Electronics > Repair

Prealigning IF transformer

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ttx450:
Hi, I am trying to prealign the IF transformers in a old tube radio.  I am not having the best of luck getting any change in the amplitude when I turn the capacitors.  Using a 455Hz signal, connecting signal gen on one side of the primary and scope on the other side of the primary and grd(s) to IF can housing.  I am not getting any changes in the amplitude of the sine wave.  I must not be doing something right?

commongrounder:
I’m curious why you want to attempt to prealign the transformers. Your making it harder for yourself. They won’t stay tuned once they are in the radio circuit. Also, the radio IF tube stages themselves are an alignment aid. Read and study some alignment instructions for a typical heterodyne radio circuit. It’s very straight forward.
Best of luck!

bob91343:
Your scope has loaded the circuit to the point where it's no longer resonant.  The transformers have to be loaded with the design load, which you likely don't know.  You need a higher impedance detector or at least one that has minimal reactance.

TimFox:
Vacuum-tube IF transformers are usually double-tuned.  You probably also loaded down the primary winding with the 50 ohm or so output impedance from your generator.  A 6SK7 pentode driving such a transformer has a source impedance on the order of 1 megohm (plate resistance), plus output and stray capacitance.

ttx450:
Hi, well I am trying to learn along the way here too, this is my first attempt of trying to do this.  Being I had the unit totally taken apart, I thought I'd try to get the IF transformers close before installing, thinking it will get them close?

I have not been able to get my AWG to work (yet), maybe I am not supplying enough voltage to the IF coil (I cannot get any adjustment).  AWG has 50ohm inputs.  I been able to get my old B&K E-220D to work and also my old function generator to work.  Both have 50ohm inputs.  Scope I am using has 1M inputs.  I did put a 1k ohm resistor on the input side from the generator. 

So what I did is not close? Seems like the screws are close to where they were.  Appreciate everyone's help. Thanks.

Edit - just got my AWG to work, I been inputting the wrong frequency in :-[

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