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SWR high when amplifier is off

Capt Crunch

Captain Crunch NJ Mobile
Nov 3, 2015
169
214
73
New Jersey
www.iamawesome.com
Here's one I can not find anything on so far, so asking here. I have a Galaxy DX979 to a TS500DX to a tuned Predator 10K-1-12 Antenna on LMR400UF coax. I noticed while the amp was off my SWR warning light came on and the radio SWR is 3 on Channel 1 and a little higher than 3 on Channel 40 with the amp off. When I tried adjusting the whip drastically it did nothing at all. When I turn the amp on the SWR drops to 1.3 on 1 and 1 .4 on 40. I have also tried a Skipshooter 5ft that's tuned already and it is and stays where I tuned it with the amp on or off going a hair higher with it on. The SWR was then checked with a meter at the radio and it does the same. I have just noticed this as I set SWR with the amp inline and off and never had a problem with it, including the 10K which was tuned months ago. I am wondering if I will it hurt the radio if I use it with the amp on as the SWR are "normal" then. It's strange to me that the other antenna is not doing that and the Predator wasn't doing that. Anyone see this before? Any help?
 
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Same issue here! 980ssb into rm203 and a Tram tbc-9 coil antenna. 1.8 with amp off and 1.1 with it on! Tried a Wilson silver load and a 102” whip and all are doing it with different swr readings.
 
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Any amplifier should pass what I call the "barrel-connector test". Placing it in line with the radio should not increase the SWR seen by the radio any more than a barrel connector and coax jumper would do.

For a long time now, that amplifier has been sold using a legal loophole. It's configured as a CW transmitter by installing a small circuit board that has an oscillator circuit with an empty space for a quartz crystal. A tiny 2.5mm jack on the rear panel is for your morse-code key.

This is a fig leaf to cover the private parts. Nobody with more than a gram of sense uses it the way it's set up from the factory. At least 999 out of 1000 of these sold will get "converted" back to being a non-legal linear by removing the parts that turned it into a pretend CW transmitter.

Here is an instruction sheet for that procedure: http://www.cbtricks.com/Amp/txstar/dx400v_dx500v/dx400v-500v_mod.htm

My strongest suspicion is that the conversion was botched, and something is connected to the amplifier's standby circuit where it should not be.

Here's the schematic of the current "DX500V" model. Bear in mind this amplifier has been produced in various versions for around 30 years or more. Yours may be a bit different, depending on the age.

http://www.cbtricks.com/Amp/txstar/dx400v_dx500v/graphics/ts_dx400v_dx500v_sch.pdf

Most amplifiers like this one use two relays. One of them transfers the antenna and radio when you key the mike. The other relay takes the receiver preamp into our out of line.

Texas Star chose a different setup. It has only one relay. As a result, the path your radio's barefoot transmit signal takes is a little convoluted. First passes through C3, a 1000pf disc capacitor and then into the input side of the relay. From there, the receive-side contact sends it into the preamp switch, back out (if the preamp is off) and then to the output side of the relay. It then comes out of the relay's output-side contact pin and into C44 a .01uf disc capacitor and from there to the antenna socket.

There is a small RF choke coil L1 connected to the antenna side of the relay. Your radio's barefoot transmit signal should ignore this choke coil, as if it were not there. If this coil was overheated the varnish that insulates the wires on it will burn away. This causes the separate turns of wire to short to each other. A damaged L1 will cause the kind of trouble you're seeing.

But that's the list of things that can be blamed if placing the amplifier in line with the radio causes the radio's SWR to rise.

Gotta be one (or more) of those.

73
 
If it is that problem why would it only be doing it with the predator 10k?
Any amplifier should pass what I call the "barrel-connector test". Placing it in line with the radio should not increase the SWR seen by the radio any more than a barrel connector and coax jumper would do.

For a long time now, that amplifier has been sold using a legal loophole. It's configured as a CW transmitter by installing a small circuit board that has an oscillator circuit with an empty space for a quartz crystal. A tiny 2.5mm jack on the rear panel is for your morse-code key.

This is a fig leaf to cover the private parts. Nobody with more than a gram of sense uses it the way it's set up from the factory. At least 999 out of 1000 of these sold will get "converted" back to being a non-legal linear by removing the parts that turned it into a pretend CW transmitter.

Here is an instruction sheet for that procedure: http://www.cbtricks.com/Amp/txstar/dx400v_dx500v/dx400v-500v_mod.htm

My strongest suspicion is that the conversion was botched, and something is connected to the amplifier's standby circuit where it should not be.

Here's the schematic of the current "DX500V" model. Bear in mind this amplifier has been produced in various versions for around 30 years or more. Yours may be a bit different, depending on the age.

http://www.cbtricks.com/Amp/txstar/dx400v_dx500v/graphics/ts_dx400v_dx500v_sch.pdf

Most amplifiers like this one use two relays. One of them transfers the antenna and radio when you key the mike. The other relay takes the receiver preamp into our out of line.

Texas Star chose a different setup. It has only one relay. As a result, the path your radio's barefoot transmit signal takes is a little convoluted. First passes through C3, a 1000pf disc capacitor and then into the input side of the relay. From there, the receive-side contact sends it into the preamp switch, back out (if the preamp is off) and then to the output side of the relay. It then comes out of the relay's output-side contact pin and into C44 a .01uf disc capacitor and from there to the antenna socket.

There is a small RF choke coil L1 connected to the antenna side of the relay. Your radio's barefoot transmit signal should ignore this choke coil, as if it were not there. If this coil was overheated the varnish that insulates the wires on it will burn away. This causes the separate turns of wire to short to each other. A damaged L1 will cause the kind of trouble you're seeing.

But that's the list of things that can be blamed if placing the amplifier in line with the radio causes the radio's SWR to rise.

Gotta be one (or more) of those.

73

If it is that problem why would it only be doing it with the predator 10k? I will try the barrel connector test, thank you. I was thinking the antenna as it only happened so far with one antenna. Here is a video of this amp
 
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If it was ok with another antenna, then maybe the connector on the 10k has a loose connector on the center pin? RF is funny sometimes. When the input match was off on my 350HD, the SWR out of the amp would be high with the dead key and come down when I modulated. Then I measured the SWR between the radio and amp and found it high.
 
If it was ok with another antenna, then maybe the connector on the 10k has a loose connector on the center pin? RF is funny sometimes. When the input match was off on my 350HD, the SWR out of the amp would be high with the dead key and come down when I modulated. Then I measured the SWR between the radio and amp and found it high.

Actually you may be correct on this although still not sure why it would go down when amp is on but I did change my 27in shaft to the 12in. I did not check the swr then because I knew where the tune was on it and I had the amp on when I checked when I switched shafts. Maybe I tightened it to much and broke it. I am a big dude and have broken 4 inch bolts off of steam piping before by accident lol. I am going to try the barrel connector and take the amp out and see what happens, either tonight or tomorrow. That will verify that at least. Next I will check the antenna. That would sk if I broke the antenna.
 
I got confused with the way it was worded. I did not notice that you changed antenna.
Ok, so I went looking for my barrel connector but its male to male so going to buy one. May take a day or so. I did mention it did not do the same thing on my other antenna but I double checked with 4 antennas just now and all do go up in SWR with the amp off. Some more than the others. Predator showed 3 the others went up to 2ish with it off, from 1.2 matched with the amp on. None did this before. I am guessing that L1 might be the culprit as you explained. I did do some skipshooting the last couple of days and it could have gotten hot. I usually have fans on it but I took them off for new ones I haven't hooked up yet. I guess I'll know when I get the connector. Thanks.
 
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Ok, so I went looking for my barrel connector but its male to male so going to buy one. May take a day or so. I did mention it did not do the same thing on my other antenna but I double checked with 4 antennas just now and all do go up in SWR with the amp off. Some more than the others. Predator showed 3 the others went up to 2ish with it off, from 1.2 matched with the amp on. None did this before. I am guessing that L1 might be the culprit as you explained. I did do some skipshooting the last couple of days and it could have gotten hot. I usually have fans on it but I took them off for new ones I haven't hooked up yet. I guess I'll know when I get the connector. Thanks.

Have you a dummy load you can put on the antenna jack of the amp instead of an antenna? If so try that see if you get a good reading with a dummy load it should show 1.2 or less when the amp is not on. even with the amp on the match should look pretty good with a dummy load instead of antenna assuming the load can handle the power.
 
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Have you a dummy load you can put on the antenna jack of the amp instead of an antenna? If so try that see if you get a good reading with a dummy load it should show 1.2 or less when the amp is not on. even with the amp on the match should look pretty good with a dummy load instead of antenna assuming the load can handle the power.
I have a small one I can try without the amp on though. Ill try it tomorrow, thanks.
 
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