UHF Radius Transmit Dies

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kcbooboo
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UHF Radius Transmit Dies

Post by kcbooboo »

I've been using a 25w 2ch Radius as a repeater transmitter, running about 6.5 watts out, feeding an external Henry amplifier. There is a fan blowing a lot of air on the Radius heatsink and it's running basically ice cold. After the repeater has been transmitting for 20-30 minutes, the output signal would sometimes just die. Unkeying the input signal and letting the repeater completely come to rest would allow the transmitter to function again at the next keyup, often for another 5-10 minutes. I replaced the Radius and the power amp on separate visits and the problem has not returned.

Today I put the Radius on the bench, aimed a fan at it, hooked it up to a dummy load and wattmeter, and keyed it up. It was drawing exactly 3 amps from a 14v supply and making 6.6 watts output. After about 30 minutes the output power, and input current, dropped in steps of 1-2 watts every 15 seconds or so, eventually ending with basically zero current and no output signal. If I unkeyed the radio and keyed it right back up, it drew 3 amps and still output 6.6 watts for maybe 5 minutes, then started dying again. Once the power and input current go down to zero, they stay there until the radio is unkeyed or powered off. The heat sink is still ice cold, by the way.

Without the fan, it does get warm after a few minutes of transmitting, but I don't think I want to run it that way for any extended period of time. 40 watts in, 6.5 watts out, the remaining power has to go somewhere.

I have not opened the radio to check anything yet. I don't think I have any way of determining if the RF board is losing drive, or the power amp is being shut down. I'm fairly sure that when I put this radio into service I did a complete (well, as complete as I could) calibration and alignment. The power amp's power output was adjusted over the full frequency range. About the only settings I did not play with were those for deviation.

My mind is open to any Vulcan mind-meld suggestions that may come along. My next step would be to swap either the PA or the RF board and try to isolate it to one or the other, although I suppose the logic board, which controls the power output, could also be at fault. I do have full manuals and lots of test equipment, so don't be shy about suggesting something.

Bob M.
Nand
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Post by Nand »

Maxtrac and Radius M216 type models monitor several items to calculate and control the output power. According to the manual, the computer monitors the final collector current through a small current sense resistor located on the final board and it also measures the radios temperature with circuitry located in the reference oscillator compartment. Both of these measurements and the key down time are used to calculate the safe power level for the PA and adjust this level if needed. Unkeying and rekeying starts the process all over again.

The GM300 series operates the same way, but it uses a temperature sensor located at the PA.

My guess is that by having the output turned down, the computer gets information that it can’t deal with. Or you just have a defective component in this area. Motorola suggest in one of their manuals that the power control circuitry is difficult to trouble shoot.

Nand.
OX
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Post by OX »

Could it possibly be your power supply crow barring? You say the current and wattage both fall to zero. What about the voltage? If the voltage sags (say > .5 VDC) while transmitting then I would suspect an over-loaded power supply or a defective power supply.

We run across this problem with security products all the time. Over-loaded or over-dutied power supplies cause all sorts of problems and they tend to last for a period of time while in over-load but eventually shut down. The newer supplies will self reset once the load is removed.

Just a thought.
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Post by kcbooboo »

I probably should have clarified the power supply situation. It's an Astron RS12 set for 14.0 volts output. There's one meter on the front panel, switchable to measure volts or amps. When the Radius dies, the current just drops slowly to zero while the supply voltage is still at 14.0 volts and the radio is still stuck in transmit with the front panel lights indicating the same channel and status. The radio was doing the same thing when it was at the repeater, but there the power supply was an RS35 with significantly more power capacity. Also, the current Radius radio at the repeater has been working flawlessly.

Well I know that the power amp is NOT getting hot, so there must be something else going on, or being sensed. It does seem like the power drop is being done in discrete incremental steps over a short period of time. This is the only radio that I've had at the repeater that has done this. I don't recall if the one that's currently in service is a 25 or 40 watt Radius, but I do know I exchanged the two multi-channel units for two-channel radios bought specifically for this purpose.

I'll have to read Nand's post a bit more carefully and also check the schematic at some point to understand what's going on.

Thanks for the thoughts, however.
Bob M.
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jim
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Post by jim »

...just a shot in the dark, but did you check the feedline, antenna, connections and lightning arrestor to see if they are breaking down under RF load? If this radio is capable, it may be possible that the protection circuit is backing the power down as it "sees" the VSWR change when RF is applied and something in the antenna system heats up and breaks down. I know you reproduced this with a dummy load, but at this point, it might be worth checking.
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Post by kcbooboo »

There's another Radius currently running at my repeater and it does not have this problem. I pulled the 2ch 25w radio after we had several long morning QSOs with several transmitter dropouts. We have had similar QSOs with the replacement radio in operation for the past two months.

The bad radio seems to operate the same on the bench as it did at the repeater, in that the power is reduced in steps to zero, after it's been transmitting for 20-30 minutes. And the heatsink is completely cold from end to end as I have a small fan blowing lots of air on it. There is a 5-pin logic board in this radio, so age may be a factor. I think a complete alignment of the unit would be my next step, followed by monitoring the power controller input line to make sure that the power is dropping as a result of it being told to do so, rather than some other component heating up and making the power go down. But from the stepped lowering of output power, I'm inclined to agree that the radio thinks it's getting hot and is backing the power down as a way to cool itself off. Whatever is sensing the temperature is obviously in error. Also I'll try heating things with a heat gun, especially the area around the reference oscillator.

By the way, this radio's RF output is only 6.6 watts because it is driving a Henry C10D70 amplifier at the repeater site, and this is all the power I need to get 60 watts out of my duplexer. Since replacing the radio, I've also replaced the power amp; it's now a Henry C10D100R amp, but still getting the same amount of drive that the previous amp got, because I forgot to bring the wattmeter with me when I replaced the amp. Current draw is just a bit more than with the old amp, so I figure I'm still well within recommended operating parameters. But these changes have nothing to do with the original problem that the Radius is shutting down.

Bob M.
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Post by jistabout »

Hey Bob, I know its been almost 2 years, but did you ever get this figured out? If so, what was the problem? Just curious......
Aww screw it. I didn't wanna fool with it anymore anyhow.
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Post by kcbooboo »

The concensus is that MaxTracs and Radiuses have an algorithm inside that cuts the transmitter power back after 30-60 minutes of transmitting, thinking that the PA must be getting hot after being on this long, even with a fan keeping it cool. It's just something that they do, and we can't disable this "feature" unless we knew enough about the codeplug and firmware in the radio.

The ultimate "cure" is to use something besides a MaxTrac or a Radius as a repeater transmitter. In my case, I put in an MSF5000 as a repeater and that IS rated for continuous duty.

By the way, GM300s actually do sense the PA temperature, so they might work longer if the PA is kept cool, but who knows what evil lurks in the minds of Motorola !

There was another discussion about the temp sensing on these radios, but it seems to have subsided.

Bob M.
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Post by EKLB »

Dont recall exactly where i saw it but there is a wire around the timer in the radio that you put a potentiometer in place and then set it for the drive to the pa for the wattage output you wanted.

Obviously you unwired the other driver circuit and put in the potentiometer from another part of the circuit that eliminated the timed tx circuitry.

Edited this in = Believe this could be the answer if you did the same as they do with the maxtrac.

Http://www.repeater-builder.com/maxtrac ... ntrol.html

EKLB
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Post by kcbooboo »

And if you look closely enough at that article, you might notice that by a strange coincidence, the author and I are the same person !

Yes, that should keep the transmitter running even after the CPU thinks the PA's getting hot. Of course, the CPU can do other things to make the radio stop transmitting, if it really wants to.

At the time I was having the problem, I didn't know about the manual power adjust modification.

Bob M.
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re: MAXTRAC Manual RF out adjust

Post by kato56 »

EKLB wrote:Dont recall exactly where i saw it but there is a wire around the timer in the radio that you put a potentiometer in place and then set it for the drive to the pa for the wattage output you wanted.

Obviously you unwired the other driver circuit and put in the potentiometer from another part of the circuit that eliminated the timed tx circuitry.

Edited this in = Believe this could be the answer if you did the same as they do with the maxtrac.

Http://www.repeater-builder.com/maxtrac ... ntrol.html

EKLB
I saw this article and I'm wondering if anyone knows of the same mod for a 2-channel Maxtrac 100 that's being used as a link radio. By looking at the pictures in the article, my 2-ch. radio's board looks different and without a schematic of either radio, it's hard to determine where to install the pot, and what components to remove. BTW, my radio's (with a fan) transmitter will "shutdown" when keyed continuously in about 5 minutes.

Thanks
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Post by xmo »

"...without a schematic of either radio, it's hard to determine where to install the pot..."
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Spent the 30 bucks - buy a manual.

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Post by kcbooboo »

If you have the model number of the logic board, I have a manual and will check to see if it can be modified per the article. There are four different logic boards listed: three have 5-pin accessory jacks, and one of those is a very early design.

5 minutes is terribly short. Are you sure you don't have the transmitter's time-out timer in RSS set for 300 seconds? I always set mine to zero. I know the radios get hot, but with a good fan blowing over it, and the power cranked down to about 1/4 of the rated output, they stay cool for nearly an hour.

Bob M.
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UHF Radius transmit dies

Post by Blitzbug2u »

The Astrons are rated peak--A PS12 is rated about 8-9 A continuous--what is the fuse value of the radio ? Could be the power supply is folding back on rxcess current draw. Just a thought---Blitz.
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