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Maratrac low band buzz on Receive

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eportel6607

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Hi Guys,
I have a low band 42-50Mhz Maratrac that emits a buzzing sound during receive and during any audio when the speaker is unmuted.
I mean the buzz sound is there when ANY audio is present. I'll explain a bit better:
If the radio is receiving a signal the buzz (higher pitch...not like a PL drone buzz sound) but the buzz is also there when the beep sounds are emitted when changing channels or working the volume up/down.

I have several of these radios and only one make this buzzing sound. If the radio is in scan and the Priority channel has activity on it the buzzing sound coincides with the flashing of the "Pri" LED indicator.
I've tried several control heads (clamshell and advanced) and several power supplies.
The radio seems to be, otherwise in working order.
Also when receiving a signal from another radio the audio seems perfectly understandable and isn't garbled or distorted...but this buzzing sound is present on all instances when audio is heard.

Any idea what could cause this?
Thanks,
Chris
 
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cmdrwill

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A possible cause may be dried out capacitors on the audio board.
Another is incorrect wiring to the battery, BOTH Red and Green wires must go directly to the battery. The Red one should have a 30 amp fuse, the Green should be 3 amp fuse.

Poor ground.
 

eportel6607

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South County, Rhode Island
Hmm. Thanks very much for the reply.
The radio(s) are on the bench. Since I have left the wiring connected to the power supply and simply swapped out the radio itself (using the same wiring harness) I would then suspect the caps. All the other radios tested were very quite when tested the same way. I have no idea where to start when it comes to the caps. Is there a board that is specific to the audio?
Thanks again,
Chris
 
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krokus

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Hmm. Thanks very much for the reply.
The radio(s) are on the bench. Since I have left the wiring connected to the power supply and simply swapped out the radio itself (using the same wiring harness) I would then suspect the caps. All the other radios tested were very quite when tested the same way. I have no idea where to start when it comes to the caps. Is there a board that is specific to the audio?
Thanks again,
Chris
The hum could be from your power supply, especially if you are loading near the capacity of the supply.

Sent via Tapatalk
 

Tech792

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Since you say you have so many of these in service, try swapping it out with another vehicle to see if the problem moves. That will help you narrow it down to the radio or the installation.
 

cmdrwill

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Hmm. Thanks very much for the reply.
[snip] I have no idea where to start when it comes to the caps. Is there a board that is specific to the audio?
Thanks again,
Chris
The audio board is a L shaped board. They plug in but be very careful the connections are easily damaged.
You could send it in we still repair Maratracs here.
 

ramal121

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Yes, if you take the bottom cover of of the chassis, the audio board will be an "L" shaped board toward the front (handle) portion of the radio.

Now here comes some trivia. At one time there was a LSB for early Maratracs concerning a buzz out the speaker when ever it was un-muted. It had to do with a data clock line (16 dot something KHz) being too close to an audio line and would bleed over a buzz when the speaker un-muted. The fix, I believe, was to break the data line and run it away from the offending audio line with a jumper. Granted this is from fuzzy memory from about 15 to 20 years ago. MOL of course does not show a LSB from this long ago unless I'm looking in the wrong spot. Any help here?
 
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