Strange problem with computer and home AM transmitter

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Strange problem with computer and home AM transmitter

Post by Brig on Mon Jun 04, 2012 5:17 pm

As I have posted previously, I've had fine results using my home AM transmitter, the same type that "Bob" on YouTube discusses (although I found the circuit long before "Bob" made his video). It has worked for me beyond my best expectations, using as audio sources an FM receiver and a CD player. I get no hum, no buzz, no "FMing" (whatever that means), no heterodyne whistle or other interfering noise, and more than enough volume. Usual transmitting frequency is about 1300 kc and I cover my entire house plus an appreciable exterior distance.

However, twice now when I attempted to use the audio output from my computer I instantly tripped the house circuit breaker. I first used the headphones jack, then tried the audio output jack on the computer sound card.

I did some troubleshooting, thinking that a voltage leak or somesuch was causing the short but I found nothing out of the ordinary. I checked two transmitter units and both produced the same voltage measurements at the audio-in connection, about 0.5 vdc, 0 vac.

Why this problem occurs only with the computer as audio source is unknown to me. Can anyone help or offer suggestions? Thanks in advance.


Brig
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Re: Strange problem with computer and home AM transmitter

Post by Motorola man on Mon Jun 04, 2012 6:40 pm

Do you have a transformer on the audio input? If there isn't a transformer there, there may be some sort of difference in voltage potential causing the issue. I haven't tried using a computer as an audio source for my transmitter, but I have used sources using A/C power with no trouble at all. I'll try it with mine tomorrow.
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Re: Strange problem with computer and home AM transmitter

Post by Motorola man on Tue Jun 05, 2012 6:07 pm

I gave my transmitter a whirl with it connected to my computer and had no problems at all. If your transmitter design doesn't have a transformer on the audio input to isolate the source from the radio, add one. If it already has one, make sure that it's not shorted.
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Re: Strange problem with computer and home AM transmitter

Post by Brig on Tue Jun 05, 2012 6:12 pm

Problem solved! No one, apparently, took note of the 0.5 vdc (actually, closer to 0.6) measurement at the input that I had noted. The reason for the voltage is that I had neglected to make certain that the "hot" terminal of the volume control is free of all original connections. I snipped a side of a resistor that was still there, tried the unit at the computer audio output again, and--lo!--no circuit breaker tripping.

However, another simpler problem was present. The audio quality was poor--too much hum. A matching transformer took care of this right away. I've had the transformer, an antiquish 5:1 unit, in a hell box for almost 35 years. I simply connected the primary side to the computer audio output and the secondary side to the transmitter audio input. So, all in all, an interesting problem and a successful learning experience. But why the other audio sources (an AM-FM receiver and a CD player) did not also create a short and trip the house circuit breaker remains a mystery.

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