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While repairing an old radio with no AM reception, I found on my multimeter that there is continuity between the primary and secondary windings of the first 455 kHz IF transformer in the circuit. I desoldered it and found that the windings are still reading 0.6 ohms between primary and secondary. Is this normal? I checked for shorts between the can and IFT coil but there are none so I am not sure why the windings would appear to be connected.

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It's not normal to have shorted coils when they ought to be only magnetically coupled by the helical tuning slug and resonate with the fixed cap to get the centre frequency at 455kHz with a bandwidth of about 15kHz.

If you can rewind ok. if not this may work http://www.rf-microwave.com/en/shop/0/211-455-khz-55-mhz-107-mhz-if-coils-transformers/2531-B-455-E-10-R.html

Or better review these specs http://www.mouser.com/catalog/specsheets/XC-600131.pdf

Always use a plastic or wood driver for the ferrite slug and very gently.

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  • \$\begingroup\$ It will not be easy to find a replacement IF transformer for the shorted one because unlike most others it has three pins for the primary and three pins for the secondary. I did contact RF microwave and they advised me that the part I was looking for was customised for that particular Panasonic radio and generic replacements would not be suitable. \$\endgroup\$ – Bob Hazra Dec 6 '16 at 23:07
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    \$\begingroup\$ i agree, it is not easy. I wonder if it was wound with 1 continuous dual loop to reduce cost with common ground on each side. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 6 '16 at 23:58
  • \$\begingroup\$ As SunnySkyGuy says they may be internally connected for convenience if the design calls for a common centre tap the connection may not matter and may even be required. \$\endgroup\$ – KalleMP Feb 4 at 22:40
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Depends on the radio. I would need more detailed info about the radio (make & model # ), but the first IF stage is your mixer/oscillator and most of the time it is a tapped inductor (that looks like the other if transformers). Later on in the integrated am/fm stereo receivers they were relocated to an external ferrite antenna stick to minimize interference with the other circuits.

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