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GR1225 Issues - Help Needed

This forum is for discussions regarding System Infrastructure and Related Equipment. This includes but is not limited to repeaters, base stations, consoles, voters, Voice over IP, system design and implementation, and other related topics.

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berkinet
Posts: 26
Joined: Fri Jan 06, 2017 11:30 am

GR1225 Issues - Help Needed

Postby berkinet » Fri Jan 06, 2017 12:04 pm

I have a GR1225 H5157B I am trying to get on the air. When it was received it exhibited a problem noted by several others: It would start to transmit and then immediately cut out. I fixed that by removing, cleaning and resoldering the two caps on the M25C24 - one was clearly cracked free of the solder pad. While I had the radio open, I also desoldered all 6 of the tabs and the two resistors. Now the radio transmits. But, only at a maximum of around 7 watts, regardless of the power setting. In the RSS "coarse transmit power adjustmen"t changing the High Power setting produces 5.8 watts at "1" and 7.2 watts at "127" while the low power settings go from 6.8 watts at "1" and 7.2 watts at "127."

Clearly something is wrong. My guess is the power transistor, but it could also be the two resistors or maybe even a logic problem.

BTW, The RSS info shows information consistent with the model number:
    Model: Name: Radius R1225
    Model Number: M44GRC90C2AA
    Band: 444 - 474 mHz
    Watts: 25 - 45
    Modes Allowed: 16

Any ideas, hints or pointers gratefully accepted.

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motorola_otaku
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What radios do you own?: 2 turntables and a microphone

Re: GR1225 Issues - Help Needed

Postby motorola_otaku » Fri Jan 06, 2017 2:46 pm

UHF R1225s were notorious for blowing the PAs, especially the 40W units. They're not continuous duty cycle-capable. Heat was always the culprit, and if it looks like components have desoldered and slid around on the board that is most likely what happened.
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berkinet
Posts: 26
Joined: Fri Jan 06, 2017 11:30 am

Re: GR1225 Issues - Help Needed

Postby berkinet » Fri Jan 06, 2017 4:59 pm

motorola_otaku wrote:UHF R1225s were notorious for blowing the PAs, especially the 40W units. ...

Thanks. That is clearly what happened. Now my question is " would a blown final cause the low power behavior I am seeing, or do they fail 100% when they go?

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jackhackett
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Re: GR1225 Issues - Help Needed

Postby jackhackett » Fri Jan 06, 2017 5:11 pm

UHF R1225s are notorious for people thinking the PAs were blown, but most I've seen were repairable by resoldering. The transistors in them are remarkably tough. Biggest problem spot is the base tab of the output transistor, gets burnt solder underneath of it sometimes. Also the caps and coils in the output low pass filter get cracked solder joints.

Not sure what the 6 tabs and 2 resistors mentioned are.

berkinet
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Joined: Fri Jan 06, 2017 11:30 am

Re: GR1225 Issues - Help Needed

Postby berkinet » Fri Jan 06, 2017 6:10 pm

jackhackett wrote:UHF R1225s are notorious for people thinking the PAs were blown, but most I've seen were repairable by resoldering. The transistors in them are remarkably tough. Biggest problem spot is the base tab of the output transistor, gets burnt solder underneath of it sometimes. Also the caps and coils in the output low pass filter get cracked solder joints.

Not sure what the 6 tabs and 2 resistors mentioned are.
I was referring to the six contacts on the actual M25C24 transistor and the 2 capacitors that the three tabs below the transistor and the two resistors that connect the two tabs above the transistor. I did remove the existing solder and resolder everything around the final transistor. However, I did not check the surrounding coils and caps.

Note that before doing that the radio would transmit for a brief period, like 500ms, and then drop carrier. After resoldering it would transmit, but at a very low power level and the power level could not be adjusted in the RSS.

So, my question is still: is that limited power likely to be a blown final, a logic problem, or something else?

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jackhackett
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Re: GR1225 Issues - Help Needed

Postby jackhackett » Sat Jan 07, 2017 7:59 am

Ok, I don't know why but I was thinking of 4 tab transistors, those ground tabs kind of merged in my mind :)

Those aren't resistors though, they're ceramic chip caps on the base (input) and clamped mica caps on the collector (output).
You can usually get away with re-soldering the chip caps, but sometimes they need to be replaced if they've been damaged by the heat.

I usually carefully lift the base tab and clean up the connections, what happens is from temperature cycling the solder under the tab cracks, creating a high resistance and arcing, which creates more heat, which melts the caps right off the board.

Also check the coupling cap between the driver and final, those will get bad solder joints as well.

And go over the low pass filter solder joints, they get hairline cracks in them.
If re-soldering doesn't fix it you might have a bad transistor, could be the final, could be one of the others. Unless you're using it out of band it's not likely to be a programming/alignment problem.

berkinet
Posts: 26
Joined: Fri Jan 06, 2017 11:30 am

Re: GR1225 Issues - Help Needed

Postby berkinet » Sat Jan 07, 2017 2:03 pm

jackhackett wrote:...check the coupling cap between the driver and final, those will get bad solder joints as well.

And go over the low pass filter solder joints, they get hairline cracks in them.
If re-soldering doesn't fix it you might have a bad transistor, could be the final, could be one of the others. Unless you're using it out of band it's not likely to be a programming/alignment problem.

That was what I was looking for. Thanks! At this point, I am thinking it is a better bet to just return this to the seller and look for another box.


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