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Author Topic: Low buzz on my VHF/UHF tranceiver  (Read 3608 times)
K2OWK
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Posts: 1061




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« on: February 18, 2014, 06:04:58 PM »

I have an old scarce Yaesu FT-4700RH transceiver. On transmit I have been told it has a very low hardly noticeable low frequency buzz. This buzz is only noticed when no modulation is present. I tried grounding my 13.8 volt switching supply to the transceiver, removed any wall warts in the line and removed a DC voltage monitor from the circuit. Buzz is still there. I was also told the buzz sounds like 60 HZ. I have monitored myself on my HT and the buzz can be heard. I am using FM and the buzz is on both VHF and UHF. I have been told not to worry about it, but it annoys me to know it is there. My HT at the same location is clear with no buzz. Does anyone out there have any suggestions as to what to try to eliminate the noise.

Thank you,

K2OWK 
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K5LXP
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« Reply #1 on: February 18, 2014, 06:26:05 PM »

Is it a PL encoder?  Try enabling/disabling that and see.

Try it into a dummy load.  That would isolate a ground loop. 

Disconnect the mike and key the rig without it, and see if it's coming in that way.

Could be the PLL too.  That would be a bit harder to resolve.

Mark K5LXP
Albuquerque, NM
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K3LRH
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Posts: 58




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« Reply #2 on: February 19, 2014, 05:04:20 AM »

....power supply?
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K1CJS
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Posts: 6034




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« Reply #3 on: February 19, 2014, 07:10:38 AM »

I would second the power supply guess.  Probably a leaking filter capacitor.
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K2OWK
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Posts: 1061




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« Reply #4 on: February 19, 2014, 05:36:36 PM »

Thanks for the help. I have tested the power supply it has less then 20MV ripple at full transmit output. I have isolated the transceiver and tried battery operation, same buzz. The PL encoder is not causing it, it buzzes with it in and out. I have not tried removing the microphone and keying the transmitter. I will try that and see what happens. I will let you know.

Thanks again,

73s

K2OWK
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KF7CG
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Posts: 836




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« Reply #5 on: February 20, 2014, 05:47:45 AM »

Is there a stray 60Hz magnetic field in the area, like say from the transceiver being placed on top of an analog power suppy or the power supply area of another radio. Dynamic microphones themselves are also susceptible to picking up induced hum. With some of the audio circuits in the modern gear offering a certain degree of automatic audio compression the audio input can be sensitive to induced hum when no true audio is present.

David
KF7CG
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K1CJS
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Posts: 6034




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« Reply #6 on: February 20, 2014, 06:05:48 AM »

One other thing you may check is the routing of your antenna cable, but first, try disconnecting the antenna cable, if you didn't already when you 'isolated' the rig, and seeing if the buzz goes away.  If it does, it may be that the buzz is being picked up by the antenna cable somehow.  If the cable is routed next to any power lines, that may be where the buzz is coming from.

If you can, try temporarily using the rig somewhere else--your mobile, perhaps?
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K3LRH
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Posts: 58




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« Reply #7 on: February 20, 2014, 11:45:34 AM »

......maybe it's NOT 60~....something else?.......  with an old rig, the noise could be coming from almost anywhere.  Is the mic balanced or unbalanced input?   How do the caps look?...Is the rig new to you?...

good luck with it.    73
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