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Help With 12AU7 40 Meter Mobile Converter

Aug 5th 2016, 17:29

NE6F

Joined: Jul 11th 2016, 18:01
Total Topics: 0
Total Posts: 0
I was about 14 or 15 years old when I constructed a single tube, 12 volt, 40 meter mobile converter connecting up to my parents 1964 AM car radio.

I remember how surprised I was when I first tuned across the AM radio dial hearing distant 40 meter signals. What I don’t remember is how I hooked up the converter to the car antenna and the car radio. I do remember unplugging the car antenna from under the dash and grabbing 12 volts from the fuse box.

Would any of you mind taking a look at the attached diagram that I have redrawn using my 50 year old hand sketched one?

Also, I think it came out of the 1950’s editions of either QST, CQ, or maybe 73. I really want to say it was from QST, but not quite sure.

I connected it up to 12 DC and was able to peek the output using a grid dip meter for the 8120 Khz oscillator stage.

The following stage has two tuned circuits that I was able to dip at what is indicated in the diagram.

Would anyone know how the converter works? Especially how it connects to an outside antenna and then to the car radio antenna input.

Also noted on my original diagram I had written 8120 – 7250 = 670. Is 670 Khz, on the AM broadcast band, the equivalent of 7250 KHz when the converter is working?

Thank you in advance for any help you may be able to provide.

73’s
Richard Schneider
NE6F (Previously WB6KUH)

https://photos.google.com/share/AF1QipPbzres3RDdf1e2dQj2X6PxF69og91qhzOGlsjvKfB1oTFPeaZBBc0wZ6fTfFSOyQ/photo/AF1QipMEdvRjMSqufwp-FwpdMwLSvlwzOZ0aPjZWdYjh?key=b0taYl9Ha21zbkk1YmlEeEZ4UnpRa2hST2xKQWNn







Aug 5th 2016, 19:27

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

The output connection to the AM radio looks OK. I'd suggest winding a one or two turn coil to around the coil that resonates at 7250kHz--grounding one end and connecting the other end to the antenna. You may need to run the filaments a day or two to get rid of the gas inside the tube that accumulates after decades of sitting around.

Mar 1952 - QST (Pg. 21)
Pointers on the Installation of Mobile H. F. Converters
Author: Barbee, Basil, W5FPJ
http://p1k.arrl.org/pubs_archive/33186

A popular AM frequency is 7290kHz.. You may want to tune around 830kHz if the oscillator is at 8120kHz.

Zack Lau W1VT
ARRL Senior Lab Engineer
Aug 7th 2016, 18:27

NE6F

Joined: Jul 11th 2016, 18:01
Total Topics: 0
Total Posts: 0
Zack, as you suggested I did two winds around the 7250Khz slug tuned inductor and connected it to a long wire antenna. I was able sweep my grid dip meter up and down the 40 meter band. I followed the signal from the dip meter on a AM broadcast band receiver. So, I know its working. Thanks for the suggestion and the additional information you provided.

Rich
NE6F
Aug 7th 2016, 18:32

NE6F

Joined: Jul 11th 2016, 18:01
Total Topics: 0
Total Posts: 0
I know it's a long shot, but if anyone comes across the acritical that features this 40 Meter to AM broadcast band converter, I definitely would appreciate it!

https://photos.google.com/share/AF1QipPbzres3RDdf1e2dQj2X6PxF69og91qhzOGlsjvKfB1oTFPeaZBBc0wZ6fTfFSOyQ/photo/AF1QipMEdvRjMSqufwp-FwpdMwLSvlwzOZ0aPjZWdYjh?key=b0taYl9Ha21zbkk1YmlEeEZ4UnpRa2hST2xKQWNn


Aug 26th 2016, 19:05

NE6F

Joined: Jul 11th 2016, 18:01
Total Topics: 0
Total Posts: 0
Update: Got it working in my truck using the vehicle's 30" car radio antenna. Nice reception from a IBS in China.

https://photos.google.com/share/AF1QipPbzres3RDdf1e2dQj2X6PxF69og91qhzOGlsjvKfB1oTFPeaZBBc0wZ6fTfFSOyQ?key=b0taYl9Ha21zbkk1YmlEeEZ4UnpRa2hST2xKQWNn

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