Prealigning IF transformer

[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

[TimFox]

The 1 megohm load from the scope is reasonable, but what is the capacitance?  IIRC, the tuning cap on a 455 kHz IF xfmr is maybe 30 pF, so the scope will strongly change the secondary tuning.
1 k from a 50 ohm generator is nowhere near high enough.  See my note above about the plate resistance of a 6SK7 (or 6BA6 for later radios).  Try a 1 megohm resistor to drive the primary from the generator output.

[floobydust]

You might have the wrong IF frequency in mind? There was 262.5kHz usually in cars.
I would see what resonant frequency you are actually getting with the test setup. I would add series resistance from the signal generator output, it's surely much lower than a triode driving the primary, to see the peak(s).

[ttx450]

The 1 megohm load from the scope is reasonable, but what is the capacitance?  IIRC, the tuning cap on a 455 kHz IF xfmr is maybe 30 pF, so the scope will strongly change the secondary tuning.
1 k from a 50 ohm generator is nowhere near high enough.  See my note above about the plate resistance of a 6SK7 (or 6BA6 for later radios).  Try a 1 megohm resistor to drive the primary from the generator output.

The scope is 1M @ 11pf.  Thanks for the information I will try a 1 Meg resistor and see how that effect the current setting.  The radio I am working on has 12SK7, will look that up too.  Thanks for the response.

[ttx450]

I looked at the 12sk7 plate resistance and shows 0.12 meg.  I switch the in series resistor from signal generator to 1 meg and got some hairline spikes and notice the frequency dropped way off.  I tried a 680k, same thing.  I went down to a 100k and got a readable signal.  I re-peaked the IF transformer and it changed very little if any from the 1k, maybe peaked sightly higher.  It was very hard to tell.  So if the plate resistance is approximately is 120k, is this what we are trying to duplicate on the input?   Learning here, sorry for the dumb questions.

[bob91343]

Rather than peak the transformers with a signal, measure the impedance of the primary while loading the secondary.  You can use nanoVNA for that if you have one.

You need a 10:1 probe on the scope.

[ttx450]

Rather than peak the transformers with a signal, measure the impedance of the primary while loading the secondary.  You can use nanoVNA for that if you have one.

You need a 10:1 probe on the scope.

Thats interesting.  I been thinking of getting a nanoVNA, but dont have one.

[ttx450]

Curious, can I use my old Triplet 850 vtvm to peak?  It has 0.5v dc setting, been thinking of trying.  Or will I run in impedance issue with it?

[bob91343]

That Triplett should be okay.

[TimFox]

I looked at the 12sk7 plate resistance and shows 0.12 meg.  I switch the in series resistor from signal generator to 1 meg and got some hairline spikes and notice the frequency dropped way off.  I tried a 680k, same thing.  I went down to a 100k and got a readable signal.  I re-peaked the IF transformer and it changed very little if any from the 1k, maybe peaked sightly higher.  It was very hard to tell.  So if the plate resistance is approximately is 120k, is this what we are trying to duplicate on the input?   Learning here, sorry for the dumb questions.

At only 100 V, that is correct, but it increases to 800 k at 250 V.  The plate resistance is the source resistance to the primary;  the load on the secondary can be higher resistance (plus capacitance), unless feeding a diode detector.

[ttx450]

I looked at the 12sk7 plate resistance and shows 0.12 meg.  I switch the in series resistor from signal generator to 1 meg and got some hairline spikes and notice the frequency dropped way off.  I tried a 680k, same thing.  I went down to a 100k and got a readable signal.  I re-peaked the IF transformer and it changed very little if any from the 1k, maybe peaked sightly higher.  It was very hard to tell.  So if the plate resistance is approximately is 120k, is this what we are trying to duplicate on the input?   Learning here, sorry for the dumb questions.

At only 100 V, that is correct, but it increases to 800 k at 250 V.  The plate resistance is the source resistance to the primary;  the load on the secondary can be higher resistance (plus capacitance), unless feeding a diode detector.

Thanks for explaining that.  Why did I have to use a lower resistance resistor to get it to read right (lower then 1m)?  On the secondary I had to go lower yet.