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Open PT primary?

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2/7/2006 11:03 AM

Open PT primary?

Sorry for the long delay. Unfortunately I don't have much time to spend on my beloved amps.

Following the TADP, I took a few ohm readings:

1) between the two ends of the primary (black wires): 7 ohm;
2) between each primary end and ground: infinite ohms (=open). This occurs with the two black wires either connected to the circuit or disconnected. In either case the primary ground wire was tied to ground.

What is this?
The TADP doesn't even mention an open reading between the primary ends!

Any help is highly appreciated.

Carlo

2/7/2006 3:19 PM

[QUOTE]between each primary end and ground: infinite ohms (=open). This occurs with the two black wires either connected to the circuit or disconnected. In either case the primary ground wire was tied to ground.

What is this?
The TADP doesn't even mention an open reading between the primary ends![/QUOTE]
That may be OK. If there is a "primary ground" lead, it's possibly a shield between primary and secondary. If so, it absolutely should be open between it and the primary leads.

If you had an open between the black wires, it certainly would not blow fuses - nothing would happen.

Did you test for shorts to frame and to the secondaries?

IF you really suspect a short inside the PT, use the shorted transformer tester at GEO,
http://geofex.com/FX_images/xform_test.gif

With a battery, a neon bulb and a resistor, you can detect any internal shorted turns with one test. The ohmmeter then tells you whether you have an open on any winding, or a short between windings.

2/7/2006 8:44 PM

Just a thought - its not a 120 volt one you are plugging into 240 supply in Italy by any chance ?
After following a link to "Regis" site re
"Replace a Fender Optoisolator's bulb?"
I noticed that he has a restored 1965 Silvertone 1482 on his site

http://bellsouthpwp.net/d/o/doctorx/silvertone%201482/

and that it has 2 black wires coming out of the transformer which I assume is the common 120v model.

Just a thought as Carlo mentioned a 7 ohms reading
between the 2 primary wires.

Perhaps Regis could be kind enough to measure
his 1482's primary and post it here...

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