FM If Transformer Rebuild

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FM If Transformer Rebuild

Postby SDS-PAGE » Fri Mar 31, 2017 10:09 am

Hey Folks,

I have recently picked up a Coronado AM/FM tube radio from a local antique store. The AM section works great but the FM section has extremely low output on only two channel it is able to tune into. I narrowed down the root cause to IF input and output transformers and possibly a ratio detector transformer having silver mica disease. I removed the transformers, checked for continuity, and removed the diseased internal mica wafers. I did the same for the AM IF cans and bridged each winding with a 100 pf cap.

Now, I am struggling to know what cap values I need for the FM IF and ratio detector transformers. I did a quick search on the web and it looks like the value is typically much less than 100 pf. To those that are knowledgeable in tube radios, could you shed some light on this for me? Measuring wafer mica capacitance is not an option for me as the wafers were destroyed during the removal process.

Thanks,

Min
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Re: FM If Transformer Rebuild

Postby DeathRex » Fri Mar 31, 2017 11:05 am

I haven't replaced any caps that have silver mica disease, because I don't know of any that have it, except a Loewe Opta Hellas 552WP, that I never could et good FM on. It uses 1000pf in the AM section and for FM on the first 2 IF transformers (not the detector) it has a 60pf, 60pf, 25pf, and 60pf. The detector has a 16pf and then 60pf. The radio bypasses the output 60pf caps when in AM mode.
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Re: FM If Transformer Rebuild

Postby SDS-PAGE » Fri Mar 31, 2017 12:04 pm

This is very helpful. Thank you!
I may try trimmer caps to see what capacitance will get me close to what I need. I don't have the appropriate equipment setup to do it properly (especially aligning FM) but I am going to rely on a very scientific method of tuning it with my ears.
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Re: FM If Transformer Rebuild

Postby kheper » Sat Apr 15, 2017 2:13 pm

Here's a brief description, regarding tuner alignment, To do it correctly, you need an FM signal generator, a scope and a distortion meter. Distortion meters (unless you tune as a profession) are prohibitively expensive.

http://www.fmtunerinfo.com/index.html#alignments

One thing to watch out for: Never use an Allen Wrench on a transformer. It'll crack the tuning slug. Radio Shack makes suitable, plastic wrenches. If the slug won't turn, don't force it. It may have wax melted in it to prevent drift. Use a hairdryer - on the hot setting - to melt the wax. Some people tune (or claim to do so) by ear, but I'd be skeptical of the results.
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