Wanted: Dynaco FM3 Selectivity Filter

[Algar_emi]

Hi. Looking to get a Dynaco FM-3 Selectivity Mods filter, or if not available I would like to know of the specs of the Ceramic filter used, probably 10.7Mhz, possibly 280Khz bandwidth. The mods can be seen on this pictures, before and after the mods. The seller seems to be gone from the original forum.

Also if no info is available I would like to get at least one 10.7Mhz Murata style IF filter, preferably 280Khz. These parts are now obsolete from the usual sellers.

Thanks
SB

 

[rayma]

Hi. Looking to get a Dynaco FM-3 Selectivity Mods filter, or if not available I would like to know
of the specs of the Ceramic filter used, probably 10.7Mhz, possibly 280Khz bandwidth.

Looks like there's just the one part on the board, can you see the part number?
This filter operates on the detected mono audio before the multiplex decoder,
not in the IF strip.

 

[kevinkr]

It's a 10.7MHz IF filter and it is in the IF signal chain, not in the demodulated audio path.

I don't not see a ground connection (middle terminal of the filter) connected so the filter isn't really doing the job I suspect.

They also have significant insertion loss and quite low impedance.. (IIRC 330 ohm is typical) For optimum performance they need to be properly terminated and look back into a matching source impedance.

Note that the installation as shown is not well implemented, at these frequencies screw terminals should not be used amongst other things.

You might find someone selling filters at electronic flea markets and ham fests. (I have seen them at several such events in recent years.)

 

[Algar_emi]

Hi Kevin, thanls for the info. There are available on ebay, in 10 quantity, but rather expensive if you add the shipping fee (more expensive than the parts really) and the US exchange rate. I'll ask around. Ham fests are getting rarer every year, disappearing hobby I guess, at least around where I live.
Found this proposed schematic to add a ceramic IF filter, to a tube circuit, on FMtunerinfo. The resistor is there to terminate the filter.

 

[kevinkr]

Worth a shot certainly if you can get the filters. I would terminate into 330 ohm unless the filter specifies 300 ohms.

 

[Algar_emi]

Found the smd version, with 280Khz BW that is said to be easier to implement in most tuner. It has a low 3dB insertion loss. SMD version will be easy to mount on a ground plane DIY small pcb, and smd resistor/cap if needed. It is also cheap at 1.80$ and stock at Mouser: SFECV10M7FA00-R0

Will try that. SB