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 Post subject: Couple of questions re. IF transformers & Local Oscillators
PostPosted: Sep Sat 11, 2021 5:29 pm 
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Location: Wake County NC
So, I run into IF transformer slugs that are stuck and stick so bad to turn them you will eventually strip out the driver slot. What can be done to free these slugs up prior to trashing the tuning driver hole? Is there anything that can be used that won't permanently screw up the transformers resonant frequency peak, etc.? I was thinking maybe a bit of graphite or some other inert lubricant. Any ideas?

I have been working on a radio learning about IF chains, and now Local Oscillators. A lot of questions have come up, for example: The local oscillator on the radio in question should generate 455kHz as the IF frequency. What does the variable capacitor in the IF tuning section tied to the Antenna, and Mixer tuning module (it's a ganged 3 part Va cap) have to do with generating the IF frequency? I assume the Local Oscillator does more than generate simply 455KHz. What product is supposed to be coming out of the Local Oscillator tuning section, actually? Why is there a variable capacitor in the mixer tuning section? I understand the need for one in the Antenna tuning section. But the function of these others is not clear. Any help would be appreciated. IF there's an online resource that explains how this setup works, and clearly, I would appreciate a link in lieu of a decent explanation.

Last question, the signal going out of the pentagrid mixer/oscillator tube goes to the first IF transformer. However, the signal leaving that transformer seems to be at about 1/5 voltage leve (signal strength) . This is a problem with the IF Transformer, yes? I understand that the dB should be about the same leaving the transformer as entering. Any information on this issue would be much appreciated.

Thanks for any help on these issues.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Sat 11, 2021 6:11 pm 
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No lubricants. Use some heat to free the slug. Some people will direct a heat gun nozzle into the top of the transformer. Be very careful not to overheat. I use an Allen wrench that I heat with my soldering iron (not over a burner—too hot) and then insert that into the slug, but do not use it to turn the slug—just heat it. I then insert a plastic tool and carefully try and turn the slug.

As far as the tuning capacitor:

If it has two gangs, one is for either the antenna tuning or the RF amplifier and the other for the local oscillator. If it has three gangs then one for the antenna, then the RF, then the LO. The local oscillator should really be called a variable local oscillator. It’s frequency is adjusted to be 455 KHz above (or below) the frequency shown on the dial. The mixer tube combines the dial frequency (which you have tuned with the first gangs) and the LO frequency to create some mixer products. The IF transformers and amplifier tubes select the product you will use to amplify as the signal moves toward the detector. The other products from the mixer are ignored (selected out) and don’t make it to the detector. The IF frequency is really a range of frequencies that include the audio information and is centered around the IF (in many radios-455KHz).

Norm

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Sun 12, 2021 2:35 am 
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For a good explanation of how a superheterodyne receiver works, read the receiver chapter of an ARRL handbook. Some older ones are on the web.

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Sun 12, 2021 9:16 am 
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Norm gave pretty good (simplified) answers to your first two questions.
I am going to try to simplify a very complex problem. (Apologies to the real engineers if I fail. :))

"Last question, the signal going out of the pentagrid mixer/oscillator tube goes to the first IF transformer. However, the signal leaving that transformer seems to be at about 1/5 voltage leve (signal strength) . This is a problem with the IF Transformer, yes? I understand that the dB should be about the same leaving the transformer as entering. Any information on this issue would be much appreciated."

Your problem is that you do not quite understand "dB". dBs are not the equivalent of "signal levels". A dB figure is the measurement of a ratio, generally but not always, of the output to input. A third letter means that the dB figure is an absolute measurement. "0 dBm" (used a LOT in radio) represents a power level of 1 mw into 50 ohms, or about .224 vRMS. -20 dBm = 10 microwatts.
40 dBm = 10 watts. It's also a convenient way of working with really small or really big power levels. When I was in the radar business, I worked with levels from about - 100 dBm (REALLY, really tiny) to about 100 dBm (1 million watts). And you have broadcast engineers here who have worked with even higher power levels.

"0 dBA" (used in audio), on the other hand, represents a power level of 1 mw into 600 ohms, or about .775 vRMS. dBs are always logarithmic.

The way mixers work makes it difficult to measure the signal level out of a mixer. The "standard" RF mixer generates four outputs: the original freq, the LO, and the sum and difference freqs. The IF amps filter out the three undesired signals. You need pretty specialized equipment to separate out the signal you want.

Now, IF the transformer has a 1:1 turns ratio, the output should be a small bit less than the input. You will always have some losses in the transformer. If the turns ratio is different, you will get more or less voltage out of the transformer. The POWER level (minus losses), however, will be the same.

John


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Mon 13, 2021 1:11 am 
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How are you measuring the IF voltages? Unless you have the right equipment, connecting something to the transformer will detune it and make the measurement meaningless.

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Mon 13, 2021 7:54 am 
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Good point, Jim, but what do you think you should use?

My first thought was a 'scope with a 10:1 probe, but even that has a little capacitance. Perhaps a high impedance RF millivoltmeter?

John


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Mon 13, 2021 9:17 am 
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Basically, I'm following the tuning directions for the radio, as written in the manual along with the tube pin voltage chart. Reading the voltages requires a high resistance VOM and I'm using a Fluke 73 and a 26III.
I've read that you should use a .2ufd Cap in line with a 10x scope probe to read the signal at points on the Oscillator assembly and the IF Cans.
As far as dB I understand that probably wasn't the proper terminology. The point still stands that the Primary side was around 5x times the signal (not DC) on the secondary side. The cap removes the DC which is essential to get the trace actually on screen. Several posts recommend this approach.
I'm trying to make sense of the oscillator output in order to determine if it is functioning properly. What I understand is with the Dial set on 1.0MHz the output should be 1455KHz.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Mon 13, 2021 5:24 pm 
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The radio itself is the best diagnostic tool you have. A second radio will "hear" the oscillator. If you have a general coverage radio that has a BFO and accurate digital readout, it is very useful for setting the oscillator. For some radios, the manual has the gain listed for each stage. This isn't really useful unless a radio just refuses to work. Normally, a person starts at the output--audio--and works toward the input--antenna. You inject progressively smaller signals as you proceed, to get the same output--volume--which can be measured with an output meter connected to the speaker terminals. The amount you reduce the signal at each stage is the gain for that stage. Inject signals at the grid of the stage you are testing. As others have stated, connecting anything to the IF transformer output de-tunes the transformer. This is okay, but the transformer must be re-tuned after you remove the connection and any results are tainted. Some other methods of picking off the signal are to wrap a coil of wire around the tube and get the signal that way or to pick it off the cathode through a small capacitor. A 30p-100p will pick off the signal where a large capacitor is not isolating enough.

Norm

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Mon 13, 2021 6:21 pm 
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Jim Mueller wrote:
For a good explanation of how a superheterodyne receiver works, read the receiver chapter of an ARRL handbook. Some older ones are on the web.


http://radioremembered.org/superhet.htm


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Tue 14, 2021 1:24 am 
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Greetings to Norm and the Forum:

Norm Johnson wrote:
For some radios, the manual has the gain listed for each stage. This isn't really useful unless a radio just refuses to work.


I'm sorry, but I can't agree with that. Block and level information is among the most useful info you can have.... unfortunately, it is rarely included with most service literature. If the radio is dead, it is usually a simple matter to locate the defective stage. The most difficult problems to solve are those where the radio is working, just not as well as it should.

Suppose you have a radio that is speced for 1/2 uVolt for 20 dB quieting.... but the actual performance is 1 uVolt. That's pretty hard to locate... all the stages are working, all the voltages will be close to correct, just something isn't quite right. Level or gain information for each stage is EXTREMELY helpful in these situations, which occur all too often. With the factory level information at hand, one can decide whether the problem is in the RF amp, the mixer, or the IF.... or perhaps due to low L.O. injection level. Sometimes, it is a combination of more than one stage.... good luck finding that without the stage gain info.

I'd much prefer to work on a dead receiver than a marginal one, every time. It is usually quicker and easier to remedy the former than the latter. Just my $.02 worth; your mileage may differ.

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Tue 14, 2021 11:39 pm 
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Oh, yes. In a dead receiver (or transmitter, for that matter), the problem is generally pretty obvious. The solution may not be, however. In a marginally performing receiver, at least ONE something, somewhere is not quite working correctly.

What is even worse is an intermittent problem. There, unless you are really lucky, all you can do is keep a very detailed maintenance log.

John


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 15, 2021 6:36 pm 
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"The radio itself is the best diagnostic tool you have. A second radio will "hear" the oscillator. If you have a general coverage radio that has a BFO and accurate digital readout, it is very useful for setting the oscillator. For some radios, the manual has the gain listed for each stage." This is a great idea. In fact I used my little spectrum analyzer and found that the (problem radio) frequency for the LO was ~1.6MHz where the freq for a working radio was ~1.3MHz. Any ideas here? Same model radio with the same control settings, BTW. Seems the broken radio LO is way out of tune? Is there a point in the LO schematic that you could use a scope to look for 455KHz IF Freq for some kind of tuning effort--given there's no speaker output from the LO. Or is the 455 freq only the result if the IFts notch filter and the LO will always present a spectrum of frequencies that you cannot use to tune the caps and coils? I've heard that the LO sends out the tuned frequency minus the IF. Not what I'm finding. More like added to, rather than subtracted. Any suggestions here would be appreciated. (i know how to tune the IFTs for 455KHz optimization and have done so in the past, successfully.)
More information is this: I can inject (sg gen) 455 modulated signal into the primary of the 1st IF and get a strong tone out of the radio speaker. I believe this is a good sign that the audio at least is working as well as the inductive coupling IF chain in the IFts. [/i]

"Normally, a person starts at the output--audio--and works toward the input--antenna. You inject progressively smaller signals as you proceed, to get the same output--volume--which can be measured with an output meter connected to the speaker terminals. The amount you reduce the signal at each stage is the gain for that stage. Inject signals at the grid of the stage you are testing. As others have stated, connecting anything to the IF transformer output de-tunes the transformer. This is okay, but the transformer must be re-tuned after you remove the connection and any results are tainted. Some other methods of picking off the signal are to wrap a coil of wire around the tube and get the signal that way or to pick it off the cathode through a small capacitor. A 30p-100p will pick off the signal where a large capacitor is not isolating enough." I assume my TinySA spectrum analyzer would suffice for picking up the LO signal? It gives the frequency as well in a pronounced peak, unmistakably different from background. Based on this the LO seems to be working but significantly out of tune. Given this I would think that the IF chain won't work without the 455KHz. This is why I need to re-tune it, somehow--with no output to the speaker.
I can inject 455KHz onto the grid of the Converter and I then get noise through the speaker. In this noise you can tune in carrier signals--but no recognizable voice. So, something seems wildly detuned. Indicates I'm not getting the requisite 455 from the LO, or it is malfunctioning otherwise, I would think.


I believe there may be multiple problems in this radio. Just trying to work through them as I can identify them. Thanks for all the help and I am making progress. Trouble shooting tips are very helpful. As I said, the IF and the Audio appear to be functioning. Thanks Bill for the link to the troubleshooting info. I'll be checking that out, for sure. Thanks Norm for your suggestions and Info as well.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 15, 2021 8:15 pm 
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Greetings to KE4CDR and the Forum:

I think a bit of superheterodyne theory is in order here. Superhet receivers came about because of the limitations of TRF (tuned radio frequency) receivers. It is hard to get multiple stages of RF amplification to "track" together as they are tuned through the broadcast band and the system is prone to oscillation and other kinds of instabilities. These problems are overcome by making an amplifier chain that is optimized for one frequency... usually 455 KHz for broadcast band receivers.

This overcomes the difficulties of TRF radios, but there is one problem.... there are no broadcast stations operating at 455 KHz. Therefore, the frequency of the broadcast station that you wish to listen to must be shifted to 455 KHz. This technique of frequency shifting is called heterodyning and receivers that employ it are called superheterodyne receivers. In order to do this, a local oscillator and a mixer are required. Let's look at the mixer first.

A mixer is a non-linear amplifier stage that is supplied with two signals of different frequencies. When these two frequencies are applied to the mixer, you get four principal outputs: You get the original two frequencies, plus the SUM of those two frequencies plus the DIFFERENCE of those two frequencies. The two inputs to the mixer stage are the desired radio station and the local oscillator. Lets take a simple example:

We wish to receive a broadcast station transmitting on a frequency of 1,000 KHz. Our radio has an IF frequency of 455 KHz. In order to receive the station at 1,000 KHz, the receiver local oscillator is tuned to a frequency of 1,455 KHz. This signal is injected into the mixer along with the 1,000 KHz signal from the radio station. The resulting output from the mixer is FOUR frequencies:

1,000 KHz. (The radio station frequency.)
1,455 KHz. (The L.O. frequency.)
2,455 KHz. (The SUM of the L.O. frequency and the radio station frequency.)
455 KHz. (The DIFFERENCE between the L.O. frequency and the radio station frequency.)

Of these four frequencies, only ONE is able to get through the IF amplifier chain.... the 455 KHz DIFFERENCE frequency. All of the others are rejected by the selectivity of the IF amplifier chain.

If we wish to listen to a radio station on a different frequency, we tune the L.O. to a different frequency, so that the difference between the L.O. and the desired radio station is 455 KHz. Usually, the input to the mixer stage is also tuned to help reject unwanted signals and some more sophisticated receivers will have a tuned RF stage before the mixer to aid in selectivity. All of these tuned circuits are "ganged" (i.e. mechanically coupled together) so that the system is optimized for the desired receive frequency.

The other component of this system is the local oscillator or L.O. This is a CW (continuous wave) oscillator that runs at a frequency that is always 455 KHz above the desired signal. Thus, for most broadcast band receivers, it runs from about 1055 KHZ to 2055 KHz. It never runs at 455 KHz or anywhere near there so you cannot make any meaningful measurements at 455 KHz..... that frequency never appears in the L.O. system. The only thing you can check for is that the L.O. is running, that it has adequate output to drive the mixer and that it tunes over the correct frequency range. That's it.

Note that it is possible to have the L.O. operate at 455 KHz BELOW the desired reception frequency, but this is not usually done in broadcast receivers for technical reasons that are best left for more advanced discussions. Also, I have left out any discussions of image response for the same reason. However, it is necessary to grasp the superhetrodyne principal in order to trouble-shoot mixer or L.O. difficulties.

Regards,

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Wed 15, 2021 11:35 pm 
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The L.O. has a provision for adjusting its frequency. The LO frequency is determined by a coil connected in parallel with a capacitor. The capacitor is actually two capacitors--one gang of the tuning variable capacitor in parallel with a small value trimmer capacitor. These are often directly below the chassis at the LO gang of the tuning capacitor. In a normal alignment, the tuning knob (capacitor) is turned to near the low end of the dial and the LO coil (part of a transformer) is adjusted with a moveable slug until the station (modulated signal of the frequency on the dial injected at the antenna) is heard from the speaker. Then the tuning knob (capacitor) is turned to near the high end of the dial and the small trimmer capacitor is adjusted until the new higher frequency signal injected at the antenna is heard from the speaker. Then back to the low end and back and forth until the radio dial matches the correct frequencies of the stations. You have to go back and forth a few times because each adjustment changes the tuning you just did at the other end of the dial a small amount. The center of the dial sometimes won't be exactly on station, but that is beyond your control after getting the top and bottom ends perfect.

Remember, if you have a broadcast receiver, the signal you are using must be modulated (30 to 40%) to hear it. A sign wave without modulation won't be heard. If you are hearing your 455 KHz IF signal then you are using a modulated signal. Same goes for the signal injected at the antenna.

Norm

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Thu 16, 2021 1:13 pm 
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Quote:
You get the original two frequencies, plus the SUM of those two frequencies plus the DIFFERENCE of those two frequencies


A suggestion: replace "plus" with "and" for us picky math geeks.
Acknowleging that JT listed the frequencies for clarity

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Thu 16, 2021 6:04 pm 
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Between Jt and Norm, these are no doubt the best explanations of LO and it's behavior that I've seen. The link provided was excellent as well. I've been to multiple web sites, YouTube videos, etc., and half of them, in retrospect, appear to not even understand the essential points of the LO as well as they claim. Most end up re hashing the obvious details and missing the inner workings and specific frequency characteristics. Thanks for filling in the blanks. These 2 posts are going into my compendium of essential tube radio information. Thanks for the clarity. I'm still working on this problem. Had to switch out a IFt this morning because the slug on the primary was stripped. But I'm back to where I was, at least. The problem seems to be shared with some misalignment issues.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Fri 17, 2021 1:27 am 
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I've been to multiple web sites, YouTube videos, etc., and half of them, in retrospect, appear to not even understand the essential points of the LO as well as they claim. Most end up re hashing the obvious details and missing the inner workings and specific frequency characteristics.
Welcome to the wonderful world of the internet. You'll find a similar situation for most other topics.

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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Fri 17, 2021 7:47 am 
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KE4CDR, the first step after you get the audio working should ALWAYS be to align the IF. Otherwise, you end up chasing your tail.

John


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Tue 21, 2021 9:46 am 
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Ok, the IF chain is working adequately and I've checked the LO which of course is working as well. Switched out on IFT for a good one and ...sound. It's good. Now I have a heck of an alignment problem with the broadcast band. Seems to be shifted over into the next band so there is some wide overlap. I did an alignment but apparently tuned on an image or a harmonic. Or, this is what I assume. When aligning the broadcast band var. caps I noticed hardly any variation in the output voltage of the radio. Should have tipped me off. If I'm missing a possibility, please chime in.


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 Post subject: Re: Couple of questions re. IF transformers & Local Oscillat
PostPosted: Sep Tue 21, 2021 10:00 am 
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Oh, yes; and thanks to all for the helpful comments and explanations. These are massively important to my tube radio education journey!! Great forum!~


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