Kenwood: Kenwood VM71A Question

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eboyce79

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Merry Christmas! I’ve been doing some troubleshooting to find the source of intermittent interference when transmitting. I visited a local ham who tested my SWR and found a loose ground and let me borrow a different antenna (improved SWR). Unfortunately, once I left his house, the problems returned. I’ve checked my grounds and my connections...no dice. Just curious if anyone else with a VM71A (or any other radio) experienced similar issues. I’ll keep tinkering this weekend.
 

AK9R

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Are you talking about the ticking sound in your transmitted audio?
 

eboyce79

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No. It’s described as “scratchy” or “ref interference.” I’m thinking it might be the mic extender. I’m going to hook up the mic straight to the radio and see what’s what. Frustrating not knowing!!
 

mmckenna

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I think if you can describe the interference for us it would help. There are a number of different types/sources of interference, kind of hard to narrow it down with no specifics.
 

eboyce79

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I’ll have to hear the interference for myself...I’ll simplex to my HT. Folks on the repeater only described it as “very scratchy” or “RF interference.” It’s intermittent and bad enough that I’m unintelligible. Once I replicate it so I can hear, I’ll report back. Thanks!!
 

AK9R

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If you have extended the mic cable using a 4 twisted-pair network cable (CAT 5, CAT 6, etc), take it out of the picture and connect the microphone directly to the radio using the supplied cable from Kenwood.

There is a known issue with the TM-V71 and TM-D710 radios and their mic cables. In addition to microphone audio, there is a pulsed clock signal going through the mic cable. The Kenwood cable shields just the mic audio line and leaves the line carrying the clock signal outside the mic audio shield. Twisted-pair network cables often have no shielding at all and depend on the twisted pairs to reduce the common mode interference. But, the pin-out of a typical network cable puts the line carrying the clock signal in close proximity to the mic audio with no shielding.

Even if your mic cable isn't picking up the clock signal, it could also be picking up RF generated by the radio. To mitigate this interference, your antenna needs to matched to the transmitter (low SWR) and your grounding needs to be in good shape.

Also, if your installation puts any kind of strain on the connectors at either end of the mic cable, that strain might mis-align the RJ45 connectors resulting in less-than-perfect contact. Make sure that the connector plugs straight into the radio with no pulling to either side or straight out.
 

krokus

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I’ll have to hear the interference for myself...I’ll simplex to my HT. Folks on the repeater only described it as “very scratchy” or “RF interference.” It’s intermittent and bad enough that I’m unintelligible. Once I replicate it so I can hear, I’ll report back. Thanks!!
Ask someone to record that for you, then upload the file, and share it with us.

Sent using Tapatalk
 

eboyce79

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Thanks for all the help and suggestions. I applied it and found the issue.

The Problem: I tried to go cheap and bought a cable from Amazon instead dropping $65 on the Kenwood extension kit. Once I eliminated the extension, my problem disappeared! I ordered the Kenwood extension kit and a new antenna for good measure.

I underestimated the need for proprietary cables and hoped a cheap alternative would work. It worked for a little while, but now I’m buying the spendy cable anyways. Lesson learned.

Thanks again for the help!!

Eric
 

eboyce79

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If you have extended the mic cable using a 4 twisted-pair network cable (CAT 5, CAT 6, etc), take it out of the picture and connect the microphone directly to the radio using the supplied cable from Kenwood.

There is a known issue with the TM-V71 and TM-D710 radios and their mic cables. In addition to microphone audio, there is a pulsed clock signal going through the mic cable. The Kenwood cable shields just the mic audio line and leaves the line carrying the clock signal outside the mic audio shield. Twisted-pair network cables often have no shielding at all and depend on the twisted pairs to reduce the common mode interference. But, the pin-out of a typical network cable puts the line carrying the clock signal in close proximity to the mic audio with no shielding.

Even if your mic cable isn't picking up the clock signal, it could also be picking up RF generated by the radio. To mitigate this interference, your antenna needs to matched to the transmitter (low SWR) and your grounding needs to be in good shape.

Also, if your installation puts any kind of strain on the connectors at either end of the mic cable, that strain might mis-align the RJ45 connectors resulting in less-than-perfect contact. Make sure that the connector plugs straight into the radio with no pulling to either side or straight out.
Spot on! Thanks!
 

AK9R

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There used to be a guy who made Kenwood mic extension cables. He found a source for the cable that Kenwood uses (it has a shield around the mic audio line in the cable) and ran a "cottage business" selling extension cables d/b/a Greenlight Labs. But, he's since moved on and I don't believe he's selling those cables anymore.
 

KD2FIQ

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I have the same radio with a homemade extension cable. I have received similar complaints of audio with some interference. I may need to pony up and get the real cable! I did put together a faceplate extension cable with great success. That works fine.

EDIT: Just to clarify, I did make a custom pinout on the RJ45 ends that paired the mic #6 pin with #3 ground if I remember correctly.
 
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KD2FIQ

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I may be mistaken with the pair used for the mic in the above post. But I know I did not follow 568B. I may have done pins 1 and 2 (pair 1), pins 3 and 4 (pair2), and so on.
 

AK9R

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In the Kenwood mic cable, pin 6 is Mic Audio and pin 5 is the Mic Audio Shield.
TM-D710_mic_cable.jpg
 
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