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 Post subject: Aligning FM receiver
PostPosted: Jun Fri 20, 2003 5:38 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 107
Location: Palmyra, Virginia
Hey guys,<P>I need help. I have a big Philco chassis from '48 that has BC, SW, and FM. I have replaced caps resisitors etc. and for a while picked up FM stations weakly. In trying to align the AM I lost the FM signals and now can pick up nothing on AM, FM and SW. <P>So my question is: I need an alignment crash-course. Will a R.F. signal generator with a range of 100 kHz to about 20 mHz work? or do I need one that can go up to 108 mHz for the FM? I have several reference books (the Johnson and the Carr books) but I need it plain and simple. Anyone willing to fill me in? Thanks! Andy<P>------------------<BR>Andy Sorrell<BR>Blacksburg, Virginia


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 Post subject: Aligning FM receiver
PostPosted: Jun Fri 20, 2003 11:47 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 477
Location: NC
Andy<P>You would probably get more responce by posting this on the electrical restoration forum.<P>Chances are your three bands all share a common IF, and you have disturbed this part of the alignment (since you are not receiving any stations at all).<P>The model number would help us to pinpoint more detail for you.<P><P>------------------<BR>Ken


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 Post subject: Aligning FM receiver
PostPosted: Jun Tue 24, 2003 3:41 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 596
Location: Buckingham County, VA. (Foothills of the Blue Ridge Mtns)
Biggest problem with FM alignment is getting the bandwidth wide enough. With traditional AM alignment you would peak each coil for maximum signal strength. You can't do that for FM, you will give up a little signal strength by tuning the coils for a wider bandwidth .. each coil is tuned slightly off center.<P>------------------<BR>


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 Post subject: Aligning FM receiver
PostPosted: Jun Wed 25, 2003 7:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3162
Location: Cockatoo, Victoria, Australia
Hi Andy, See if you can pick up a TV / FM sweep and marker generator. I think you should find one for low price at swap meets, Ebay, etc. It will need an oscilloscope unless it has one built in. With that aligning FM and TV sets will be a breeze.<BR>Before worrying about aligning the FM, concentrate on getting the AM section going. You'll only need your current oscillator for that. Try feeding it into the IF stage first and when that's working OK move to the RF and converter stages.<BR>Don Black.<BR>PS it produces an RF signal that sweeps up an down in frequency across the circuit you're aligning to display a frequency response curve. The marker displays a pip at spot frequencies to calibrate the response.<BR>Don Black.<P>------------------<BR>


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 Post subject: Aligning FM receiver
PostPosted: Jun Wed 25, 2003 6:38 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 26
Location: Carlin, Nevada USA
Andy, I think you're going to find that you have dual IF transformers, with a 455 khz winding for AM/SW and a 10.7 Mhz (or just maybe 4.3 Mhz, there were a few) winding for FM. The 455 khz IF would just be too narrow band for decent FM. You might have to take the cover off an IF can to identify the windings, but I'll bet what happened is you accidentally tweaked the FM settings trying to improve the AM. If you can identify the FM adjustments, you can do a rough alignment without a signal generator by first peaking all the transformers for maximum signal, then going thru and tweaking one DOWN in frequency just a hair and the next UP in frequency, or if the thing has separate IF transformers for FM with two slugs or trimmer caps, set one adjustment UP in freq and the other DOWN on each transformer. This will probably give you enough bandwidth for decent FM. If you have something like a 6H6 dual diode tube for the FM detector, don't play around much with the transformer preceding it. That adjustment will make a huge difference in your signal quality. Of course, the best thing is to get a sweep gen and scope, but this might be a cheap and dirty method of getting you going for a while.<P>Ron<P>------------------<BR>


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 Post subject: Aligning FM receiver
PostPosted: Jun Wed 25, 2003 7:21 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 107
Location: Palmyra, Virginia
Hey Guys,<P>this is a philco 48-1276 console. I have an oscilliscope I picked up (for free!) this past week, I just have to make sure it works. -- Andy<P>------------------<BR>Andy Sorrell<BR>Blacksburg, Virginia


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 Post subject: Aligning FM receiver
PostPosted: Jun Wed 25, 2003 7:43 pm 
Moderator

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3162
Location: Cockatoo, Victoria, Australia
Hi Andy, What's the make and model of your oscilloscope? Have you got the schematic and service information for it?<BR>Don Black. <P>------------------<BR>


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 Post subject: Aligning FM receiver
PostPosted: Jun Wed 25, 2003 11:30 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 107
Location: Palmyra, Virginia
Hey Don,<P>I'll have specifics when i get off work, but it is very large (weighs at 100 lbs) and i would assueme from the 60s It was from a university suplus sale. -- Andy<P>------------------<BR>Andy Sorrell<BR>Blacksburg, Virginia


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 Post subject: Aligning FM receiver
PostPosted: Jun Thu 26, 2003 1:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 107
Location: Palmyra, Virginia
Hey Don, <P>It is a Tektronix Type 565 Dual Beam Oscilloscpe. I'm not sure if it works or not, but I do not have the scheatic for it. -- Andy<P>------------------<BR>Andy Sorrell<BR>Blacksburg, Virginia


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