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How to build a 455khz oscillator for a signal generator..

Discussion in 'General Electronics Discussion' started by Kennynva, Jul 14, 2016.

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  1. Kennynva

    Kennynva

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    Jul 14, 2016
    Does anyone know how to built this...without the crystal...would like to tune the IF transformers of old tube radios..

    Thanks..
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Without a crystal, how will you know the output frequency is accurate?

    ak
     
  3. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    If you don't mind spending fifty bux, maybe this sweep generator from China will do the job. You don't simply "peak up" the interstage IF transformers, you stagger tune them to provide the receiver bandwidth you need for good selectivity and audio quality. To do this, you need a sweep generator that sweeps about five kilohertz on either side of the 455 kHz IF frequency. This Google results page has lots of links to RF sweep generators. If you think a single-frequency RF signal generator will do the job for you, just Google for 455 kHz RF signal generator.

    Years ago I bought an RF sweep generator to align the IFs in television sets. You inject the swept RF into the first stage and look at either the demodulated output or the RF output using an RF probe on an oscilloscope whose horizontal sweep is synchronized to the sweep generator sweep. Below is a picture of the EICO sweep generator I used for TV sets. Obsolete for the TV technology used today.

    [​IMG]

    Quartz crystal accuracy is not usually necessary to align the IF transformers in a single-conversion superhet receiver with 455 kHz IF.
    If desired, a 100 kHz crystal oscillator and its harmonics can provide "markers" for calibrating the tuning.
     
  4. dorke

    dorke

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    Jun 20, 2015
    Hop,
    What are those strange "RF connectors " ?
    never seen them before.

    That EICO can still be used for alignment of "Hi-FI" receivers in it's "A BAND"
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    used to be very common on budget gear of the early years, had more than one bit of gear with that style
    not sure what they were called ?
     
    73's de Edd likes this.
  6. 73's de Edd

    73's de Edd

    2,937
    1,245
    Aug 21, 2015
    Sir Kennynva. . . . . . . . . . . . .


    Messrs . . . Hop, Davenn et Dorke . . . . .

    BEHOLD . . .ye olde tyme . . . . . .

    Amphenol 75 series microphone connectors:

    Amphenol Mike connectors.png


    You say . . . . .

    " I would like to tune the IF transformers of old tube radios.."



    Do I detect a plurality there ? . . . . . . . .or if its being just one specific model.
    of AM radio . . . . at THIS time . . .and if you can give me its brand and model #.

    Requirement 2 is that you have an AM radio station operating at 910 Khz that
    you can pick up.
    ( Or give me your ZIP code so that I can research from FCC files / data .)

    I can Edd-u-cate you how to do it NOW with NO signal generator at all, and being
    aligned to a 900~ (hertz) degree of accuracy.




    73's de Edd
     
    Last edited by a moderator: Jul 16, 2016
    davenn and hevans1944 like this.
  7. Alistaire Berry.

    Alistaire Berry.

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    Sep 19, 2016
    I think they look like amphenol scc connectors. Quite common in the 50s.
     
    davenn likes this.
  8. Kennynva

    Kennynva

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    Jul 14, 2016
    Thank you Sir Edd...I have a Leader LGS-16 signal generator now, and trying to understand the alignment of my Hallicrafters SX-71 communications receiver...I dont get the part of the IF alignment the second step..if you look at the schmetic for me...and explain the part about the crystal that is built in, and aligning the adjustment "U" to get the 455khz right..I am completely lost...I can get the rest of the alignment in the second section..thanks for all that helps me...

    Kenny KE4HVE
     
  9. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Kenny, do you have this service manual? I doubt your Leader LGS-16 has the required 250 Hz resolution and stability to accurately find the crystal filter resonant frequency, which will be quite sharp and somewhere between 450,000 Hz and 460,000 Hz. Step 2 is adjusting the IF transformer secondary tuning to match whatever this actual crystal frequency a particular receiver happens to have. It will nominally be 455,000 Hz (the second IF frequency) but production variations will cause variations about the nominal value. Properly tuning the secondary of the IF transformer that drives the crystal will guarantee the best bandpass performance of the crystal filter and produce the best selectivity the receiver is capable, nominally about 2500 Hz.

    The Note in the manual says this is factory adjusted and that it is unlikely an ordinary bench frequency generator will have the required resolution and stability to "find" the crystal resonance and tune the secondary of the IF transformer to match it. Best to just proceed to Step 3... unless you have already messed with Adjustment "U" in which case you are pretty much screwed. The Leader LGS-16 does not have a good reputation as a precision, stable, RF signal source.

    I suppose with modern circuit design you could build a 450 kHz to 460 kHz oscillator with sufficient resolution and stability to find the crystal resonant frequency and thereby tune the secondary of the IF transformer, per the procedure outlined in the manual. After all, you don't need any accuracy, per se, from this signal generator... just resolution (250 Hz) and stability. Perhaps a Wien Bridge oscillator could be designed to work between those two frequency limits. They do produce a nice sinusoidal output with low harmonic distortion.

    Hop AC8NS
     
  10. Kennynva

    Kennynva

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    Jul 14, 2016
    It was already adjusted when I got it...so I would need to buy a digital or more up to date rf generator to do this? Could I just turn that screw for best reception now since it has been adjusted now..?? I think it is close now...I did the best I could with the leader generator..I can hear the pitch get very loud or high, in a certian spot with this generator...Thanks for your time...I was wondering how to do step 2...
     
    Last edited by a moderator: Feb 27, 2018
  11. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    I wouldn't go that far. This is just a very simple adjustment that requires the ability to set the RF excitation frequency applied to the grid of the previous IF stage very precisely. Not accurately, just precisely, and with some short-term stability while you tweek the core. You just need to match the crystal resonant frequency to the transformer secondary resonant frequency. Failure to do this will broaden the bandwidth of the receiver and pretty much render the crystal filter useless for its intended purpose: narrow the receiver bandwidth to reduce noise and adjacent signals you don't want to hear. This is especially important in the crowded ham bands of today where it seems there is an SSB signal every kilohertz or so. If you are into CW, you can build a simple audio filter to improve CW reception, but that doesn't do much to reduce the noise level. Even a simple one-crystal IF filter is better than none at all to maximize the selectivity of your receiver and reject (attenuate) signals nearby in frequency to the one you are trying to listen to.

    If I were doing this, I would probably just cobble together a 450 kHz to 460 kHz sinusoidal signal generator that has about 250 Hz (or better) tuning sensitivity,,, less would be better as you want to "sneak up" on the resonant frequency of the crystal while slowly tuning the transformer to match it and provide the necessary "shoulders" on the bandwidth. That's why the instructions say to "rock" the signal generator while slowly adjusting the core in the secondary of the IF transformer. The rocking action provides a small frequency variation as you tune past the resonance frequency of the crystal, and all turning the slug does is center the bandwidth of the transformer (which is considerably larger than the resonance peak width of the crystal) symmetrically about that peak.

    When its done right, you end up with about 2500 Hz bandwidth with the crystal in the circuit, more bandwidth when it is out. Later versions of crystal bandpass filters used multiple crystals to obtain very sharp "skirts" on the bandwidth... some as little at 300 Hz or so for CW reception. Collins went in another direction and used mechanical filters to obtain the same end results, building them so well that they could create SSB signals from DSB modulation by simply filtering out the unwanted sideband. It was much easier to create a DSB signal with a suppressed carrier than to synthesize a SSB signal with a suppressed carrier using quadrature mixing of two quadrature RF and two quadrature audio signals. Today its all done digitally.,, but I digress (as usual).

    A ferrite loop stick and a small variable tuning capacitor of maybe 20 to 50 pF would allow you to construct a Hartley oscillator that could tune over 450 kHz to 460 kHz with acceptable tuning resolution. You might have to parallel some fixed mica caps with the variable cap and play with the number of turns on the ferrite core to get the tuning range you need along with the resolution. It would help to also have an inexpensive frequency counter while putting this together and that would also allow you to write down the actual crystal filter resonance frequency when you find it. This will allow you to re-adjust your cobbled together oscillator to exactly that frequency during subsequent IF alignment steps.

    It's been many years since I have farkled around with classic Colpitts, Hartley, and Pierce RF oscillators, but I don't recall that any rocket science is required. Most NPN small-signal transistors will oscillate at a half-megahertz given proper positive feedback. Google some circuits, play around on your bench until something emerges that works "gud enuf" for this one-time alignment job. Need I mention that there is some good information available from the ARRL Handbook (any edition after 1960)? Lots of free information available online if you are an ARRL member.

    I see you are a fairly recently licensed amateur radio operator. Do you have an electronics background? Are you comfortable with building prototype circuits, using them as needed, and then abandoning them to go onto something else? I don't wanna disparage it (becuz now I are one), but many new hams are appliance operators. They get their ticket, buy a rig, and get on the air. Nothing wrong with that IMHO. Not everyone has deep pockets so we all get by with what we can. I would have loved to have owned your vintage Hallicrafter when I was getting started in this hobby. I hope you enjoy many hours listening with it after (not if) you get it aligned and purring. Something about the warm glow of vacuum tubes and the smell they emit that brings back fond memories...
     
  12. Kennynva

    Kennynva

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    Jul 14, 2016
    You are exactly right about the smell and the tubes...I repair and built every kind of circuit I can..I have a breadboard I use alot...and not scared to built things...That is all I do every day..build things...then use them then start over and build again...thanks for the long thread..I enjoyed all of it...When I use my generator I do get a louder sound from the radio speaker at or around the crystal freq..is this what your talking about??? I know my generator is not the best but I do hear that sound...So this radio depends on that crystal to work properly right??? and for it to be (aligned) right...can I just get a more accurate crystal that is more modern and use it...??? Would this help?? The crystal is in a little black box..which seems to split in two parts....I have read somewhere where someone (cleaned) there crystal..what is that all about..???

    Again thanks..

    73's
    Kenny
     
    Last edited by a moderator: Feb 27, 2018
  13. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Some pictures of the crystal in its little black box might help. The pictures in the Hallicrafter manual I found online are not very clear.

    No, a "more accurate" crystal is not going to help. You need to get the secondary of the IF transformer tuned to match the crystal you have. With the crystal out of the circuit (there is switch on the receiver that does that) the IF bandwidth is fairly broad, several kilohertz at least, and that is determined by the RCL characteristics that establish the "Q" of each transformer winding... nothing you can do about that, it is what it is. When you add the crystal to the mix, it increases the effective Q of the tuned circuit by a huge factor... Q of about 100 or so for the transformer and a Q of about 10,000 or more for the crystal. The result is a much narrower band of frequencies that will pass through to the final IF amplifier, and that will determine the receiver bandwidth at any given frequency you tune the radio to. The "swishing" noise you should hear, as the signal generator frequency is "rocked" across the crystal resonant frequency, is your audible indication that the transformer slug is properly adjusted. The BFO should be on when this tuning is performed.

    Adjusting radio receiver IF transformers for "maximum loudness" is a time-honored method, but it isn't very useful for matching the very sharp response of a quartz crystal to resonate with the transformer driving it. Ideally, you want the combined response of the transformer and crystal to be maximized at the IF or approximately 455 kHz and then fall off rapidly on either side. Problem is, the crystal doesn't always resonate exactly at this "ideal" frequency. Later you will pick a frequency for aligning the rest of the IF transformer primary and secondary windings, and this frequency only has to match, fairly closely, whatever frequency the crystal happens to be. If it doesn't match, then the narrow bandwidth of the crystal filter will not be centered on the bandwidth of the the other IF transformers, resulting in decreased sensitivity when the crystal filter is switched in. Bottom line is this: you can't change the crystal's resonant frequency, but you can find out what it is in Step 2 and then use that frequency to tune everything else to match.

    Crystals inside cases with removable covers should never require cleaning, but some hams (including this one) would sometimes open the case and use a soft lead pencil to rub some graphite onto the surface of the crystal. The added mass of the graphite would lower the resonant frequency of the crystal. Handy if your supply of crystals is limited for a crystal-controlled transmitter (as all Novice rigs were back in 1966 when I got my Novice ticket) and you wanted to move your transmit frequency a few kilohertz to avoid QRM from other "rock-bound" Novices. The General class hams could use VFOs and transmit wherever they pleased in the Novice bands, so "pulling" a crystal by this method was not terribly useful. Later, crystal-based frequency synthesizers allowed Novices to operate with more frequency agility, but by then my Novice license had expired and I "retired' from amateur radio to pursue other things electronical.
     
    Last edited: Sep 21, 2016
  14. Kennynva

    Kennynva

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    Jul 14, 2016
    Long time no hear from I hope you are well....I found this thread, and read all of it...and enjoyed all of time you invested in the question...I still, if you believe it...dont fully understand the crystal thing...and if you answer back to me on hear, i have another question about it...thanks...

    Kenny
     
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