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 Post subject: Signal generator
PostPosted: Oct Fri 23, 2020 4:51 am 
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Location: St. Louis, MO
I just picked up a signal generator. Not sure how to use it yet. Do I need an oscilloscope to pair with this, or can I still align without one? Sorry if that seems basic. I’m trying to learn how to align a radio now.


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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 5:26 am 
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Location: Austin, Texas
You don't need an oscilloscope yet. You do need a voltmeter. An analog VTVM (vacuum tube volt meter) works the best for radio alignment. You will be looking for maximum or minimum readings as you make adjustments and that is much easier to do with a meter pointer than trying to keep up with a digital value.

A frequency counter is another good item to have since the dial on a signal generator is often not as accurate as we would like. With a frequency counter you can set the signal generator to exactly the frequency you need.

What make and model signal generator did you get? Some are accurate enough that you don't need a frequency counter.

Jay


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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 5:56 am 
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"Elements of Radio Servicing," chapters 5, 6, 7.

https://antiqueradios.com/archive.shtml


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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 10:17 am 
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There are a number of different types of signal generators out there. Some are good for radio alignment and others have different purposes. If you tell us what generator it is, we’d be in a better position to tell you if it is suitable first, and then give some pointers on checking it out and using it.

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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 3:49 pm 
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Also tell us if it's an AM or FM radio you want to align.

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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 5:17 pm 
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Joined: Feb Mon 17, 2020 5:51 am
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Location: St. Louis, MO
Here is the signal generator i bought. Seems capable and recommended as a good model from the seller who restores radios. GW Model: GRG-450B

I would like to practice on a Zenith 6D629 AM only radio since the alignment instructions seem easier than the others that i have. Eventually I would like to align a RCA Victor 3-RF-91 that i recapped. The FM plays well, but the AM plays at a really low volume. The RCA’s alignment instructions are a little more advanced.

I tired to YouTube some people doing alignments, but all of them had scopes with a lot of focus on the waves. Made me think I needed one since I couldn’t find any instructions on using a signal generator as a stand alone.


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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 9:41 pm 
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Dejablue wrote:
Here is the signal generator i bought. Seems capable and recommended as a good model from the seller who restores radios. GW Model: GRG-450B

I would like to practice on a Zenith 6D629 AM only radio since the alignment instructions seem easier than the others that i have. Eventually I would like to align a RCA Victor 3-RF-91 that i recapped. The FM plays well, but the AM plays at a really low volume. The RCA’s alignment instructions are a little more advanced.

I tired to YouTube some people doing alignments, but all of them had scopes with a lot of focus on the waves. Made me think I needed one since I couldn’t find any instructions on using a signal generator as a stand alone.


theres many many videos on youtube that are helpful. I watch many flavors, as there are many way to do something.

This guy is smart though uses many unorthodox approach, many simplified methods to fix radios & tvs... I enjoy him, not so much him turning the radios to loud when listening or adjusting, but past that you can learn a lot. https://www.youtube.com/channel/UCw2Sbt ... QTbIRI6cqA enjoy watching him get elc shocks ..lol

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Last edited by ttx450cap on Oct Fri 23, 2020 11:05 pm, edited 1 time in total.

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 Post subject: Re: Signal generator
PostPosted: Oct Fri 23, 2020 10:26 pm 
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Location: Long Island NY
OK, so now we know what we're talking about. This is a great little generator to use for aligning AM radios, shortwave radios, and the simpler FM radios. Have you tried it out to see if it works yet? With a short cable connected to the output, laid near a working radio, you should be able to tune the signal generator in if you set it and the radio to the same frequency. With the generator modulation on, you should hear a tone when the radio picks it up. If the modulation is off, you should hear almost perfect quiet as the unmodulated carrier from the generator will pull the AVC down in the radio when they are on the same frequency and make the background noise and hiss disappear.

The other things you will need are a test cable to get the output into the radio, and an output indicator of some kind. The best indicators to use are analog meters. A VTVM can be used a couple of ways; a VOM or AC voltmeter can be connected across the speaker. If you only have a digital multimeter you can use that, but it's not as easy to see when you've peaked the IF transformers up as it is with an analog meter. An oscilloscope can be used as an indicator if you are making u-tube videos and you want them to look impressive but it won't work any better than an analog meter.

The references outlined above will take you through the details, but basically you start out by injecting the IF frequency (eg. 455 kHz) into the converter stage of the radio with the modulation on. The output indicator goes across the speaker terminals. If you have a VTVM you can connect it across the AVC bus in the radio. The output level of the generator is set to the minimum level that produces a usable indication on the meter. You then adjust the IF transformers for maximum output, reducing the signal generator level as you go along. If you connect the meter to the speaker voice coil terminals you have to turn the volume control all the way up, otherwise the AVC circuit in the radio will try to keep the output level where it is set and you won't see as much change when you tune the IFs. If a VTVM is connected across the AVC bus the volume control setting becomes irrelevant since you're using the AVC voltage itself to measure the signal levels.

Once the IFs are peaked, next step is to disconnect the signal generator from the converter stage and loose couple it to the antenna by laying the output cable nearby. Set the generator to 1600 kHz or as near as possible without interference from stations, and set the radio to the same frequency. Then peak the oscillator trimmer for maximum output on the indicator. Finally, I prefer to turn the signal generator off and tune a station in near 1400 kHz, then peak the antenna trimmer for maximum volume. The actual alignment instructions are here http://www.nostalgiaair.org/pagesbymodel/111/M0025111.pdf and you'll see they are pretty close to what has been said already.

Four things to be aware of. First, when you connect the signal generator to pin 6 of the 14Q7 (input grid), you have to put a blocking capacitor in series with the generator. The blocking cap can be anything from 0.0001 uF to 0.001 uF. It keeps the generator from swamping the DC bias voltage on the grid. Second, when aligning the IFs, always go back and forth between the adjustments. Many of them interact! Pay attention to how much you are turning the trimmers or slugs. Things should peak up within a quarter to half a turn if everything is well. If you find yourself turning the adjustments round and round, something is wrong and you need to figure out what is going on before proceeding. Third, this radio is an AC/DC type with a "floating" chassis. It is possible to work on this kind of set without an isolation transformer, but it would be safer for your signal generator, the radio, and you if you did use one. Especially since your generator has a standard IEC power inlet with three-wire (grounding) cord. Finally, alignment should be done only after you have completed any repairs, capacitor replacements, or tube replacements you intend to do. Alignment should be considered a last step to fine-tune a radio that already works and all repairs have been completed. If you then go back and change tubes or other parts the alignment may change and need to be touched-up again.

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"Hell, there are no rules here--we're trying to accomplish something!"

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 Post subject: Re: Signal generator
PostPosted: Oct Sat 24, 2020 1:35 am 
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Location: Dallas, TX
I have basically the same generator made by B & K. It has an output you can attach a frequency counter to that has a more constant and higher output level than the normal output. The signal level needed for alignment is often too low for a frequency counter to work well.
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 Post subject: Re: Signal generator
PostPosted: Oct Sat 24, 2020 4:14 am 
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Joined: Feb Mon 17, 2020 5:51 am
Posts: 122
Location: St. Louis, MO
These comments are excellent! Thank you. I’m going to dive into this using these references and instructions. Thank you all!


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 Post subject: Re: Signal generator
PostPosted: Oct Sat 31, 2020 10:49 pm 
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Dejablue wrote:
I just picked up a signal generator. Not sure how to use it yet. Do I need an oscilloscope to pair with this, or can I still align without one? Sorry if that seems basic. I’m trying to learn how to align a radio now.


this is another guy I could not think of his name. He's good and good teacher. Has some frugal repair videos that starts out with basics/and basic eq.
https://www.youtube.com/channel/UCUEo3L ... g/featured

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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 1:13 am 
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Joined: Feb Mon 17, 2020 5:51 am
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Location: St. Louis, MO
ttx450cap wrote:
Dejablue wrote:
I just picked up a signal generator. Not sure how to use it yet. Do I need an oscilloscope to pair with this, or can I still align without one? Sorry if that seems basic. I’m trying to learn how to align a radio now.


this is another guy I could not think of his name. He's good and good teacher. Has some frugal repair videos that starts out with basics/and basic eq.
https://www.youtube.com/channel/UCUEo3L ... g/featured



Thank you!!


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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 2:46 am 
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Dejablue wrote:
ttx450cap wrote:
Dejablue wrote:
I just picked up a signal generator. Not sure how to use it yet. Do I need an oscilloscope to pair with this, or can I still align without one? Sorry if that seems basic. I’m trying to learn how to align a radio now.


this is another guy I could not think of his name. He's good and good teacher. Has some frugal repair videos that starts out with basics/and basic eq.
https://www.youtube.com/channel/UCUEo3L ... g/featured



Thank you!!


I see he has a interests in astronomy , I use to be when young.. hobby I cannot even think about now.. very cool.

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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 10:33 am 
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Do beware with that combination. It looks like it was built for a counter, however do check with the Modulation / Tone on and off. If it actually outputs modulated signal from that dedicated plug: Many counters don't like it and will give a false reading.

Marc


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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 4:59 pm 
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Location: Dallas, TX
Marcc wrote:
Do beware with that combination. It looks like it was built for a counter, however do check with the Modulation / Tone on and off. If it actually outputs modulated signal from that dedicated plug: Many counters don't like it and will give a false reading.

Marc

Sure, you have to turn the modulation off when checking the frequency. The counter output is simply the signal before it goes through the attenuator.
EDIT: I stand corrected. I just checked. Al least on mine, the counter output is NOT modulated when the normal output is. I should never post before I have my coffee.

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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 6:12 pm 
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Joined: Feb Mon 17, 2020 5:51 am
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Location: St. Louis, MO
Thanks Marc and Notime!


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 Post subject: Re: Signal generator
PostPosted: Nov Mon 02, 2020 11:34 pm 
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With the Generators at entry level and there are 3 here salvaged (2workers one needs a transformer) there is no provision made for a counter or much else: Extremely basic.

Silicon Chip Magazine Australia has a project one which is more like a comms receiver where the readout on the display is correct.

Always with these things it is better to be wise before the event. With time & the nature of the test equipment it can drift and have no compensation. Which is why I use a counter. That said with all equipment, it does pay to check it occasionally, things can go wrong.

I have found it an advantage to use the audio on my cheapy, as the sync signal for the oscilloscope. Much more stable oscillograph as if the audio drifts the oscilloscope will compensate.

+1 on Notime with signal strength for the counter: True

Marc


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 Post subject: Re: Signal generator
PostPosted: Nov Wed 04, 2020 10:30 pm 
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That should be a great simple SG for what you want to do. I really like the "freq monitor" output. If it works as described by another poster, you can set and monitor the frequency accuracy regardless of the RF output level or whether the RF is modulated or not. You should pick up a frequency counter that corresponds to the frequency range of the radios you plan to work on. The dial needle will not be as accurate as you will probably wish and the freq counter will allow you to dial it in exactly. While you could look at the wave forms (modulated and unmodulated) with an o-scope it isn't necessary for what you want to do. I second the VTVM. You will place the leads across the speaker coil and monitor the VTVM needle as you "tweak" the IF transformers according to the alignment instructions. You will simply set the IF transformers to the peak needle reading. The value is not important. You will then have a basic consistent set of test equipment for AM and simple FM radios. You can add as you learn and want more sophisticated items. I do enjoy tinkering with the oscilloscope and if you get into restoring test equipment you will need one. Have fun, Doc


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 Post subject: Re: Signal generator
PostPosted: Nov Wed 04, 2020 10:48 pm 
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In Australia the "Astor" branded radios in particular had a dedicated plug (fitted with appropriated blocking caps) for IF alignment with a meter.

As on several occasions, do check the counter by heterodyning against a known precise frequency eg. WWVH. Quality Communications receivers with digital readout, can be handy for this.

Marc


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 Post subject: Re: Signal generator
PostPosted: Nov Fri 06, 2020 1:19 am 
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Location: St. Louis, MO
Dejablue wrote:
I just picked up a signal generator. Not sure how to use it yet. Do I need an oscilloscope to pair with this, or can I still align without one? Sorry if that seems basic. I’m trying to learn how to align a radio now.


What's up, St. Louis buddy!

There isn't much to a signal generator (SG). Just think of a signal generator as an extremely low power AM radio station transmitter that at most can only broadcast a few feet and again at most only produce one sound without help.

I'll try to keep it simple, but you will need to understand at least one wave: a sine wave. Just think of the shape. If a radio, AM or FM, picks up that signal that's shaped like a pure sine wave, then that radio station will be dead silent. Why? Because it doesn't change. You gotta change the wave somehow to make it sound different. That base wave is called the "carrier wave" and changing it is called "modulation". In the case of AM radio, that's Amplitude Modulation, where amplitude is how tall the wave is.

As is, your signal generator will only put out that unchanging wave at the frequency on its dial. With that, you can build a little antenna for your SG and have a nearby AM radio pick up the station you created. The problem is that it will be quiet. You can tell that the radio is receiving the station, but how do you know that it's receiving it well? To do that, you'll need to add some sound to that station. We'll need to modulate it. SGs have some built in. Remember how I said that at most they'll only produce one sound? That's the internal modulation. For most SGs, it's 400 Hz. If your radio is working right, then you'll hear that steady tone.

But what if you want to hear different sounds? It's possible to hook up an external modulation source and have your SG send out that sound instead.

RF level is important. You don't want to broadcast a signal too strong right next to a radio. You could throw off your alignment by doing this. Use that control to turn down your SG's output power as necessary. Frequency monitor is nice too. You can hook a frequency counter up to that and make sure that the dial needle is accurate. In other words, if the SG's dial says that it's on 455 KHz, then the frequency counter should say "455 KHz".

Here's the manual if you don't have it yet:
https://www.artisantg.com/info/instek_g ... manual.pdf


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