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41-295 I.F. harmonics across dial - Printable Version

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41-295 I.F. harmonics across dial - mgutierres - 03-29-2013

Here in Mpls 1500 AM is occupied by KSTP, and very powerful station, and on my 41-295, it resonates on 590 and to a lesser extent on 1045, approximately.

Is there something I can do, short of adding a wave trap, to prevent the I.F. from resonating like that? I've done a complete re-cap, and replaced most of the resistors as well, but except for the pushbuttons, I've pretty much left the mica caps alone. Should I look more closely at the mica caps near the oscillator?

A friend with a 40-201 reports the same problem, only worse. I re-capped that one, too, so I'm willing to admit fault if I did something wrong.

Anyone else have this problem?

RE: 41-295 I.F. harmonics across dial - BrendaAnnD - 03-29-2013

You didn't do anything wrong... sometimes it just happens. The image (not harmonic) at 590 is the primary image for 1500 (1500 - 910), and a strong station (doesn't even have to be a 50K station, we had one in Caldwell, ID that ran 1000W at 1490 that used to clobber my favorite station out of Boise on 580) will show up in both spots on the dial because it simply overloads the input tuned circuit (antenna loop plus tuner). Sometimes, you can re-orient the radio to minimize this effect, if the two stations' towers are not in a direct line from you. As far as 1045, all I can think of is two strong stations in the area "beating" and producing that frequency.

RE: 41-295 I.F. harmonics across dial - mgutierres - 03-29-2013

Thanks BAD, I was kinda hoping it wasn't my fault. I still might try fooling with some kind of wave trap. I recently re-capped an RCA 13k that does have one, and it works like magic in suppressing its 460 khz I.F.

Mike

RE: 41-295 I.F. harmonics across dial - exray - 03-30-2013

That's not the same thing, Mike. The wavetrap in the RCA is designed to knock out coastal marine stations that used to broadcast close to the IF frequency.

For you to reduce the image at 590 you need to attenuate the 1500 signal with a trap.

Has your set been aligned accurately? Image rejection is accomplished in the RF/Antenna stage. Its never 100%, though. Sets with a tuned RF amp stage will perform best in regard to image rejection.

RE: 41-295 I.F. harmonics across dial - BrendaAnnD - 03-30-2013

Hey, Bill.. good to see you.

RE: 41-295 I.F. harmonics across dial - mgutierres - 03-30-2013

Thanks for the correction. I think the 295 and the 201 are aligned fairly well...I used an Eico 324 which was zeroed in to 455 khz with a Sony 2010, and all the IF coils were peaked using a VOM attached to the AVC line. They are very sensitive and great DX radios, both the 295 and 201.

RE: 41-295 I.F. harmonics across dial - exray - 03-30-2013

Sounds like you are good there. These radios are using their original loop, correct?

I've got a quickie idea you could try if you can scrounge up an old backboard loop antenna and variable cap from any old AA5 radio.

Howdy, Brenda. Long time no 'see'.

RE: 41-295 I.F. harmonics across dial - mgutierres - 03-31-2013

I have a plastic AM loop with twin leads wrapped around it, and I just nabbed a little 2-gang condenser for my parts bin. Will those work?

RE: 41-295 I.F. harmonics across dial - exray - 03-31-2013

Probably. What I'm suggesting is to hook up the loop and capacitor in parallel and place it close to the Philco loop with no actual electrical connection. Then tune the cap of your newly built wavetrap to around what would be 1500 (almost completely open) and see if you get a dip in the KSTP signal or the image at 590.

It oughta work. I've got radios sitting together on shelves that will often do this.

RE: 41-295 I.F. harmonics across dial - mgutierres - 03-31-2013

I'll give it a try and let you know what happens. Thanks for the suggestion!

RE: 41-295 I.F. harmonics across dial - exray - 03-31-2013

Shoot, to make it easier if you have any old radio with a built-in loop and can physically place it close to the Philco's loop it should do the same thing. The second radio remains turned off.

RE: 41-295 I.F. harmonics across dial - mgutierres - 04-01-2013

I'll try it with my old Transoceanic and it's "Wavemagnet". The tuning cap hooked to the loop didn't make much difference, except a bit of static at 1500 when I tuned past it.

RE: 41-295 I.F. harmonics across dial - exray - 04-02-2013

It should 'goof' with the 1500 signal as you tune by. May distort, may clip some audio like selective fading does, etc. What we want to see is how effective the trap is at cleaning up 590 regardless of how it trashes 1500.

Tweaking the spacing between the two antennas will give a deeper/sharper notch vs a not so deep/but broader notch.

RE: 41-295 I.F. harmonics across dial - mgutierres - 04-02-2013

It did work better from the Transoceanic, especially on 590 to attenuate the 1500 signal. I did have to do a little do-si-do dance with the two receivers to get an effect, but it is noticeable.

RE: 41-295 I.F. harmonics across dial - exray - 04-03-2013

Cool! Now something permanent if you want to. I have a strong local at 1370 and have used a small toroid coil and trimmer inline with the antenna for trapping. Easy to hide away and doesn't require a dance for best placement. Downside is that it will affect reception +/- the tuned frequency so tuned for 1500 you may lose other stations say 1470-1550.