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900 Mot. PA SGTF1020B

This forum is for the discussions targeted at converting various models of Motorola equipment to operate in the 900MHz Amateur Band.

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Joined: Sun Nov 25, 2001 4:00 pm
What radios do you own?: Too many

900 Mot. PA SGTF1020B

Postby PETNRDX » Sat Jan 05, 2008 9:47 pm

Got one of these in a round about way from the person selling them on Ebay.
Sort of surprized that it works quite well.
Ran basic tests prior to making some modifications suggested on the VHF South Website referenced on Ebay.
I can get near 150w out with 28 vdc, and drive around 4 watts on 927.
Seems to me that 28 v is a bit high for these.
Still get over 100 watts with 24 v with 4 in, and for appx 1 watt drive I get around 30.
The one I have has a single RF board attached to the heatsink, and a small single stage circulator that is also attached to the single RF board.
The one the Ebay seller references on the VHF South website has a "stand alone" circulator fastened to the heatsink connected by coax jumpers.
And that Website states that the circulator won't work at 927.
Seems to me that it also states this model PA won't work at 927, but there was hardly any TX out difference between 902 and 927.
I am pretty sure that the circulator is WAY off its design specs, but the power output, and current draw are in specs, so I am going to use it for a bit and
see how well it works.
The heatsink gets warm after several minutes of Tx at 60 watts out.
But not hot enuf that I have a concern.
So, heres the questions.
Anybody familiar with these?
I have a couple of 28 V nomial supplies, but none that will go as low as 24 (Ferro-resonant) anyone know if these will take 28v?
Anybody have experience removing or bypassing the internal circulator?
I kinda hate to remove it since it is integral to the power control, but I have several dual port isolators for actual protection,
so I could take out the circulator, use an external dual port only, and drive it to only around 60 watts.

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Re: 900 Mot. PA SGTF1020B

Postby n1oty » Sun Jan 06, 2008 5:49 pm

A group of us have a number of 900 repeaters either actually up or in construction. We have modded a few of these with the circulator on the board. We leave them in, run the amp off 24 volt regulated supplies, limit transmitter drive to about 3 watts in order to achieve about 100 watts out of the amp. We do add a cooling fan to the heat sink of the amp as a precaution. They work great in repeater service.


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Joined: Sun Nov 25, 2001 4:00 pm
What radios do you own?: Too many

Re: 900 Mot. PA SGTF1020B

Postby PETNRDX » Sun Jan 06, 2008 6:37 pm

Sounds good.
That was what I had been hoping.
Thanks a lot for the reply.
I am going to continue using it at about 60 watts out then.

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Re: 900 Mot. PA SGTF1020B

Postby digus » Thu May 29, 2008 7:43 am

Hello All,

Just found this forum - a bit late I suppose. I have a few of these amps that I had originally bought to setup a secondary data link to an IRLP repeater. The seller accidentally sent a mismatched "A" model, so I decided to experiment with it. I did bypass the onboard circulator/isolator on the "A" unit, but I have no way to test the results. The circulator in these is the usual stack of magnets and brass rings. I removed the 4 screws on top and removed everything inside, soldered a solid copper wire across the input and output of the circulator and replaced the lid and screws. The circulator body itself is soldered directly onto the amp's heatsink and would be almost impossible to remove without damaging the unit. Anybody know of a cheap way for me to get into 900Mhz so I can test my modifications? Anybody want to test them for me? I'm not sure if the original data link plan is going to happen now or not...


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