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In Reply to: RE: 10.7 mHz Discriminator Transformer Repair posted by BFitz on September 21, 2011 at 12:02:54
Thanks for that suggestion. I joined that group and just downloaded and read the article. I have been in contact with Edward J Engelken of San Antonio who has done some of these repairs. He is mentioned at the bottom of the last page in regard to responses to the "Farago Letter". Ed is into restoring antique radios and has been a great deal of help in this project.
There are many variations on exact construction of IF and Discriminator and/or Ratio Detector transformers. Each one may have to be taken as an individual example although there are lots of similarities between manufacturers. The GL3654577 discriminator transformer in my tuner was made by Automatic Mfg. Co. There were no numbers on the schematic to identify pins, so I had to make a detailed drawing before I removed the unit from the chassis, and again at each stage of disassembly. Otherwise I would have made a mistake on reassembly. The one unknown in this unit is the value of the coupling capacitor between the 6BN6 plate circuit and the center tap of the secondary winding. Since the capacitor was shorted, I had no chance to measure capacitance as recommended in the article. Now it will be a matter of tacking across different values underneath the transformer afte reinstallation in the chassis. I have ordered an array of small pfd value disc ceramic 5% NPO capacitors to try one by one. When I get the right value the alignment should be very close as no adjustment slugs have been moved.
Follow Ups:
Today I received a set of various small value capacitors to try on the tuned circuits of the discriminator transformer. I had left the slugs of the discriminator untouched during the repair so that when the correct value of capacitor would be placed in the circuit it would restore near normal operation when the right value was selected. It turned out the correct value was 27 pfd for both the coupling capacitor and the capacitor across the primary winding.
The original mica wafer capacitor that was built into the base incorporated dual capacitors, one for the primary winding and the other for the coupling capacitor.
I believe these values would be typical for 10.7 mHz IF and Discriminator transformers in other brands of tube type FM tuners.
Valuable assistance was provided by Edward J. Engelken who has been active in antique radio repair for quite some time.
I used ceramic capacitors of 5% NPO type for the replacement parts.
Joe
Thanks for posting this information. It should help people with Citation III tuners as well, as the problem would be the same. I have a Citation III here that has the same problem.
Bob
Bob;
I was glad I found the solution and gladly posted the information. Yes the Citation III-X has the same exact discriminator transformer as does the F500X FM Tuner. There were a series of AM/FM Stereo receivers that Harmon-Kardon made that likely used the same discriminator parts too.
Today I went through a complete alignment using the instructions that came with the tuner kit. At one point the unit began to make static again and I suspect one of the IF transformers is giving trouble in the same way. If it becomes more consistent, I may be able to track down which stage is getting flaky and repair the IF transformer for it also. If and when that happens I will post the information here. For now it is operating normally though.
Joe
As a final update on this issue, my unit although working, had weak audio and very poor swing of the center of channel tuning meter. RF and IF tubes had been replaced with new tubes and realignment done. I researched technical information on the Foster-Seeley Discriminator circuit and paid particular attention to any mention of the coupling capacitor between the plate of the limiter stage and the secondary of the discriminator transformer. Discussions refer to the capacitor as C8. Nowhere was a value for the capacitor stated. I had used a 33pFd, which was similar to the value used across the secondary of the transformer. The only thing mentioned about the capacitor was that its reactance was low at the operating frequency. That 33pFd value calculates to about 450 ohms at 10.7mHz. I had a 560pFd mica capacitor on hand and tacked it across the coupling capacitor. Immediately the audio strength improved and the tuning meter had much greater deflection when tuning across a station. A 560pFd has a reactance of about 26 ohms at 10.7mHz. I finally settled on using a 1000pFd capacitor which has a reactance of about 15 ohms at 10.7mHz. The value of the capacitor had no effect on the tuning of the primary or secondary of the transformer.
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