiter. Basically, you wire a receptical to be in series with the light bulb. The light bulb and your amp become two resistors in series. If the amp has no resistance, then all the voltage is dropped across the light bulb and it burns brightly. The current to the amp is limited by the wattage of your bulb, which protects your amp. Some people use a small wattage bulb for initial start.

[wsaraceni]

thanks. ill look it up.


would forgetting that ground fry the PT? ive been looking at it over and over and cant see anything else i missed.

also, did i do the rectifier incorrectly? it looks like the diagram to me.

[Merlinb]

I've smoked transformers without killing them. Disconnect it and check for shorts, then try powering it with no load and see what voltages it gives. You *might* have gotten away with it if you didn't let it run for too long...

[wsaraceni]

viola.

so i guess the schematic is wrong? it has the right side of the picture in it.

[mlp-mx6]

Can you post a link to the schem?

If it looks like the one on the right, the schem is wrong.

[wsaraceni]

http://www.scribd.com/doc/2474233/Match ... ar-Effects

oh well. i guess its just a $50 learning experience at this point. time to make a light bulb thingy next!!!

[mcrracer]

I found this schematic on Prowess site. It is correct.
http://www.prowessamplifiers.com/schema ... robox.html
Maybe I don't know what I'm doing but I do not see a schematic on the site you linked. I sure hope they bring Schematic Heaven back!