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Author Topic: Did I damage my radio?  (Read 10042 times)
KE7DJQ
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« on: September 14, 2013, 03:15:02 PM »

I didn't realize my FT-757GX radio was tuned to 20m when I tuned up another radio next to it on the same band.  It was only for a second before I stopped after hearing the feedback.  Now I think I hear distorted audio from the radio. 

Is one second long enough to fry something?  I didn't have an antenna connected to the radio.
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VE7REN
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« Reply #1 on: September 14, 2013, 03:40:16 PM »

well its not a good thing to have happen,,,hehe.. ive done it a few times myself over the yrs,more then enough.. never had anything happen so far,so ide say nope....but next time pay attention ok.....from experience..good luck
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AA4HA
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« Reply #2 on: September 14, 2013, 04:00:06 PM »

... Now I think I hear distorted audio from the radio...

If this problem has persisted after you transmitted into the receiver I would say that you have damaged your radio. The clue is that "I hear distorted audio".

It may be fix-able. See if you can receive weak stations like you used to. That will point out of you zapped the pre-amp (first RF stage). You may have messed up a mixer or even the audio amp (distortion).

There is no lower end, "safe below this duration" for semiconductor devices. Even a mistake that lasts for milliseconds can be enough to do in a solid state device.

Find someone who has some test gear and some knowledge to see if they can check receiver sensitivity with a signal generator (or even another, known-good radio transmitting on the same frequency at exceedingly low power).
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WX7G
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« Reply #3 on: September 14, 2013, 04:34:36 PM »

With no antenna connected there should be no damage. The radio is fine.
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AC5UP
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« Reply #4 on: September 14, 2013, 06:19:31 PM »

Ditto.

The chances of coupling enough RF into the center pin of a buck-nekkid SO-239 to damage the front end / mixer / IF / detector stages is extremely unlikely.

OTOH.......... If the acoustic feedback was severe enough it is possible the speaker cone has warped. Or the audio amplifier output trannies aren't exactly complementary any more. Modern amplifier chips are remarkably damage resistant but this is an older radio. The speaker cone rubbing the magnet is a more likely scenario, but you'd have needed to howl the bejeesus out of it to make that happen.

If you can lay hands on an external speaker that might give you a better idea if you're hearing things. If an external speaker sounds equally distorted that means it's either an amplifier issue or you're paranoid. A signal generator and 'scope would be the definitive test.
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N3DT
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« Reply #5 on: September 14, 2013, 07:06:29 PM »

The 757GX is a tough little radio, unless you socked RF straight into the antenna it's probably fine.  As I remember it was listed as one of the better ones to have in case of EMP nuclear attack.  I still have mine, it's hardly worth selling.

Check it out thoroughly before deciding.  Probably a simple FM sinad or even just a basic MDS check will tell.  You'll need a calibrated generator, like a service monitor.  Or just compare it to a known good RX.

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KA4POL
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« Reply #6 on: September 14, 2013, 11:01:03 PM »

What usually happens when you overload the receiver you fry the input. This would result in a deaf RX. As you had no antenna connected chances are slim you did that. The distorted audio would by any means not be the result of damage to the RF input stages.
It is basically possible that RF got into the audio amplifier, a MB3713 IC. Turn the audio down and check it with headphones to narrow the problem down. It could well be the speaker that suffered from overload.
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W1JKA
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« Reply #7 on: September 15, 2013, 02:21:28 AM »

     If you knew how many times I have done this to my MFJ Cubs (antenna disconnected) with 75w from my IC-7200 then MFJ wouldn't have such a bad reputation, yes Cubs are working fine.
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KE7DJQ
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« Reply #8 on: September 15, 2013, 05:46:24 PM »

Thanks for the replies guys.  I just plugged an external speaker into the radio and the audio is back to normal.  Briefly going back to the built in speaker gives tinny, distorted sound.  I am much relieved to have the radio working great.
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KA4POL
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« Reply #9 on: September 15, 2013, 10:00:14 PM »

Time to replace the internal speaker.
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WB2EOD
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« Reply #10 on: September 16, 2013, 08:04:31 PM »

This is the risk of being able to operate more than 1 radio at a time.
It is possible to damage receiver even when NOT directly coupled to the transmitting antenna.  You may have gotten off cheap this time.
I have 2 adjacent operating positions.  After a careless and costly millisecond, I purchased a good quality selector switch so that the  antenna system can only be connected to one radio at a time, the opposite one is grounded. I used a signal generator to test the isolation.  A 60 over 9 signal on the "selected" radio didn't move the meter and and was barely audible on the opposite receiver.

73
WB2EOD
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G3RZP
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« Reply #11 on: September 17, 2013, 03:44:19 AM »

I had a tuned loop receiving antenna for 160. Fortunately, I did some tests before connecting it to the receiver. With 400 watts to the vertical 75 feet away, I got 80 volts across the loop output...

That would certainly do the receiver no good.....
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WB6DGN
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« Reply #12 on: September 20, 2013, 08:20:10 PM »

Quote
With no antenna connected there should be no damage. The radio is fine.

Agree with others.  I don't think that any RF caused damage occurred.  If anything, and if the volume was fairly high,  the feedback MAY have damaged the speaker although I think that, even that, is a stretch. 
As is often the case, when one "expects" a problem, one can easily hear (or see) it even if it really hasn't occurred.  Even knowing that, I find myself doing that every now and then.  I then spend and extra hour or more "chasing a ghost" until I realize that the problem is in my head.
So...I'd swap speakers or use headphones, and if it sounds OK that way, I chalk it up to "experience".
Tom

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