Magnavox IF transformer help

[jdh2161]

I've been trying to replace/fix some IF transformers due to silver mica disease that is causing problems with my pre-amp. I'm having difficulty finding schematics or values for these specific transformers and would love some help if anyone has any info. The four transformers are as follows:

Magnavox 360611-1

Magnavox 360747-1
I've found that this is a 10.7mc IF (same as Merit FM-256)

Magnavox 360748-1
10.7mc ratio detector (same as Merit FM-257)

Magnavox 360781-2

Any info would be greatly appreciated. Thanks!

 

[glen65]

What model and chassis number are you working with?
Do you have an L/C meter, or something else that measures inductance and capacitance?

 

[jdh2161]

Unfortunately I don't have a capacitance meter or a way to measure the original capacitance on my own. The pre-amp model(chassis) number is Magnavox 5914-10. I haven't been able to find a schematic anywhere for this particular model.

 

[dcgillespie]

JD -- A capacitance meter won't help anyway if the internal caps are already toast.

I've done similar repairs, and about the best you can do is center the slugs, and then use equal value external caps on each side of the transformer, going up in value while watching the level presented to the first limiter stage. Start with about 25 pF. As you go up in value, the signal level will increase, and at some point, start to decrease. When you find the peak, that's the value you want to tack in on each side of the transformer. Then, align the IF strip as you normally would.

This should work for the ratio detector transformer as well. The internal series output cap (if included) associated with these transformers is usually a discrete device, so that cap shouldn't be an issue either way.

I've done this procedure often enough on other pieces of gear, that then when I did finally come across a good exact replacement transformer, installing it in place of the "repaired" transformer produced no change in performance at all.

I hope this helps!

Dave

 

[Trio]

If you have the schematic for the chassis, that will give you some idea of what to look for. It is rare to locate the specific data for just an RF/IF transformer of that vintage. The above repair is the best method I have encountered myself with those when they are actually a failed capacitor issue.

Also, if your leads on the 2 you do not have match for, are only 2 or 3, depending where in the circuit they are located, they may just be RF traps.

 

[glen65]

Unfortunately I don't have a capacitance meter or a way to measure the original capacitance on my own. The pre-amp model(chassis) number is Magnavox 5914-10. I haven't been able to find a schematic anywhere for this particular model.

I wasn't going to suggest measuring the capacitance. Since the caps are bad you wouldn't get the reading you need anyway. I was going to suggest measuring the inductance of the coils.
Now the cap values used in the transformers roughly range anywhere from 20 to 150+pf depending the other characteristics involved. Just an example for simplicity, lets say that you were to measure 4.4 microhenries on both the primary and secondary of the transformer with the slug centered. You already know what the frequency is supposed to be (10.7mhz).
You could then go to an online calculator to find the capacitance.
http://www3.telus.net/chemelec/Calculators/LC-Calculator.htm

At that point all you need to do is fiddle with the capacitance number until you've reached the resonate frequency. In the example I gave it would be 50pf.

 

[jdh2161]

Thanks everyone for the advice. I'll let you know how things go.