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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 5:18 am 
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DaveInNC wrote:
They'll probably hum for me as well.

Yes, but if they can carry a tune, it'll be an achievement... :wink:

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 9:11 pm 
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I kept searching for a technical explanation of the phenomenal we have been seeing and finally found it in a book on building low power transmitter by Rudolf Graf and William Sheets. The TH and any other low power transmitters will hum if the house AC wiring is part of their grounding system. The grounding scheme done by Flinx should work at any frequency: it essential decouples the AC wiring from the antenna by using a large metal surface for the ground. It forces the antenna return current to mostly hit the metal surface rather than the AC wiring.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 9:22 pm 
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Interesting.

I'm tempted to try a metal sheet under my antenna to see if it improves the signal any. I already have low hum, but to have it non-existent or just lower than it already is would be better.

My antenna is simply a 4-5' telescoping antenna mounted to a wood block with a pi net antenna tuner on the wood block.

Would the metal sheet just be placed under the wood block and connected to the ground of the antenna tuner which connects to the transmitter's ground?


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 9:29 pm 
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bb.odin wrote:
I kept searching for a technical explanation of the phenomenal we have been seeing and finally found it in a book on building low power transmitter by Rudolf Graf and William Sheets. The TH and any other low power transmitters will hum if the house AC wiring is part of their grounding system. The grounding scheme done by Flinx should work at any frequency: it essential decouples the AC wiring from the antenna by using a large metal surface for the ground. It forces the antenna return current to mostly hit the metal surface rather than the AC wiring.



Thank you, Mine definitely had the hum until I accidentally grounded it to my desk. I knew it worked I just did not know exactly why. Maybe later I will experiment with a dipole antenna arrangement ground on one side and antenna on the other using old 300 ohm rabbit ears. or something like a j-pole.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 9:30 pm 
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A baking sheet works.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Wed 25, 2019 9:39 pm 
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Perhaps mine doesn't hum much because the case of my transmitter is metal.

EDIT:

The idea of putting metal under my antenna didn't work for me.

Interestingly enough my RCA K-80 hums less with the power cord ground of the transmitter disconnected, but my R-390 hums more with it disconnected.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Thu 26, 2019 2:19 am 
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This is a good explanation for hum WITH a wall transformer (and I recall reading suggestion from at least one person to bypass the diodes in this case.)

However the reference does not offer any explanation for when the unit is powered by a battery, as I and Marvin did. I don't remember if Binh did it as well. I noted zero difference in hum, in a careful A/B comparison, battery vs. wall wart. Recall in this experiment there was no reference to the AC, even at the input (which was connected to a battery-powered iPod.)


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Thu 26, 2019 2:27 am 
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The RF is getting into the ground wiring.

If I read that post right, the ground plane is supposed to help prevent that from happening.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Thu 26, 2019 3:06 am 
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My TH made no difference in hum when powered by a battery. The unit was out of the metal case at the time and the test was done in not too big a space in the basement. The unit had a poor ground, the DC ground on the board is it. The antenna current has to find a way to return to the board and its path still encompasses the house wiring. There are many non-linear devices on the wiring ( LED bulbs, CFL lights, switch-mode power supplies...) which cause the wiring impedance to be modulated by the AC. My little transmitter on battery had almost no hum because it has a vertical dipole, telescopic up on one leg, lamp post down on the other, unit in the middle.

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Thu 26, 2019 3:42 am 
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bb.odin wrote:
My little transmitter on battery had almost no hum because it has a vertical dipole, telescopic up on one leg, lamp post down on the other, unit in the middle.

Sounds like it's on stilts... :wink:

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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 28, 2019 5:56 pm 
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fifties wrote:
bb.odin wrote:
My little transmitter on battery had almost no hum because it has a vertical dipole, telescopic up on one leg, lamp post down on the other, unit in the middle.

Sounds like it's on stilts... :wink:


It only needs one. Thanks to you, I'll name it Avocet.

Was looking for a large metal surface to use as ground for the antenna inside the house. It finally occurred to me what would be a better one than the metal ducts. Attached the TH's metal case to the return air duct with an 18-gauge wire. Big drop in hum with a battery radio! It worked well at 820kHz and 1,670kHz. No reduction in hum on a AC operated radio but now it's much pleasant to listen to music with a battery radio. The test was done with a vertical and a horizontal antenna. I didn't bother to isolate the ground prong of the AC adapter - I'm pretty sure that the air duct is grounded at the air conditioning system.

Did another modification of the audio amplification circuit. Changed R231 and R232 to 10.0K and R230 to 1.00K so that the stereo line input is mixed to mono and attenuated by a factor of 11. Changed R212 to 680 Ohms and C209 to 1uF so that the AGC pre-amp has a nominal gain of 11. Essentially the line input has no gain applied before it reaches the modulation amp input - it's actually attenuated by about 2dB in the audio pass band. Compared to the original gain, the AGC pre-amp gain is attenuated by a factor of 13. To allow the internal electret microphone MC200 to be useable, its load resistor seen by the pre-amp input needs to be increased by 13 times. Fortunately the microphone's load is splitted into two resistors R234 (470 Ohms) and R235 (4.7K) which attenuates the mic's output by a factor of 11. It's sufficient to swap R234 and R235 to magnify the mic's signal by 10. An external microphone may not be useable unless it has a built-in preamp with a gain of about 20dB. All ceramic capacitors in the audio circuitry are replaced with mylar or film capacitors. No hum is heard with the built-in monitor when the volume is cranked up to maximum.

Power supply mods: 220uF 16V cap paralleled with C330 for the 5V rail; C207 increased to 220uF for the pre-amp's 5V supply; C334 increased to 220uF 35V for the charge pump's 12V rail; 100uF 16V cap paralleled with C309 for the PLL's 3.3V supply; C224 increased to 100uF for the internal microphone's supply.

For line input, the 3 dB pass band is from 45Hz to 4.5kHz with a total gain of about 36dB. Final audio frequency response:


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Dec Sat 28, 2019 7:59 pm 
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Try isolating the ground of the power supply since the air duct is grounded and see if that reduces hum.


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Mar Sun 01, 2020 11:07 pm 
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Greetings from Poland!

Folks! You are great! I have read the whole thread and I modified my TH 5.0. This transmitter have brilliant sound now! Much Better than the best european AM transmitters.

Yes... The best way is -
Stuck to c301 leg, 0,1 uF polypropylene capacitor and completly remove the c337 tantalum capacitor.

There is my sound reception from TH 5.0
I don't have any noise, hum, even on the top of the band.
I have good CB Radio 13,8 V Power Supply.
Only connection reel to reel recorder to radio, caused slight hum Yes, the AM reception was recorded on 50 years old tape!!! Line output of my radio, give too weak signal to computer, and some computer can make noise.

youtube.com/watch? feature=emb_logo&v=5u_BKNH_f6c


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 Post subject: Re: Talking House AM Transmitter MODIFICATION IDEAS HELP?
PostPosted: Mar Thu 12, 2020 9:15 pm 
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But now I gave a 0,15 uF capacitor.

However, I think, that this is better way.


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