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Author Topic: Kenwood TKR-750 Min Power  (Read 6178 times)

KA5IPF

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Kenwood TKR-750 Min Power
« on: May 29, 2018, 02:35:25 PM »

Does anyone know how low it can be turned??? We are getting a 2w input amp and need to know what attenuators I may need. I can't get at the repeater at the moment to find out.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #1 on: May 29, 2018, 05:40:02 PM »

Service manual specs says it'll go down to 25W.  Power adjustment is VR1 on the final unit.  I'd say you probably can't go under 10W without it getting nasty (at least that's my experience with GE's 40W MastrII).
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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #2 on: May 29, 2018, 07:14:19 PM »

I know it will go to 15w as that was what I set it to last time on the bench. Don't remember trying to see how low it would go.

Ok here's the rest of the story and hoping for some ideas.

At the site we have a TKR-750 feeding a Henry 100w amp and the output is fed to a set of Wacom cans and then to a DB antenna thru 3/4" heliax.
The repeater itself is clean. I noticed the amp was oscillating (picket fence) sometimes. If a put a Bird wattmeter inline in from the amp to the cans it quit. Take the wattmeter out and picket fence. And then found the second harmonic. Using the service monitor at the site the second harmonic is 30 dB greater than the primary when the amp is online. This was being monitored by an HP SM off the air. Removed the amp for service. Evidently changing feedline length stopped the picket fence but gave another problem. I removed the amp to see what it did at the shop.

At my shop on both the service monitor and the spectrum analyzer the second harmonic is 49-52 dB down from the primary out of the amp @ 96w out. All Ok. I was using a KW TM721 for power set to 14w at the shop, it was clean.

My only thought is the cans have somehow changed and have more reflected power and that is causing the amp to oscillate or whatever. I had a circulator at one time I could have put in line to confirm (between amp and cans) my thought. It got sold when no-one felt it was needed. I could put the Bird dummyload on the output of the cans and then the output of the amp and monitor off the air what is happening. I could also check the cans for alignment and return loss.

Any ideas on the problem? All feedlines are double shield Teflon coax.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #3 on: May 31, 2018, 07:54:47 AM »

What kind of insertion loss do you have through the cans individually?  Once of them may have developed "tin whiskers" causing weird stuff to happen.  I've even cracked the cheap pot-metal SO-239 connectors on the cans causing horrible losses, too.

Also, how much power is your exciter driving the Henry with?  If you're underdriving either amp (internal repeater PA or the Henry), I've seen spurs and 2nd harmonics go crazy.  Also, check the coax between the repeater and the amp.  You can use your HP SM on tracking gen to do this (just insert it between ANT and GEN).  If it has 3-6DB loss (and I've had teflon jumpers get this lossy), you may be underdriving the Henry.  If your amp or repeater has N connectors, check the bifurcated center pins.  I've had them spread out causing terrible connections, too.

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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #4 on: May 31, 2018, 08:32:04 AM »

All good suggestions. I don't remember the insertion loss, it's been too long. Best I think is abt 1.5-2 dB. I have not had a chance to check them again. The amp is a Henry repeater amp showing 10-20w input for 100 out. It on the bench it needed 15w to make 100 so before reinstalling it I set the repeater for 14w and the repeater was clean. It is rated for 100% duty @ 25w with a max out of 50w. The original cable from the amp to cans was the large Teflon, I don't remember the spec. I used a short RG-142 which should handle 100w with ease.

A little background. The repeater was down in power at my house and several other places as well.  I checked the power out using a Bird 43 inline from the amp to the cans it was 100w. Removed the Bird and checked with SA. Using the SM at the site it showed the repeater/amp was oscillating, picket fence. We pulled the amp to check it. At the shop it was fine except for the relay. It would not pass the signal when the amp was off line. I found an identical relay and replaced it. All was well. We put it back online and and checked the repeater on the SM in SA mode using an antenna. When spanning 10 MHz it looked like a picket fence. I changed the coax from the amp to cans and fence gone. Several locol hams reported the power was still not normal so I expanded the SA to look at the second harmonic (off air). It was 30dB stronger than the primary. Amp back to shop where it worked perfectly. Both of those trips we were time limited so had to work fast. Now we have plenty of time. (Schools out)
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #5 on: May 31, 2018, 10:24:09 AM »

I wonder if this is a case of the repeater-to-amp or amp-to-can jumper length somehow making it self oscillate.  Adding the wattmeter would change that length, hence stopping the self-oscillations.  Quarter wavelengths in the 2m band are around 19" long, perfect for a jumper length to be exactly 1/4 or 1/2 wavelength, even at a 2nd harmonic.
« Last Edit: May 31, 2018, 10:31:40 AM by K4JJL »
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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #6 on: May 31, 2018, 10:37:03 AM »

I considered that and we installed the RG-142 jumper. That got rid of the picket fence. As to whether or not the 2nd harmonic has been there all the time I don't know. I only looked at the big picture after the fence went away and the power level was still not up to par.

Keep thinking, we are scheduling a work "day" soon and have plenty of time.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #7 on: May 31, 2018, 11:04:32 AM »

After rereading one of your previous posts...

Best I think is abt 1.5-2 dB.

That's kinda crappy for a duplexer.  It's eating up almost 1/3 of your TX power (and your receive sensitivity).  I try to keep mine under 1dB.  However, if you drop your insertion loss, you may get some bad desense if your filters aren't tight enough.  Also, are the coax jumpers between the cans cut to length to match your frequency?  If they're surplus duplexers from a commercial application, they may be too short (or possibly damaged from moisture if it's not a conditioned site).
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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #8 on: May 31, 2018, 11:32:09 AM »

The cans were purchased from Wacom and set for our frequency. Desense is not a concern as the repeater has a separate receive site. This site is TX only. The low pass side of the cans is tuned to the packet racket frequency. It was not active during any of the tests. I can probably reduce the loss factor but I don't think that's part of the problem.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #9 on: May 31, 2018, 01:09:00 PM »

packet racket frequency

Glad to see someone else calls it "packet racket".  ;D

I challenge you to turn off the packet and see how long it takes for someone to notice.
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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #10 on: May 31, 2018, 01:35:28 PM »

I don't know how long but it could be a while. We also have Echolink and NWS has a direct link. Makes it hard to turn it off for a few days in storm season. Looks like we are going out in the AM to try and figure out something.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #11 on: June 01, 2018, 12:42:28 PM »

Any luck with it?
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W1BR

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RE: Kenwood TKR-750 Min Power
« Reply #12 on: June 05, 2018, 11:29:33 AM »

I have seen repeater amps develop instability problems when looking into duplexer loads, which can be highly reactive off the TX operating frequency.  The solution was adding a ferrite isolator between the amp and duplexer TX port, which is good practice anyways.

Pete K1ZJH
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KA5IPF

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RE: Kenwood TKR-750 Min Power
« Reply #13 on: June 05, 2018, 12:26:16 PM »

I thought about that but don't have one at the moment. I checked the amp at my shop and it was clean. At the site I checked repeater and am, clean. Then hooked to cans and checked output of cans, clean. When connected to the antenna the second harmonic goes up.
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K4JJL

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RE: Kenwood TKR-750 Min Power
« Reply #14 on: June 05, 2018, 01:44:10 PM »

When connected to the antenna the second harmonic goes up.

Probably doesn't need saying, but I'm gonna say it anyway: what's your reflected power look like between amp and cans as well as between cans and antenna?
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