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 Post subject: Unreliable Output - Field Strength Meter
PostPosted: Dec Mon 23, 2019 6:05 am 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
I figured out how to get my field strength circuit working (https://www.airplanesandrockets.com/ele ... 54-man.htm) but I am dealing with very WEAK transmitted signals. As such, I added an amplifier stage to the original circuit.

The new diagram looks like this:

Attachment:
IMG_6082.JPG
IMG_6082.JPG [ 101.34 KiB | Viewed 535 times ]


I have a very odd issue, where the set occasionally works extremely well. It will show a 1-1.5 mA dip on the meter, and clear audible signal in the headphones. Then, for no reason, it becomes very quiet, showing dips of no more than a tenth of a mA, for the same signal. The set is sitting perfectly still- No jostling or movement. The power supply for 45 V is steady, and it's not an issue with the D cell powering the filaments.

The set amplifies a sound signal extremely well, never showing this issue. To add a sound signal, I place my input on the intersection of the 330 pF grid capacitor and the 1M grid bias resistor on grid 1 of the first tube, and ground the other end of the audio signal. The meter will show dips up to 2.5 mA. Touching the intersection with a finger produces a very loud hum, and the meter drops to zero (as expected).

Because of this, I am guessing my issue is related to the LC combination and/or the 330 pF capacitor. None of these components are faulty, and I can't figure out why in the world the signal would occasionally be excellent then suddenly, and for no apparent reason, drop so drastically. The set normally operates on that subpar level.


A little backstory: I am trying to get relative readings of transmissions on frequencies from 800 MHz to 5 GHz (typical cell phone and wifi technology). The coil would be tuned for way down at 30 MHz, but I am hoping that it instead acts as a choke for higher frequencies while letting most RF pass through to ground. Without the variable capacitor, I can hear the signals, but with the same problem described above. Adding the capacitor and cranking it to highest capacitance gave a slightly stronger signal reading in my test set. Once assembled on the board, however, it seems to make no difference (and I still hear the transmissions the same).


Here are images of the set as it is right now. The brass strip is the common ground, which the voltage supply negatives are connected to, as well as the bottom of the LC circuit.

Attachment:
IMG_6078.JPG
IMG_6078.JPG [ 276.54 KiB | Viewed 535 times ]


As you can see here (with no signal) the 5000 ohm resistor almost perfectly centers the I-70 5 mA meter. The I-70 is a typical ammeter, with the distinction of no values being printed on the dial. It also is at zero to the right, not the left as most meters are. This way, when you are tuned to maximum signal, the needle is furthest to the right ("highest" signal reading, but lowest current).
Attachment:
IMG_6077.JPG
IMG_6077.JPG [ 305.93 KiB | Viewed 535 times ]

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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Mon 23, 2019 6:39 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10213
Location: Santa Rosa, CA
Quote:
I haven't gotten that far yet- I can't get the meter to show any output, even when I add a strong signal to the grid.


If your input circuit is tuned to 27 MHz, you won't get much result at 1000 KHz.

Rich


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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Mon 23, 2019 6:43 pm 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
Rich, W3HWJ wrote:
Quote:
I haven't gotten that far yet- I can't get the meter to show any output, even when I add a strong signal to the grid.


If your input circuit is tuned to 27 MHz, you won't get much result at 1000 KHz.

Rich


Hi Rich,

That whole issue was resolved by having a higher plate voltage. It's also important to note that my test signal is past the tuned circuit, added directly to grid 1, and not on an RF frequency.

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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Mon 23, 2019 8:48 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 23682
Location: Somers, CT
I am not sure how that last stage works as a detector and meter driver. One would expect
to see a RF amplifier, followed by a detector to demodulate the audio and drive the DC meter.

Odd circuit.

Pete

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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Mon 23, 2019 9:21 pm 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
Peter Bertini wrote:
I am not sure how that last stage works as a detector and meter driver. One would expect
to see a RF amplifier, followed by a detector to demodulate the audio and drive the DC meter.

Odd circuit.

Pete


My hope is that the first tube still acts as the detector. The second tube just amplifies the output signal for a louder volume in the headphones, and stronger meter dips.

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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Tue 24, 2019 12:31 am 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
Interesting update:

I can remove the LC circuit and still have the same reception. I replace it with a straight wire to ground- The antenna is then connected to ground. What in the world??

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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Tue 24, 2019 7:03 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 10213
Location: Santa Rosa, CA
Not surprising.

L/C wasn't tuning anything of interest in the first place.

Rich


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 Post subject: Re: Unreliable Output - Field Strength Meter
PostPosted: Dec Tue 24, 2019 11:09 pm 
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Joined: Feb Sat 16, 2013 5:08 pm
Posts: 957
Location: Ft. Collins, CO.
My reliability issue was due to the first tube. Something was wrong with it internally. I replaced it and now have no issues.

Perhaps someone can explain this to me, though... If I stretch the antenna a certain direction, I pick up an FM station! 96 MHz. I'm not too surprised to hear a signal at that frequency, but I am surprised it's an FM broadcast station.

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