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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 3:44 am 
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Location: Lexington, KY USA
You might try jumpering the AVC line to ground, rather than connecting a battery that provides a negative voltage to the line.

In truth, with the terrible RF noise we have now, the full gain of many sets is useless, so perhaps optimizing the alignment closer to a normal level of gain is not so bad.

Radios with different AVC circuits will respond differently. Interesting area of experimentation.

Do watch out for a limited number of in and out turns built into some of the adjustments. You don't want to wear the sucker out playing with it.

Ted


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 12:14 pm 
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There's a neat trick you can do to align radios using the AVC bus that is simple to do and will often improve the fidelity of a radio after alignment. I thank my old friend Charles Blanding (now SK) for this one.

First, get the radio into a reasonable state of alignment using the traditional signal generator and meter method. Doesn't matter if you put the meter on the AVC bus or the audio output.

Next, connect a DC coupled oscilloscope across the AVC bus. Feed modulated RF into the first IF or converter stage as you would in any case, and take care to use a low level signal that produces perhaps half a volt or so of AVC voltage (the exact voltage is not critical). You can use a VTVM if you don't have a scope but it's easier to see what you are looking for on a CRT.

Then, rock the IF slugs or trimmers gently back and forth through the resonant point. What you will see is a dip as the AVC voltage goes more negative at resonance. You want to find the place where maximum dip occurs and leave each trimmer at that point.

What you will find is, because this is a "dynamic" method, the trimmers or slugs will end up in slightly different places than they did when you did the "traditional" alignment. With the IF bandwidths lined up rather than their peaks, better fidelity usually results.

Quote:
Alignment should to be performed when the circuits are at maximum gain. Therefore I do not use a battery which will set the gain at less than maximum. Then I keep the signal generator low enough so that only a slight deflection of the meter is observed as each stage comes into alignment.

I also use the audio meter method but the procedure of keeping the signal generator output and resulting AVC voltage at a minimum is the same.


One problem with connecting a battery across the AVC bus is the Miller capacitance of the tubes. This is the intrinsic grid-plate capacitance multiplied by the gain. By holding gain fixed with a battery, you get a perfect alignment at one AVC voltage, but you may not be on peak at any other signal level because the Miller-C changes. To me it is preferable to align at maximum gain (i.e. no battery and the lowest signal input level that can be detected). If there's enough signal to engage the AVC and lower the gain, slight misalignment is not going to be noticeable.

It is to be noted that there were some elaborate receivers made with complex amplified AVC loops and AFC in some cases, where the temporary connection of a battery across the AVC bus is called out in the alignment procedure. Obviously if it is part of the procedure, follow it. They've already figured out the impact of the battery and have you adjust for it accordingly.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 3:54 pm 
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I vote for the DC scope on the AVC bus... There is NO friction or hysteresis in a scope tube and you see rather than listen to the peak... works like a champ!!!

Jim

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 04, 2018 4:56 pm 
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Chris108 wrote:
To me it is preferable to align at maximum gain (i.e. no battery and the lowest signal input level that can be detected).
That's the reason for the battery.

It locks the receiver gain at (quite close to) maximum.

You should never run a tube at zero bias because the grid might draw current. Using the battery guarantees that this will not happen, without significantly reducing system gain.

- Leigh

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 1:04 am 
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Hello Guys,
well this is a old post but it has a lot good info.

Sincerely Richard


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 2:20 am 
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I've found that with any IF alignment method, I like to listen AND have my scope on the audio output so I can see a couple of cycles of the modulating tone and make sure it's symmetrical.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 4:50 am 
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Hello jtiner,
For sure I don't normally do that for Am But FM or when i used to repair VCRS and Beta machines .

Sincerely Richard


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 2:36 pm 
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Yes an old post but a good one.

There are a few problems with the idea of using the AVC voltage as an indicator of signal level for an alignment.

The first thing you need to consider with an alignment, is : Under what conditions do you want the radios alignment to be optimal ?

The answer is obvious. In the final analysis it is when a distant station can just barely be received, out of the background noise.

Under this condition, there is generally very little activity at all at the detector as it is just barely over the threshold of conducting on the signal comping out of the last IF transformer. Also, not so much the AA5, but other radios, like those made by Philco, for one example the 1939 model 38-7, had a delayed AGC, in any case the AGC is inactive at this low level.

So the AGC voltage itself is not the ideal electrical potential to monitor the alignment. Yet having said that , nor in fact is the output of the detector either, which makes a mockery of the notion of performing the alignment with an audio tone modulated carrier, which is the customary method. But that is better than the AGC line.

By far and away the best place to monitor the signal level is at the output of the IF transformer prior to the detector diode and look at the IF frequency waveform on the scope. But, the thing is, you cannot connect the scope probe directly there , even a x10 probe, because it de-tunes the IF transformer, due to its capacitance. But, you can do the "trick" that is not in the books. If you clip the scope probe onto the plastic insulation of the wire leading out of the final IF transformer to the detector, it forms a fraction of a pF gimmick capacitor (less than one pF). This doesn't significantly alter the IF tuning. Then you wind up the gain on the scope to see the IF carrier frequency, and watch that while you do the alignment. It is much better than using a modulated carrier and it can be done with a very low level input signal that is barely out of the noise and the tuning peaks are very easy to resolve.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 3:15 pm 
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Then there is G. N. Goldberger's philosophy, which argues for alignment with the AVC voltage clamped to roughly the average AVC voltage of the local stations to which the owner listens. Goldberger was Chief Engineer of Precision Apparatus Company, and elucidated his ideas in his booklet Servicing by Signal Substitution and incorporated them in the AVC substitution feature of the Precision E-200 series of signal generators.

His argument is that if the radio is aligned for DX reception of signals below the AVC threshold, it will perform less than optimally when local reception is the norm. The shift in alignment resulting from the higher AVC voltage could introduce more distortion due to unequal sideband amplitude into the detector, more interference from the sidebands of adjacent stations, and relatively more noise from RFI sources (the station carrier could be reduced in amplitude since it is not at the peak of the IF selectivity curve).

Here's a link to a scan of Servicing by Signal Substitution:
https://www.byan-roper.org/steve/manual ... tution.pdf
His AVC Substitution Method is thoroughly described on pages 30 through 42.

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Last edited by stevebyan on Sep Tue 21, 2021 5:47 pm, edited 1 time in total.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 5:14 pm 
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Joined: Apr Fri 20, 2018 6:55 am
Posts: 173
Location: San Rafael, CA. 94903
Thanks Steve, This will be a fun (and educational) read.
Tried downloading the PDF with no luck. "A drawing error occurred". Seems to be uncorrectable.
Read most of it on your site, but wanted to study it more. Dang!

73,
-marc


Last edited by mbruvry on Sep Tue 21, 2021 10:43 pm, edited 1 time in total.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 6:18 pm 
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In most of my comment on aligning radios, I have pointed out that the AGC is not your friend. Too much signal into the set will see AGC operate and you will never get alignment right. This is the idea behind nobbling it.

It is also worthy of noting as to how its applied. If the set has SW, it is quite possible that AGC is only applied to the Pentagrid on BC band and no other tubes. AGC de-stabilises all but 6SA7 in octals on SW. In that scenario I calibrate on SW.

We all have slightly different ways of going about things. I use an Oscilloscope as the meter, as it shows me the actual signal shape, revealing distortion & coupling issues. I have meters with a dedicated input socket for calibrating by voltage. That voltage method may be helpful here?

Marcc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Tue 21, 2021 9:58 pm 
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stevebyan wrote:
Then there is G. N. Goldberger's philosophy, which argues for alignment with the AVC voltage clamped to roughly the average AVC voltage of the local stations to which the owner listens. Goldberger was Chief Engineer of Precision Apparatus Company, and elucidated his ideas in his booklet Servicing by Signal Substitution and incorporated them in the AVC substitution feature of the Precision E-200 series of signal generators.

His argument is that if the radio is aligned for DX reception of signals below the AVC threshold, it will perform less than optimally when local reception is the norm. The shift in alignment resulting from the higher AVC voltage could introduce more distortion due to unequal sideband amplitude into the detector, more interference from the sidebands of adjacent stations, and relatively more noise from RFI sources (the station carrier could be reduced in amplitude since it is not at the peak of the IF selectivity curve).

Here's a link to a scan of Servicing by Signal Substitution:
https://www.byan-roper.org/steve/manual ... tution.pdf
His AVC Substitution Method is thoroughly described on pages 30 through 42.


I have heard that argument before, but going against it, is that as the signal level increases and if it does cause de-tuning with AGC action , de-tuning is better tolerated and much less noticeable from the radio user's perspective, than when the signal is weak and the tuning critical. Of course if all you have in your area are strong local channels that you want to hear. I might agree more.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 4:40 am 
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Do note on superhets. there can be more than one, different alignment. There is mechanical for the dial, the IF, then the antenna & oscillator, and often if there is a preceding RF stage it and the mixer have to be on the same frequency. Direct coupling (No external antenna) should be done with a blocking cap in series with the lead, to avoid wiping out any tube bias.

Volume high helped keep signal low and provide voltage for the meter if you went that way & took signal of the Output tube plate via a 0.1mfd blocking cap meter on AC.

Marcc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 5:12 am 
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Hello Acornvalve ,
Yes an old post but a good one. for sure ! there is a lot great info here and like the rest of you I use to do a
lot more alignments then I have lately but winter is coming I plan to get down and dirty and catch up on some my radio restorations .

Sincerely Richard


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 7:03 am 
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Just looking back on this. If you are calibrating the Signal Generator with a Frequency Counter. My Fluke and another will not calibrate correctly if the units are fed modulated signal. Therefore you calibrate no tone / modulation. I will on my setup, leave the counter connected and unit connected to the set to avoid capacitive & loading effects dragging it off frequency.

Marc


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 5:13 pm 
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Hello marc,
I do just that I leave my fluke bench freq counter connected to the signal generator.

Sincerely Richard


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 6:34 pm 
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Quote:
There are a few problems with the idea of using the AVC voltage as an indicator of signal level for an alignment.

The first thing you need to consider with an alignment, is : Under what conditions do you want the radios alignment to be optimal ?

The answer is obvious. In the final analysis it is when a distant station can just barely be received, out of the background noise.


The problem is, there is no "one size fits all" answer here. For one thing, you have to consider the kind of IF transformers in the radio. Prior to about 1938, every radio--at least in the US--had air core IFs with trimmer capacitors to resonate the primary and secondary windings. Starting in the late '30s, iron dust cores with much higher Q's were introduced on some radios. In some of these the cores were movable; in others the trimmer capacitors remained. Later on in the post war era, they went to ferrite slug IFs, with the slugs both internal and external to the coils, and the trimmer capacitors disappeared.

Now with the old air core IFs, it was pretty hard to misalign a radio unless you really didn't know how. The trimmer capacitors had no practical impact on the the bandwidth of the IF system, and Q's were pretty low, so it didn't really matter how you did the alignment. As long as all the IFs ended up peaked on the same frequency the radio would be as sensitive and have as much fidelity as it was ever going to. This is where the cheat of "aligning by ear" was in its heyday.

With powdered iron cores the Q's were considerably higher, so one had to take better care to line the transformers up properly, otherwise sideband clipping (= distortion) would result. Also, since the Q's are so much greater it is necessary to take Miller capacitance in the tubes into account, since that can change the alignment between no-signal and full signal conditions. As mentioned in the Precision manual mentioned and elsewhere, the idea is to align at a level that produces a nominal AVC voltage, or apply one from an external bias supply, so the radio will be aligned for normal use. One could also align for DXing or for strong local stations if that was preferred.

That changed with iron dust and ferrite IFs with movable slugs. When you move the slugs, it changes not only the resonant frequency of the coils, but also the coupling between them and therefore the IF bandwidth. If you peak up for maximum signal only, you may end up with a lot of distortion as the sidebands in the signal get clipped. The whole point of the method I outlined, where the adjustments are rocked to produce the best dip in the AVC voltage is about avoiding sideband clipping, since the lowest part of the dip is where you are on frequency in the middle of the passband. Poor man's sweep generator basically.

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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 10:01 pm 
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Chris108 wrote:

The problem is, there is no "one size fits all" answer here. For one thing, you have to consider the kind of IF transformers in the radio. Prior to about 1938, every radio--at least in the US--had air core IFs with trimmer capacitors to resonate the primary and secondary windings. Starting in the late '30s, iron dust cores with much higher Q's were introduced on some radios. In some of these the cores were movable; in others the trimmer capacitors remained. Later on in the post war era, they went to ferrite slug IFs, with the slugs both internal and external to the coils, and the trimmer capacitors disappeared.

Now with the old air core IFs, it was pretty hard to misalign a radio unless you really didn't know how. The trimmer capacitors had no practical impact on the the bandwidth of the IF system, and Q's were pretty low, so it didn't really matter how you did the alignment. As long as all the IFs ended up peaked on the same frequency the radio would be as sensitive and have as much fidelity as it was ever going to. This is where the cheat of "aligning by ear" was in its heyday.

With powdered iron cores the Q's were considerably higher, so one had to take better care to line the transformers up properly, otherwise sideband clipping (= distortion) would result. Also, since the Q's are so much greater it is necessary to take Miller capacitance in the tubes into account, since that can change the alignment between no-signal and full signal conditions. As mentioned in the Precision manual mentioned and elsewhere, the idea is to align at a level that produces a nominal AVC voltage, or apply one from an external bias supply, so the radio will be aligned for normal use. One could also align for DXing or for strong local stations if that was preferred.

That changed with iron dust and ferrite IFs with movable slugs. When you move the slugs, it changes not only the resonant frequency of the coils, but also the coupling between them and therefore the IF bandwidth. If you peak up for maximum signal only, you may end up with a lot of distortion as the sidebands in the signal get clipped. The whole point of the method I outlined, where the adjustments are rocked to produce the best dip in the AVC voltage is about avoiding sideband clipping, since the lowest part of the dip is where you are on frequency in the middle of the passband. Poor man's sweep generator basically.




Most IF transformers (double tuned types typical for tube radios)are constructed by the manufacturer so that the bandwidth or bandpass characteristic is determined by the spacing of the coils, in conjunction with the resonance profile of each coil to attain the correct mutual coupling.Generally they selected close to critical coupling. It does not matter whether they are air cored, peaked by dust iron cores or by making the capacitors variable, when each coil is peaked, you are stuck with the bandpass characteristic that the transformer manufacturer created. You can broaden it with parallel resistance but you cannot make it more narrow, unless you add a Q multiplier.

There is just the "oddball case" in that if you have a permeability tuned type, for each coil on the same coil form, you have have two physical positions of the slug to get each coil to the correct frequency, and generally when that happens the slug can be positioned to be projecting out either side of each coil. This gives three combinations which have three different mutual inductances between the two coils, when both are peaked. Generally it states in the alignment instructions that the first peak as the slug is moving into the former from each end is the correct one. But one or both of the slugs can be tuned in further. What happens then is that the gain can increase and the bandwidth narrow or you can get a dip in the center of the bandpass response.

There are few tube radios that allow this possibility but there are some, as many of the IF's for early tube radio use that did use dust iron cores retained the adjustments as capacitors, Philips made some but they cleverly avoided this trouble and ambiguity by having the adjustable slugs beside each other rather than pointing towards each other in the one former. So the slug positions do not alter the mutual coupling. Of course in "most" transistor radios the problem was avoided by using a single tuning slug.

I only have one radio in my entire collection where the IF coil slugs project toward each other in the one form and can have the outcome of IF tuning slugs being in the 3 different relative positions of mutual coupling, and this is the Eddystone EC-10. It is more unusual to see double permeability tuned IF in transistor radios, i have another the Normende Clipper, but again they solved the issue by having the slugs side by side.

It is more important to sweep the IF than most people would think as it can sometimes show up unexpected anomalies in the response that you might not pick up easily otherwise.


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Wed 22, 2021 11:55 pm 
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There are variables built into the IF transformers that we have little control over. I would note that even in early times, there were Oscillographs on the data sheet showing what the signal should and should not look like. That also applies to "Riders" book on the application of Oscilloscope's and likely many others.

Cathode-Ray Tube At Work
John . F. Rider (1935)


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 Post subject: Re: using the AVC to align an AA5 - first time questions
PostPosted: Sep Thu 23, 2021 5:39 am 
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Marcc wrote:
Don't forget there is a commonly used AGC, & then there is "delayed" AGC & these are often sourced differently.
Marc

Marc, I don't think that delayed AGC is a factor in an AA%.

John


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