[50mhz] adding AM to an old FM receiver - questions

[cboone at earthlink.net]

An ex neighbor of mine (WA5EFI) HAD one of those Link base stations.....it was xtalled on 52.525....had a distinctive ker-blunk sound on the air as it keyed or unkey (I dont recall which)

Adding AM to an FM rcvr is not easy...for AM, you need a IF chain controlled gain by AGC derived from the AM demod stage..You CANNOT use the limiters in the FM stage as they are there to eliminate AM noise (and thus AM signals)

You would have to tap off after the mixer and xtal filters BEFORE any limiter stage...then go thru a normal AM IF and detector with the AGC being made by the signal or audio loudness and fed back to the IF as gain level control voltage...(preferably you want to also AGC the RF front end...but that should not be an issue) FM radios run FLAT OUT in gain....140db or more in gain from the front end to the disc..which is why FM has soo much white noise...AM does not have the "white noise" level audio wise that a FM does with no signal...Also remeber to use a narrower AM filter in its IF stages (FM is 20kHz wide but AM is only 6 kHZ wide)

Chris
WB5ITT

-----Original Message-----
>From: "Patrick J. Jankowiak" <recycler at swbell.net>
>Sent: Sep 14, 2010 10:12 AM
>To: 50mhz at mailman.qth.net
>Subject: [50mhz] adding AM to an old FM receiver - questions
>
>Hi All,
>
>I mostly lurk here, having little time due to work, but soon my antenna 
>will be up and I want to participate in the 6 meter fun.
>For 6M, besides some very low powered gear, I have only a Link 250UFS 
>base station, which is FM. It takes crystals, but I can substitute 
>existing external VFOs for the TX and RX crystals. The Link is 250W and 
>uses a 250TH final. It has separate TX and RX.
>
>My question is whether anyone has successfully added or modified an FM 
>receiver of this general kind so that it could receive an AM signal.
>
>The RX has a string of two  I.F. stages, acting as limiters. Maybe some 
>ACG? I am not sure what to do and I do not want to hack up the set 
>irreversibly.  (for  SSB  an external detector could be added once it is 
>capable of AM)
>
>complete info here - click on the logo: 
>http://www.bunkerofdoom.com/kd5oei/link/linkindex.html
>The unit in question is the larger one - the 250UFS. However, it has the 
>newer receiver, the "Ed. 7" receiver shown in the material for the 
>smaller 50-watt unit.
>
>For a direct look at the receiver in question please look here:
>
>There is a GIF of the schematic here: 
>http://www.bunkerofdoom.com/kd5oei/link/linkmenu/link_50ufs/l0000000114.gif 
>but it is not very good.
>
>The 23MB PDF file has a much better diagram on page 15 of the PDF:
>http://www.bunkerofdoom.com/kd5oei/link/linkmenu/link_50ufs/Link_50UFS.pdf
>
>
> Thanks for taking the time to look at the diagram if possible.
>
>Patrick KD5OEI