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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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29th Jan 2019, 1:33 pm | #1 |
Hexode
Join Date: Sep 2003
Location: Cheltenham, Gloucestershire, UK.
Posts: 298
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IFT Checker
My eye was caught in this thread (on "IF can transformers for homebrew superhet")
https://www.vintage-radio.net/forum/showthread.php?p=1115224#post1115224/ by the 'two terminal oscillator circuit' here: https://archive.org/stream/73-magazi...e/n49/mode/1up I've accumulated a lot of IFT's over the years; now seemed like a good time to try to put them in some sort of order. The two fet circuit really is very simple; (as usual) I made a few mods: i) 18V seems overkill. Mine works well on 9V. ii) I have loads of 2N3819s, but in oder to select 'good' ones - if necessary - I decided to make them pluggable using some 0.1" header iii) the circuit was most unreliable until I added a 0.1uF decoupler across the supply line iv) I didn't need the facility for adding capacitance in parallel, so ditched the switch and 150pF capacitor v) I used a pair of pieces of copper at right angles to allow quick connection of an IFT by just resting the appropriate legs one per plate vi) I fed the output directly into a frequency counter module I'd had lying around for some time. Total current consumption is about 35mA (10mA for oscillator; 25mA for the counter). I've been able to run thru' tens of IFTs (valve and transistor; 465kHz and 10.7MHz in a matter of minutes), so I'm pretty happy. Two photos attached. dave
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Dave Teague |
29th Jan 2019, 2:56 pm | #2 |
Heptode
Join Date: Nov 2018
Location: London SW16, UK.
Posts: 655
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Re: IFT Checker
Brilliant Dave! Mine failed. I used two new 2N5245 and identical components as the 73 magazine. I connected it to an unkown transistor IF can and used 9v. There was no oscillation.Then I increased from 9V to 18V, it burnt out both FETs.
Well I will have to try again. I used to build many tesla coils and I built this 555 oscillator circuit which can determine the resonance frequency of a LC tank circuit: https://www.rmcybernetics.com/scienc...sla-coil-tuner I have tried it with a transistor IF can with a scope ( It can work reliably without scope). It works OK with IF cans but would not be as easy as the 73 magazine circuit. Sometimes the peaking is not that sharp depending on the Q etc.. Also the frequency sweeping with function generator and scope technique is rather time consuming.. Last edited by regenfreak; 29th Jan 2019 at 3:02 pm. |
29th Jan 2019, 3:40 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: IFT Checker
That's nifty! I particularly like the idea of using header-sockets for the transistors - an idea which I will no doubt use myself in future.
I wonder how your circuit would handle the kind of 'combined' AM/FM IFTs that have both a 455-470KHz and a 10.7MHz transformer, series-connected, in the same can? |
30th Jan 2019, 12:26 pm | #4 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: IFT Checker
It would probably pick whichever frequency had the highest dynamic impedance, which would almost always be the lower one.
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30th Jan 2019, 2:26 pm | #5 | |
Hexode
Join Date: Sep 2003
Location: Cheltenham, Gloucestershire, UK.
Posts: 298
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Re: IFT Checker
Quote:
Based on a very small sample (a pair of IFTs from a Ferranti U1040) - yes: the 465 kHz resonance was the result. dave
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Dave Teague |
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30th Jan 2019, 3:46 pm | #6 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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Re: IFT Checker
If you short out the 465kHz secondary then it might do 10.7MHz instead. Not possible if both sides simply put the two tuned circuits in series, so you can't get at the join.
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