Trouble with Kenwood TM-D710A

[m_lars]

I am new to the ham radio world and just bought a Kenwood TM-D710A off eBay. It came with a power supply and mag mount antenna. I set it up in my basement to start learning how to use it while I wait for the parts to install it in my Rover. I tried twice to send a message via aprs and keyed it up maybe twice for a matter of seconds. After I got an swr meter I tested the antenna and found it to have a very high reading. I pull out the multi-meter and find there’s no continuity on the center conductor. I set it all aside until my cables and antenna for the truck arrive. After installing everything I test with the swr meter again and once again get a very high reading. I shut kit off and start checking for a short or continuity. I pull the antenna off the NMO mount and find a short across the center conductor and ground. So now I test every cable and connection, only to find the meter goes off when I touch the center connector on the back of the radio to the chassis of the radio. Now I’ve been in contact with the seller and he’s playing dumb convinced I’m doing something wrong.
Should the center conductor of the radio output open to ground? I don’t think so, but this dude ignores what I’ve said about that small fact.

 

[RockyMountaineer]

There shouldn't be any continuity between the center conductor and ground. Who is the mounts manufacturer? I would start there. If you have a manufacturing defect it should be warrantied.

Outside I'd say go buy new cabling and fix it right.

 

[Ubiety]

I keep a cheap mag mount antenna around for testing issues like this. Helped me figure out a similar problem in my JK recently. Never took the time to figure it out all the way but knew there was something wrong in the cable/antenna and later tested antenna on VNA and it was ok. Replaced cable/NMO mount and got a new antenna while I was at it and all is well. JK has a TM-D700; have a couple of them and have found them to be extremely resilient to my stupidity.

 

[m_lars]

There shouldn't be any continuity between the center conductor and ground. Who is the mounts manufacturer? I would start there. If you have a manufacturing defect it should be warrantied.

Outside I'd say go buy new cabling and fix it right.

I guess I wasn’t clear, I threw away his antenna that didn’t work. Everything else is new and has no issue. I have continuity on both positive and ground and no short between them when disconnected from the radio. As soon as I hook up the radio (or test it directly with a meter) I get the short.

 

[Ubiety]

Interesting, just did a continuity check on the D700 in the "shack" and it has continuity between the radio's antenna connector center pin and the chassis. Also has continuity between antenna connector outer ring and chassis. This seems wrong but the radio works great. Maybe Kenwood has some special circuitry to handle this? Tested with radio off, antenna disconnected, radio antenna conductors only.

 

[Kent R]

I am new to the ham radio world and just bought a Kenwood TM-D710A off eBay. It came with a power supply and mag mount antenna. I set it up in my basement to start learning how to use it while I wait for the parts to install it in my Rover. I tried twice to send a message via aprs and keyed it up maybe twice for a matter of seconds. After I got an swr meter I tested the antenna and found it to have a very high reading. I pull out the multi-meter and find there’s no continuity on the center conductor. I set it all aside until my cables and antenna for the truck arrive. After installing everything I test with the swr meter again and once again get a very high reading. I shut kit off and start checking for a short or continuity. I pull the antenna off the NMO mount and find a short across the center conductor and ground. So now I test every cable and connection, only to find the meter goes off when I touch the center connector on the back of the radio to the chassis of the radio. Now I’ve been in contact with the seller and he’s playing dumb convinced I’m doing something wrong.
Should the center conductor of the radio output open to ground? I don’t think so, but this dude ignores what I’ve said about that small fact.

@brien

 

[M Rose]

I guess I wasn’t clear, I threw away his antenna that didn’t work. Everything else is new and has no issue. I have continuity on both positive and ground and no short between them when disconnected from the radio. As soon as I hook up the radio (or test it directly with a meter) I get the short.

So I have a question... positive and ground being the power wires? Or are we talking coax?

 

[J.W.]

I read the post that he is getting continuity between the chassis and the PL259 antenna connection on the back of the radio.

Buying the radio used, it’s tough to know how it’s been treated but it’s harder than people think to burn out a transmitter with the wrong antenna. You can definitely do it with enough time but most modern radios have circuitry that temporarily reduces the power as the SWR raises. Think of it like a small pump trying to push water in a pipe uphill. As the water gets further up the hill, the pump requires more power to maintain the same flow. Reducing the power, reduces the strain on the pump (but you move less water / have less transmit power).

@m_lars have you tried a plain simplex test with a second radio to see if the Kenwood is transmitting at all?

 

[m_lars]

So I have a question... positive and ground being the power wires? Or are we talking coax?

Coax

 

[m_lars]

I read the post that he is getting continuity between the chassis and the PL259 antenna connection on the back of the radio.

Buying the radio used, it’s tough to know how it’s been treated but it’s harder than people think to burn out a transmitter with the wrong antenna. You can definitely do it with enough time but most modern radios have circuitry that temporarily reduces the power as the SWR raises. Think of it like a small pump trying to push water in a pipe uphill. As the water gets further up the hill, the pump requires more power to maintain the same flow. Reducing the power, reduces the strain on the pump (but you move less water / have less transmit power).

@m_lars have you tried a plain simplex test with a second radio to see if the Kenwood is transmitting at all?

There is continuity between the center conductor of the PL-259 antenna connector and the threaded outside of the connector as well as to the chassis of the unit.

I haven’t tried it because I’ve been worried about burning it up. I’ve got an HT, so I could hook it all back up and try.

 

[J.W.]

I would transmit with the HT and see if the mobile can receive it first. If it’s good, then try the other way real quick. Do that on low power of course but it should be very little risk to the radio for just a couple of seconds transmit with the correct antenna.

[edit] forgot to state the obvious, if you've got continuity between the inside and out of the PL259, there is definitely an issue!

 

[Ubiety]

I would transmit with the HT and see if the mobile can receive it first. If it’s good, then try the other way real quick. Do that on low power of course but it should be very little risk to the radio for just a couple of seconds transmit with the correct antenna.

[edit] forgot to state the obvious, if you've got continuity between the inside and out of the PL259, there is definitely an issue!

A perfectly good Kenwood D700 of mine exhibits the same continuity issue between center pin, chassis and outer ring. Have not tested the other in the Jeep. Took it down to test and it is now back up happily iGating and digipeating APRS.

Agree with @J.W. to try RX on the D710 and if that looks good try a low power TX on the D710. When I was troubleshooting the issue in the jeep I did just that for first TX on the new cable/antenna.

 

[J.W.]

A perfectly good Kenwood D700 of mine exhibits the same continuity issue between center pin, chassis and outer ring. Have not tested the other in the Jeep. Took it down to test and it is now back up happily iGating and digipeating APRS.

Now you’ve got me second guessing myself. Current should not pass unimpeaded between the center pin and the collar of a PL259 connection, right? I’m going to have to go test one of my radios in the morning now haha.

 

[M Rose]

Now you’ve got me second guessing myself. Current should not pass unimpeaded between the center pin and the collar of a PL259 connection, right? I’m going to have to go test one of my radios in the morning now haha.

If the antenna is connected, you will have a short between the two parts of the connector because of the antenna... the antenna becomes the conductor between the shield and the feed wire. Should be close to 50 ohms...

 

[M Rose]

Just remember I built a J-Pole that hits 1.5 SWR all a crossed the 2 Meter Band

 

[J.W.]

If the antenna is connected, you will have a short between the two parts of the connector because of the antenna... the antenna becomes the conductor between the shield and the feed wire. Should be close to 50 ohms...

No no, the PL259 connector on the radio. No antenna.

 

[M Rose]

No no, the PL259 connector on the radio. No antenna.

He says the problem occurs when the cable is connected to both radio and antennas

 

[M Rose]

I just checked chassis to center pin on three radios and get 50 ohms without coax

 

[J.W.]

He says the problem occurs when the cable is connected to both radio and antennas

the meter goes off when I touch the center connector on the back of the radio to the chassis of the radio.

Can't have an antenna connected if you are touching the center of the connector on the back of the radio, right? That's why I was thinking without any antenna connected. I may have just read it wrong. If you guys are getting continuity, I must be mistaken, I'll have to do some testing tomorrow. Still seems like there should not be current between the center and the collar (not the chassis). But again, I have been wrong about less.

All of this is moot if the radio is transmitting. Will have to wait to hear @m_lars findings.

 

[Ubiety]

I'll check the Jeep radio tomorrow as well, and retest the APRS gate as well. This time I'll

Can't have an antenna connected if you are touching the center of the connector on the back of the radio, right? That's why I was thinking without any antenna connected. I may have just read it wrong. If you guys are getting continuity, I must be mistaken, I'll have to do some testing tomorrow. Still seems like there should not be current between the center and the collar (not the chassis). But again, I have been wrong about less.

All of this is moot if the radio is transmitting. Will have to wait to hear @m_lars findings.

Definitely no antenna installed when I tested.