America - Restoration 3

This is a pictorial example of the procedure which I have used to repair intermediate transformers in which the capacitors are suffering from silver migration.

1) Start by identifying the radio. Look for a model number on the chassis or cabinet, or if that is a no-go, get the tube list, take a photo if possible, and post the tube info and radio description on an antique radios "newsgroup" or "forum". Someone is sure to help.

The idea is to get the values for the I.F.capacitors, and hopefully from a schematic. If this is not possible, and if no-one is able to help with identifying the values, then you're going to have measure the inductance of each individual coil, and calculate the capacitance using formulas for capacitive and inductive reactance at the I.F. frequency. Now that we've scared you to death with the possibility of having to know the theory and math behind resonance, have a look at the following mechanics of the operation.

2) Identify the offending transformer (this one looks suspicious) Usually this is done either by using a modulated signal generator and your ear, or a signal generator and an oscilloscope to trace backwards from the audio stage, until you pass a stage where there is no more noise either audible or viewable on the scope. Typical intermediate frequency transformer:

3) Be sure that you're unsoldering the correct transformer, that it is indeed an IF transformer, and don't get them backwards when you flip the chassis over ( this leads to expressive language later!). The transformer bases are sometimes marked with one or more coloured dots to identify what the connections are, and where they go.

If your schematic is not coded, or the transformer has no marks, it's a good idea to mark the transformer, and your schematic, and possibly even the chassis (for orientation) so as to make the reassembly easier. (Especially if you have to wait a while for the capacitors, and have a tendancy to forget like I do!)

IF xfrmr bottom side:

4) This next photo is what the transformer looks like before it is pulled apart. Now for those of you who take things literally, "pulling" is a relative term. "Gently disassemble" is probably more accurate.

Note: the clip is taken off before you remove the can from the chassis. It's true, I wouldn't lie to you.

IF xfrmr out of chassis:

5) Following is the transformer out of the can, but with the brass rivet still holding the parts of the capacitors together. I've found that the best way to remove the rivet is with a very sharp drill bit and light pressure, but high speed on the bit. Others have had better success using a hot soldering iron to the melt the rivet out of the plastic base plate. This method may be easier if you are not able to drill cleanly and carefully. The soldering iron method involves putting the round tip from a hot soldering iron into the hole in the rivet, and pushing gently while the heat from the iron melts the plastic around the rivet. The
rivet will push up into the framework of the transformer, and then can be picked off the soldering iron tip with a pair of needle-nose pliers.

6) Here it is with the rivet drilled out, and the plastic capacitor retainer plate removed. All that is left are the two top contact strips, the mica spacer/insulator, and the two lower contact strips (not visible under the mica).

Rivet drilled out - cover plate off capacitor:

7) And once again, this time with the mica out and both contact strips showing. Lots of migration showing in this puppy! It was going to need a passport soon!! Maybe even some unemployment insurance!

Capacitor mica removed:

8) To keep the contact strips from touching, I curl them a bit. If the
replacement caps WON'T fit in the can, then I glue (hot or epoxy) the strips tight into place, and solder the replacement caps onto the underside after the transformer is reinstalled into the chassis. If the replacement caps DO fit in the transformer, then..........see the next photo.

Ready for new capacitor:

9) This is "any old capacitor" which I used for the photo. It's now soldered into place and ready to go back together. In your radio, make sure that you have the correct value. You can test these transformers and capacitors out of the radio by putting an RF signal of the correct frequency (obviously) through the primary coil, and hooking your scopeacross the secondary using a detector probe. With this hookup, you can verify that the circuit resonates properly, and you can also peak the transformers before they go back into the radio.

New capacitor soldered into place:

10) Here we have the transformer with new capacitors installed and the assembly slid back into the can, with the metal retaining tabs bent backinto place.

IF xfrmr assembled - ready to reinstall into chassis:

11) This shows how the clip will attach to the transformer and hold it into the chassis. It's probably a good idea to put the transformer into the chassis before installing the clip! It's easier that way!

How retainer clip fits:

Parts Needed:

42 Series #425 40 – 200 PF Ceramic Trimmer Capacitors

85 – 100 PF 250V Ceramic Trimmer Capacitor

Bob Krueger, AB7CQ

Web Administrator

RPTR 1: 146.920/146.320 PL 123.0 (WIRES-X Room 28187)

RPTR 2: 444.600 / 449.600 PL 100 (LAN Linked)

Simplex IRLP Node: 7515 PL114.8 (146.540)

Email: ab7cqradio@ebidpal.com

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