Miniature I.F.Transformers 2nd I.F. Repair ~                                                                    

By Bill Jones

The repair of the first I.F. transformer has been covered in another article on this site.  These miniature transformers may cause a problem because of their built in tuning capacitors. The capacitors in question normally have a value in the order of one hundred pico-farads.  They are constructed on one small piece of mica that is slightly smaller than the I.F. transformer base, and the capacitors are in the base of the transformer.  The mica is silver plated on each side and the silver is placed on the mica to form two capacitors, one for the primary of the transformer, and one for the secondary.  One edge of each of the capacitors is physically close to each other. Tuning for these transformers is accomplished by means of a ferrite slug. 

Because of factors that are not clear, some radios that have this type of capacitor   will be found to have a silver migration from one capacitor to the other capacitor forming what I call a “migration resistor”. The resistor is unstable and causes a radio to be very noisy.   I would guess that the formation of the migration resistor is due to the voltage between the primary and secondary along with other factors – such as humidity and perhaps other elements such as sulphur in the air.  The problem may only be present in transformers made by certain manufacturers, but I do not have enough data to confirm such suspicion.

I do find a number of radios that use the miniature I. F. transformer to have this resistor between the primary and the secondary of the transformer.  This resistance is usually in the range of one to twenty megohms and the radio is quite noisy even when the resistor is in the order of twenty megohms.  The resistor can cause the radio to be noisy at times, but the radio may work perfectly at other times.  I also find radios that will not work at all because of the migration resistor. 

This resistance will provide a positive bias on the grid of the first I.F. and the bias may be as high as two or three volts if the first I.F. tube has a cathode resistor. The second I. F. may also be bad, but the voltage on the diode detector that is caused by the transformer will not be as large as the grid bias caused by a bad first I.F. transformer. With no I.F. signal present there will be a small positive bias on the detector diode plate if there is a migration resistor problem.  This voltage value will normally be to less than six tenths of a volt. 

The I.F. signal itself will, in a properly operating radio cause the diode to conduct very slightly on the positive peaks of the signal and the diode acting as a rectifier will provide a negative voltage on the diode plate. To determine if there is a migration resistor present may be difficult but there are several ways to check this. One easy way is to remove the wire going to the diode detector plate and measure the voltage on the secondary of the I.F. transformer.  The voltage should be close to zero and not positive.  Another means is to short the grid of the second I.F. tube to ground and if the noise is still present the problem may very well be the transformer.  You must of course be certain that there is no noise from the audio amplifier itself.  Use of a dummy tube that has good filaments but having the plate and screen pins removed is a good way to check the second I.F. transformer.  With the dummy tube inserted the noise will be present with a transformer that has the migration resistor.  Of course, if you are working with an AC set it is only necessary to remove the second I.F. tube and the noise will not go away.

 I prefer a vacuum tube voltmeter to make these resistance and voltage measurements because a normal volt/ohm meter will have a low resistance which makes it difficult to impossible to make such measurements.  The same type of noise is - on rare occasions – caused by the audio output transformer going bad.  I have not seen this problem in ages, but it did happen often in the early years of radio.

There are several ways to solve this migration resistor problem. It is only necessary to remove the mica inside of the transformer can and replace it with two capacitors of the correct size. I like the ceramic capacitors because they are small and can be placed outside the can or inside the can.  The capacitor size is often found on the schematic of the radio. However, if you have a new transformer this would be an easy way to go. I do not usually carry these transformers and also do not like to go inside the transformer to remove the mica because it can sometimes get difficult because the base and capacitor may be held together with a rivet.

I have found an easy way to repair these radios by leaving the transformers alone. The repair merely   allows the migration resistor to bring the secondary of the I.F. transformer up to the B+ value of the primary of the transformer. With the secondary at the B+ value of the primary the migration resistor is unable to affect the voltage at the grid (first I.F.)  or the voltage at the detector diode of the second I.F. This means was shown in the previous article on the repair of the sets with a bad first I.F. transformer.  I inferred that the same means could be used to repair the second I.F. but did not go into the details.  I have had some inquiries as to exactly how this repair of the second I.F. should be done and I thought that I should provide a schematic for those interested.  The schematic is shown below with a description of the schematic.  Component values are suggested along with the reason for the values. While the values are not critical the reason for their selection is given and the reason will provide a guide for selection of the components.

Questions or Comments? Please e-mail me at whnj@att.net  Thanks, Bill. .