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 Post subject: Atwater Kent Tuning Accuracy
PostPosted: Jun Thu 13, 2019 11:52 am 
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Joined: Dec Sun 13, 2015 2:17 am
Posts: 142
My AK70's dial has never been right, so yesterday I decided to attempt correcting it. After making sure The set is lined up mechanically, The instructions said to find an oscillator that puts out four different frequencies with 1500 being the highest. Then, the instructions said to inject a 1500 kHz signal at the long antenna post with the tuning indicator set to 1500. i followed all of this, but my results were always the same: the dial getting progressively more inaccurate the further below 900 kHz i went. The best i could do Is get the top and bottom of the dial to be right but not the middle, which I've never had happen before. Needless to say, I have questions

Why does the alignment data want an oscillator used that has four frequencies when only one is used?

How did I get the top and bottom of the dial right but not the middle?

Was this radio's dial ever accurate?

Below is the link to the schematic information.

http://www.nostalgiaair.org/Resources/454/M0001454.htm


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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Sat 15, 2019 5:15 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1975
Location: Costa Mesa, California
Tuning a radio with a LC (inductance/capacitance) resonant circuit like the AK and all early radios, usually required varying either the inductance or capacitance. Most used a ganged air-variable capacitor. The function for resonant frequency is non-linear so that makes having a somewhat linear dial complicated. Sometimes the blades of the capacitor were shaped to achieve this, sometimes component values were adjusted and arranged to create a somewhat linear dial spread. Then the dial is marked to read accurately with all the components and wire leads in place. Future radios are produced all the same so the dial reads accurately.

Now, many, many years later, component values have drifted, wires get shifted, coils shrink and swell, so the dial markings no longer match the actual frequencies as tuned by the current set of LC conditions. Aligning antique radios is going to be a compromise, even if you try and replace components and recreate the original values and arrangement of wire leads. The problem is that you are stuck with the original dial. That can’t be remarked to match the radio. The radio must be adjusted to match the dial. Because the math function is non-linear, because you are adjusting multiple tuned circuits, that makes it a very tricky problem to get perfect. Which is why your best solution will still be a compromise.

Norm

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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Sat 15, 2019 6:36 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5815
Location: Near Brandon, Iowa
The four-frequency RF generator was specified to see if the individual sections on the tuning cap gang were unmolested. If they retained the original factory setting, each frequency should be received at the correctly-indicated point on the dial. If the tracking was bad, the individual sections had been monkeyed with (rotated relative to the other rotor sections of the gang) and the ability to achieve tracking via the compensating caps could not be assured. The set had to pass this test before there was any point in trying to adjust it. If the set passed this test, it was necessary only to peak the compensators at a single frequency (1500 KHz) to ensure correct tracking.

Each of the RF amp stages is identical in design but since each stage is a physical entity, no two stages will behave exactly alike. That's why the ganged tuning cap was made with individually-adjustable sections so that they could be tuned for maximum amplification at a given frequency.

I suspect that someone well-meaning but ill-informed had gone through and reset all of the tuning cap sections to rotate in perfect mechanical sync with each other. This basically erases the factory stage synchronization and pretty much guarantees poor dial tracking.

I'm pretty sure that someone has cooked up a "recovery method" to approximate the original factory adjustment procedure; but I've never had occasion to look for one.

You could try setting the compensator caps to minimum capacitance (i.e., screwed all the way loose- CCW), then aligning at the band midpoint frequency (1000 KHz) with the main shaft locked in place at that indicated frequency. Loosen each cap rotor element in turn (starting with the last stage) and rock for highest speaker output (RF signal generator set at 1000 kHz, tone-modulated, minimum RF output level needed to obtain a tone from the speaker). Lock the last-adjusted one in place and repeat for the next stage up the string.

At the end of this "procedure" you should wind up with a receiver that has maximum sensitivity at band midpoint. It will almost certainly "roll off" sensitivity at both band ends; but if the result is acceptable (both sensitivity- and indicated frequency-wise), I'd call it good and leave the compensators alone. Adjustment of the compensator caps at frequencies near the band endpoints can be attempted, with the caveat that each compensator can only add capacitance to the stage it controls, so its effect is to tune that stage lower in frequency as it is adjusted clockwise. You should be able to leverage this point as you develop an alignment procedure for the set. But it might be worth the effort to first see if someone else has already cooked up a procedure for the 70.


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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Sat 15, 2019 10:42 pm 
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Joined: Dec Sun 13, 2015 2:17 am
Posts: 142
Thanks a lot for taking the time to answer my questions this deeply. A lot of the mystery has been taken out of the issue. I was able to get the set to where it's dial is accurate at all points from 610 to 1490. The only number not right is 710. AM 710 out of Kansas City comes in at 700 on the Ak's dial. That still seems odd to me considering all the others are right.


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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Sun 16, 2019 2:15 am 
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Location: Maryland 20709, USA
Have you read the Service Notes for the 70 ?

A particular arrangement of grid leads is required to obtain proper tracking.

- Leigh

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http://www.AtwaterKent.info
Click "Grebe Stuff" for Synchrophase info


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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Sun 16, 2019 1:14 pm 
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Joined: Dec Sun 13, 2015 2:17 am
Posts: 142
I'm glad you mentioned this because I forgot about another observation i had. I did read the service notes about the grid leads having to be exactly parallel. Mine are, but just to see what would happen, I moved them around with no change to tracking. How does moving the leads around not change the tracking?


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 Post subject: Re: Atwater Kent Tuning Accuracy
PostPosted: Jun Tue 18, 2019 4:32 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5815
Location: Near Brandon, Iowa
CR220 wrote:
Thanks a lot for taking the time to answer my questions this deeply. A lot of the mystery has been taken out of the issue. I was able to get the set to where it's dial is accurate at all points from 610 to 1490. The only number not right is 710. AM 710 out of Kansas City comes in at 700 on the Ak's dial. That still seems odd to me considering all the others are right.

That IS odd. I would expect the radio's response to be relatively linear over the included frequencies if the indicated frequencies near the band ends match the actual ones. Maybe one or more of the RF amp stages have unusually high gain at a certain frequency.. this could probably be adjusted out if you have the patience to fiddle with it.

If you have an oscilloscope and RF signal generator, you could profile the gain response of each stage as you swept through the entire frequency range it is capable of tuning. Knowing where gain peaks and dips exist for each stage should allow you to set the tuning cap segments' relative postions for uniform response over the whole tuning range. Unfortunately, this may come at the cost of overall sensitivity. It looks to me like the tradeoff is having a radio that accurately indicates its tuned frequency, OR having a radio that is very sensitive over just a portion of its tuning range. The factory settings were probably the best compromise that could be obtained.


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