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Old 29th Jan 2019, 1:33 pm   #1
dave_n_t
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Default 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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Old 29th Jan 2019, 2:56 pm   #2
regenfreak
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Default 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.
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Old 29th Jan 2019, 3:40 pm   #3
G6Tanuki
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Default 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?
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Old 30th Jan 2019, 12:26 pm   #4
G8HQP Dave
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Default 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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Old 30th Jan 2019, 2:26 pm   #5
dave_n_t
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Default Re: IFT Checker

Quote:
Originally Posted by G8HQP Dave View Post
It would probably pick whichever frequency had the highest dynamic impedance, which would almost always be the lower one.

Based on a very small sample (a pair of IFTs from a Ferranti U1040) - yes: the 465 kHz resonance was the result.


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Old 30th Jan 2019, 3:46 pm   #6
G8HQP Dave
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Default 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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