Yaesu FT-101 preselectors? Convertible into standalone HF preselector? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: May 08, 2022, 05:05:51 pm
content: I keep seeing these Yaesu preselector pats which contain three tunable inductors.They are fairly inexpensive and so as an experiment I want to buy one and see if I can do the above with one.Has anybody ever done something similar? I want it to have high Q and to use it to cut out splatter on the HF bands..I'm going to attach a photo of one below in a sec..They cost under $20 used just as parts... Some of the high end HF receivers have preselectors, such as Hagenuk. seems very nice.. and useful with weak signals.

author: A.Z.
author_rank: Super Contributor
timestamp: May 08, 2022, 05:48:00 pm
content: Why don't you just build your own ? A simple but effective one would just require a selectable L connected to a varactor for tuning, optionally you may follow it with a preamp/attenuator stage, nothing fancy and will work just fine; if you need an example to start, just search for "steber preamp arrl"

author: cdev
author_rank: Super Contributor
timestamp: May 08, 2022, 08:34:42 pm
content: Quote from: A.Z. on May 08, 2022, 05:48:00 pmI have been thinking about that but I dont want to reinvent the wheel so to speak. Ive built several preselectors and its a lot of fun. A Magnetic Loop is basically a preselector.

author: A.Z.
author_rank: Super Contributor
timestamp: May 08, 2022, 08:41:40 pm
content: Quote from: cdev on May 08, 2022, 08:34:42 pmdid you even check for that Steber circuit ?

author: cdev
author_rank: Super Contributor
timestamp: May 09, 2022, 12:03:52 pm
content: I did now and am checking it out..I can vouch for the usefulness and efficacy of varicap diodes (in my case BVAM109s) for making a tuning box work well, in my case I built Chris Trasks design sans preamp (Its alreadysuperstrong just with the tank circuit alone, perhaps because of the extra selectivity I should try adding that. It is really sharp.)The signal coming out of this loop is so strong I often cant handle it. Especially the bible thumpers and radio Havana.. its around as strong as a signal as Ive ever seen. It makes my SoftRock go nuts. Winter nights I have been able to pick up what must be very weak signals, people on boats on the other side of the world, etc. Very cool.The only problem is the need to use it as a loop. Often I just want the sharpnesss. Without the need to rotate the loop outside withe the speaker pointing out the window.Are you ever in the US? I think you would like to see this thing. Its very easy to make one. (with varactors)I think I am going to regardless get one of the preselectors and try to use it. I would like to use all three inductors though so I am going to need to know a bit more about the Yaesu radio they come out of.. (I have the manual, somewhere) I am specifically looking for the inductance of the three coils and their range. With that info I might be able to use RFSim99 to figure out the best filter arrangement to take advantage of it. Higher impedance looking into it would be good. I should make a web page about this project if I do it. I suppose.. why not..? They are affordable, and might turn cheap RTL based radios into significantly better ones. with a few other components.Since I already have most of the parts, I think I am going to build the "Steber" preselector in your PDF, thank you. I think I can just substitute the varactor for the ones that I have. the voltage range and capacitance are close enough. Maybe should get some more, even, they are very useful. One can also use some LEDs, especially old ones. So I just need to get the FET, thats all. I have the other transistors. Need to get another laser printer. Thank you for what looks like a potentially fun and easy project.. (since I already have so many of the parts, might as well give it a shot)

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: May 09, 2022, 09:19:50 pm
content: The part you have a link to is not really a "Preselector " in itself, but is part of the tuning of a FT 101. It is a gang of three adjustable coils, It adjusts three separate tuning sections in the FT 101. As I recall only one of the coils is in the receive section the other two are in higher voltage transmit sections.It works in LC circuits. The radios used different capacitors (mostly trimmers) engaged with the band switch to make LC circuits appropriate for the different bands, Then fine tuning is done with the "slugs" of these big inductors with the "preselector knob". This system is used with coils like this in other old rigs also. Collins used a similar system. Other rigs used a variable capacitor "gang"Part of it purpose is tuning the RX band, hence the name preselector. (have to call it something, I guess). I think when we refer to a preselector today we are referring to a front end filter or "roofing" filter, which these were not any part of.To use it as a tuner or filter you would have to add various capacitances in different bands to get it to resonate at the correct frequency.Besides these things are old and cranky and can break.I think the other suggestions sound much better than using this thing.I have built a mag loop. for receive only it is quite a good thing, and easy to make. again it is just an LC circuit.