Vintage IF Transformer Repair

What is an IF Transformer?

An IF, or Intermediate Frequency, transformer is a tuned air core transformer used in just about all analog Superheterodyne receivers in past and current AM and FM designs. Most Superheterodyne receivers have two or more IF Transformers to increase signal gain and selectivity. Selectivity is the ability of a radio receiver to focus on one broadcast while rejecting others that are close in frequency as the desired one.

Superheterodyne radios take an incoming broadcast and convert it to an intermediate frequency using a process called Heterodyning. IF Frequencies have been standardized on the broadcast bands with 455Khz used for AM and 10.7Mhz used for FM. Heterodyning is used as it is more efficient and cost effective to design an RF amplifier for a small window of frequencies than to design one efficient across and entire broadcast band. Circled below is the case of a typical IF Transformer. It is mounted to the chassis using two clips. One can be seen in the picture below.

How can I tell if one is bad?

This blog is dedicated solely to the IF Transformers used in vintage radios that employ electron tubes and not transistor designs. The IF Transformers used in transistor radio designs are typically too small to service and there is still a ready supply available, making it more cost and time effective to just replace.

Like common power transformers, an IF Transformer has a primary and a secondary winding. The difference between power and IF Transformer is:

• Power transformers typically have a different number of turns between primary and secondary windings, thereby, increasing or decreasing the voltage and current amplitude across the secondary winding. IF Transformers typically have a 1:1 ratio where the number of turns on the primary and secondary winding are equal.
• Power transformers operate at low frequencies,  typically between 60 and 120 Hz and require heavy steel laminated core to concentrate the lines of magnetic force into the secondary windings. IF Transformers, due to their high operating frequency, can work efficiently with an "air" core which is no core at all, just a cardboard form to hold the primary and secondary winding in place.

The easiest way to check if an IF Transformer is OK is my measuring the DC resistance of the windings. There should be a very low resistance (typically less than 50 Ohms) between the lead wires connected  to each winding.

While it is best to try to completely remove the IF Transformer from the circuit to test the winding resistance, if you consult the schematic of your vintage radio, you may find you can isolate the winding from the circuit to test by simply removing the electron tube connected to the winding. 

Using an Ohmmeter and testing the resistance with the electron tube removed, I had determine that the secondary winding in the Second IF Transformer had an "open" or broken connection.

​"Old Time Radios! Restoration and Repair" book on Amazon

I consult this book often during radio restoration. I grew up in the transistor and rectifier era and this book taught me a lot about vacuum tube and selium rectifier technology.

Repairing a Vintage IF Transformer

The only time I would recommend repairing a vintage IF Transformer is when a replacement one is not readily available available in your junk box. 

Step 1 Removing the IF Transformer from the Radio Chassis:

First, document the IF Transformer's connections so you know how to reconnect it into the circuit. The IF Transformer's connections enter the bottom of the chassis through four holes, circled in the picture below. Next, desolder the IF Transformers connections. The insulation on several wires has crumbled to the touch. I will either have to replace those wires or use heat shrink tubing to insulate.

An IF Transformer is typically enclosed in an aluminum case. There are tabs on the case that attach if to the radio's chassis through slots. The tabs are bent so as to prevent them from coming out of the chassis slot. The IF Transformer's tabs are sometimes soldered to the chassis. Regardless, the IF Transformer case tabs must be freed.

Below is the typical "guts" of a vintage IF Transformer. In this picture the IF Transformer leads have already been removed. The leads are to be replaced as the wire insulation became brittle and has fallen off. The two rings on the cardboard form are the primary and secondary windings wound in "Universal", fashion, which achieves both high inductance and Q in a very small space. The wire used in the winding is of a special type called Litzendraht, or Litz wire for short. Litz wire consists of many thin wire strands, individually insulated and twisted or woven together, following a carefully prescribed pattern. Litz wire reduces what is know as "Skin Effect" and better conducts alternating current at high frequencies. 

The primary and secondary winding leads connect to terminals on the white ceramic block at the top. This ceramic block contains two compression type trimmer capacitors. One is connected in parallel to the primary winding and the other in parallel across the secondary winding. The capacitance can be varied between 75 to 160 pf by a screw on top.  The screw compresses two metal plates against a mica insulator and the capacitance is varied based on the the screw tightness. In addition, the white ceramic block contains a fixed 120pf capacitor with separate leads that needs to be wired into the radio's circuit.

Step 2 Calculations:

Two calculations need to be performed before winding the new coil. First, we need to determine the required Inductance of the coil. Then, the number of turns required will need to be calculated in order to achieve the required Inductance.

We need the new coil to be wound so that it has the proper Inductance to achieve resonance at 455Khz, when wired in parallel with the trimmer capacitor located at the top of the IF Transformer. As mentioned, 455Khz is the IF Frequency used in standard AM broadcast band receivers. 

To the resonant frequency, the parallel capacitor and Inductor will provide a high impedance, all other frequencies it will act as a very low impedance or a short. Only the resonant frequency will be passed to the following detector stage.

The trimmer capacitor can be varied between 75pf and 160pf, so we use the mean value of 117.5pf for Capacitance. Using the Online Tank Circuit Resonance Calculator listed below, and by plugging in 117.5p (pf) as the Capacitance and 1m (mH) as Inductance, you can see that the resonant frequency is  0.4643 MHz or 464.3 kHz. This is close to the required 455 kHz resonance frequency required.  If we plug in 75p (pf) as Capacitance with a 1m (mH) Inductor the resonant frequency changes to .5812 MHz or 581.2 kHz. In addition, if we plug in 160p (pf) as Capacitance with a 1m (mH) Inductor the resonant frequency changes to .3979 MHz or 397.9 kHz. This indicates that we can adjust the trimmer capacitor between the values to 75pf and 160pf and be able to achieve resonance at 455kHz. 

OK, so we have determined the coil to be wound should provide an Inductance of 1mH. Next we need to determine how many turns of magnet wire are required to achieve this goal. I plugged the following values into the ONLINE MULTILAYER AIR CORE INDUCTOR CALCULATOR, listed below:
Inductance: 1mH
Coil Inner Diameter: .25 inches
Coil Length: .5 inches
Wire Gauge: 26AWG
The online calculator returns the Number of Turns (N) to be 375. Now we have all the required information to wind the new Secondary IF Transformer winding.

Online Tank Circuit Resonance Calculator

ONLINE MULTILAYER AIR CORE INDUCTOR CALCULATOR

Step 3 Creating a new Secondary IF Transformer Winding:

In my case, the secondary winding located toward the bottom of the IF Transformer had an open in it and needed to be replaced. I unsoldered both the primary and secondary winding leads from the white ceramic block containing the trimmer capacitors then detached the cardboard tube so as to make it easier to handle. I then used wire cutters to cut through the defective secondary winding in order to remove it. See the results below: 

I will not be replacing the defective secondary winding using Litz wire wound in a Universal pattern. This requires a special winding tool that I do not possess. Instead, I will create a spool towards the bottom of the cardboard form and wind multiple layers of 26 AWG magnet wire to achieve the same results. I fashioned the ends of the spool from two plastic bottom caps. See below:

I then used SuperGlue to attach the spool ends to the cardboard form. The distance between the spools ends is a 1/2 inch. The spool is centered at the location of the old secondary winding.   

Based on my calculations, 375 Turns of 26 AWG magnet wire needed to be wound on the homemade spool. I used a tiny drill and drilled a hole at the base of the spool on the bottom end. I then threaded the wire through it so that 4 inches was exposed, this is to be one lead of the coil. I then wound as many turns and I could fit across the spool placing a piece of masking tape over the layer when done. The next layer was wound in the opposite direction. Below is a picture of my first winding layer covered in masking tape.

Below is a picture of the completed new secondary winding. I drilled a second small hole then threaded the wire through it to make the second coil lead. 

The next step was to cut the leads of the new secondary winding to the proper length.

I then used an X-Acto knife to scrap off the varnish on about 1/8 inch from the end of each lead exposing the copper. The next step was to solder one coil lead to a terminal of a trimmer capacitor.

I added a 22 Ohm resistor in series with the other lead, in order to get the desired DC resistance of the Litz wire winding I was replacing. The other end of the resistor was soldered to the other terminal of the same trimmer capacitor.

I also had to solder the leads of the primary winding to the second trimmer capacitor.

As mentioned, all of the insulation on the leads of the IF Transformer had rotted and cracked off, leaving exposed wire. I replaced the IF Transformer leads with "Bell Wire" soldered to the trimmer and fixed capacitor's terminals. 

Below is a picture of the finished product:

The next step is to stuff the IF Transformer "guts" back into the aluminum case. Make sure that the Fishpaper that insulates the aluminum case is still in place. Also make sure the trimmer capacitor adjustment screws can be accessed from the top of the aluminum case. The last step is to install the brass retainer strip that centers the transformer assembly in the case. See below:  

Step 4 Testing and Initial Alignment:

I connected the primary winding of the repaired IF Transformer to my Eico RF Signal Generator. I then set the RF Generator to an unmodulated  .02 Volts Peek to Peak 455kHz signal. The secondary winding was connected to my Oscilloscope. I then proceeded to adjust the screw of each trimmer capacitor, with a non-metallic alignment tool, for maximum amplitude on the scope. 

Step 5 Install repaired IF Transformer into Radio Chassis:

The next step is to install the repaired IF Transformer back into the radio chassis. Here is a picture of the wiring from the bottom. I have circled the openings where the IF Transformer leads enter the chassis. You will notice a total of six wires. The Yellow and Black leads connect to the Primary IF Transformer winding while the Purple and White leads connect to the Secondary winding. The two additional red/white striped wires connect to a fixed value 120pf capacitor built into the same ceramic block that also houses the IF Transformer's trimmer capacitors. 

Step 6 Final Testing:

Time to power up the chassis and see if the repaired IF Transformer works correctly in the actual circuit. Success! I could immediately start to hear broadcast stations once the electron tubes warmed up. In order to perform final adjustments, I picked a broadcast station in the center of the tuning dial then adjusted the trimmer capacitor's adjustment screw, one at a time on both IF Transformers, for maximum station volume. Below is a video of the initial power up of my RCA 15X AM Radio with a repaired IF Transformer. 

Conclusion

This blog instructs you on how to repair a type of IF Transformer typically used in vintage tube radios. I would only recommend repairing an IF Transformer if one was not immediately available or if the IF Transformer was specialized and a suitable replacement could not be found. Repairing an IF Transformer is a good  way to save an old radio from the scrap heap.