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Motorola RKR1225 repeater drops tx pwr


JCase
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Greetings all,

 

I’m running a Motorola RKR1225 40 watt repeater that should be transmitting 30+ watts after duplexer but I’ve only ever realized 27.5 watts however, that’s not the main issue. When the unit first starts transmitting my test meters (tried three different ones) all show 27.5 watts. But, the longer the repeater remains transmitting, the lower the transmit power goes. It’s a slow but steady drop from 27.5 watts down to 23 watts. I’ve double checked all of my connections and my SWR meters all register 1.00 (using a DM-408B and temporarily using a 35’ run of LMR400 since I’m hoping to relocate the system to a local hospital for better range and saturation and decided to save my heliax cable for the new location. I just can’t figure out why the constant drop in tx output power. As always, I appreciate any and all suggestions and look forward to the replies.

 

John

WRDM373

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Radioguy7268 & WRAK988,

 

Thank you both for your response and invaluable information. I’ve been trying to learn as much as I can about this model and I’m surprised that this is the first time I have heard of the TX power reduction “feature”. I can’t thank all three of you that responded enough. This all makes sense now. My friend/tech, I’m assuming adjusted the power setting to 27.5 intentionally, in an attempt to minimize the chances of overheating and since the power is a slow & steady reduction that returns to the full 27.5 on each transmission, it would make sense that it has been programmed to do so for the reason you both pointed out. I did re-check all of my connections again with a focus on my cables and everything seems tight and secure. I’m sure my friend/tech intended on informing me of this feature and to be honest, he may have on the day I picked it up. I either don’t remember or it’s possible I was too busy babbling with his wife (most likely) or since my visits were usually spaced apart by several months, and as friends do, there was always a lot of catching up to do, some show and tell, some gifts and more. My bad for not paying closer attention and taking better notes.

 

Whether I receive good news or bad, I always receive the information I need from all of the supportive members on this site and I can’t thank everyone that responds, enough, for taking the time to to monitor and offer guidance by sharing their knowledge and experiences with the rest of us.

 

Until next time (and I’m sure there will be plenty of “next times”, stay healthy, well, safe and happy !

 

John

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I don’t know what the issue is, but the first thing that is running through my head is ‘Is the transmitter output truly remaining constant or is changing too?’

 

If the transmitter power is in-fact remaining constant while the insertion loss of the duplexer is changing, then perhaps a cable to or connection within the duplexer is warming and changing impedance.

 

If the radio is actually changing, then we know the issue is related to the repeater.

 

If the SWR is located at the radio and the duplexer characteristics that are changing while transmitting, would expect to see some subtle movement in the SWR.

 

Just food for thought.

 

 

Michael

WRHS965

KE8PLM

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Hey Michael,

 

Thanks for the response. I never thought about disconnecting the transmitter’s cable from the duplexer but I’ll work on that today and see what kind of readings I get directly from the transmitting side. Unfortunately I don’t have a scope or any real advanced equipment and currently don’t have the programming software that would allow me to tune the unit much less any other “serviceable” options. And the SWR didn’t budge during the transmissions, not even a twitch. The only thing that moved/changed was the slow & steady drop on the power meter. Thanks again for the feedback. I hate the obvious, I’m thinking I’ll end up having to take it to a local tech’s shop and pay a small fortune to fix the issue. I miss my friend and tech every day but days like today remind me just how fortunate and blessed I was to have him in my life for so long.

 

John

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The R1225's were famous for burning up if you ran them at high power. Someone probably did you a favor by turning down the finals to limit the power output. The sales brochure said they'd do a 50% duty cycle at high power, experience says that even at 25 Watts, anything close to a 50% duty cycle would kill them.

 

Lots of the later Maxtrac/Radius mobiles that the R1225 series were based off of had a thermistor controlled limiting circuit that would turn down the power as the amp heated up - kind of a self limiting feature to keep the radio from burning itself up. I haven't dug into the manual for one of those R1225's in probably 15 years - so I can't say for sure that's what is causing the pull-back that you're seeing, but it seems like it might be. Make sure you're keeping the fan on & blowing cold clean air across the heat sink.

 

Either way, if you want more than 20 watts out of your R1225 repeater, you should either get an external amp or go find a nice used MTR2000. I'd also say if you're trying to crank more than 20 watts out of your duplexer - what are you talking to? Portables at 4/5 watts? How far can they talk back in? Transmit power sounds sexy, but it's rarely the limiting factor in a repeater system.

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Another thing to note is that as the 1225 heats up it does slowly reduce TX power. (Not sure if all 1225's do this or just later units) I believe this was motorolas way of preventing the finals from cooking over long TX times but don't quote me on that.

It is important to note that the 1225 is only 100% Duty cycle at 20W. Anything higher and its only 50% duty cycle. If your talking on the repeater for more than 5 minutes out of 10, you're likely gonna kill the finals in a short time.

 

*Edit* I just saw Radioguy said the same thing, sorry for the repeat.

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PS- I just wanted to add - yes, the internal cooling fan for this unit is set to run 24/7. In addition, the rack cabinet it is housed in came with a single top mounted fan that runs 24/7, I added two more, one on each side, that run 24/7 and last but nit least, I added a fourth fan for added protection, that is connected to a separate thermostat and will trigger at a preset temperature. Hopefully, this is enough to avoid or at the very least, minimize the chances of overheating.

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Also remember there is loss thru a duplexer. Cheaper the duplexer the more loss. Depending on how it was tuned I have had them come in the shop with almost 3 db of loss. That's half your power. The only true way to know is to have your duplexer swept and see what the loss is on it. If its got high loss or off frequency that can cause the PA to heat up and drop power. Quick way to test is to drop the radio to low power and see if it does same thing,. As other said you dont want to run any 1225 on high power if you want it to last. I've replaced hundreds of PA's over the years. All my GR1225 run at 25 watts max. My low power units go into TPL amps.

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Greetings kb2ztx,

 

Thanks for your response and information. Since I lack the equipment, knowledge and skills to achieve much of the service items listed by all who responded, I am in the process of attempting to find a qualified local technician with affordable rates and plan on having him/her/them perform a full system check in order to confirm everything is programmed and tuned for optimal, safe, successful and reliable operation before relocating the system to a new location. This will not only provide peace of mind for me, but will also hopefully provide the certification required for installation at the hospital reassuring the host authority that my equipment meets factory specs.

 

Again, a huge thank you to all who responded. Now on to my next post.......... I told y’all there would be a “next time” lol

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