WX5812
Member
I have a uhf executive mastr ii converted into a repeater. Is there any way to remove the PA and use it as a uhf amplifier?
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Mastr executive II PA
- Thread starter WX5812
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WPXS472
Member
It has been a while since I worked on one of those. But, if I remember correctly, the PA can be used independently, though it is not trivial. As I remember, there is a RCA, or phono receptacle on the PA for the RF input. It only takes about 200 milliwatts to drive it to full output, so if you are driving it from most any kind of radio, you will have to insert an attenuator to keep from destroying the first amplifier transistor. These, and the venerable MASTR II
WX5812
Member
Oh interesting. Thanks for your reply. It would be nice to get some use out of this old thing even if only the pa gets used
WPXS472
Member
I had a software glitch and didn't get to finish my reply. I wasn't even aware that it got posted. If it is a mobile PA, you will need to either turn down the power considerably, or add fan cooling. The mobile PA wasn't designed for repeater duty. A local small time two way shop had a couple of Exec II repeaters on the air for years,and they performed very well.
WX5812
Member
Yea I have plenty of fans to keep it cool. So obviously the output is already wired for a so 239. So the input is one of the 2 RCA jacks?
WPXS472
Member
It has been a while. I don't remember there being two RCA jacks, but yes, it will be an RCA jack. There is a jumper between the output of the PA and the lowpass filter. I had to resolder that one more than once because it gets pretty warm under power. It wouldn't hurt to check it before using.
WX5812
Member
Ok I will check that out. I plan on putting this between a maxtrac and antenna since The MaxTrac gets super hot if TX more than 60 seconds.
WX5812
Member
prcguy
Member
I've used Master II repeater PA's with other exciters and its no big deal. The mobile chassis was not designed for continuous duty repeater use and you have to turn down the power and use some fans.
WX5812
Member
Is there a pot for that. I didn’t see one but I didn’t look very hard either
WX5812
Member
I have an attenuator that I saved from the scrap years ago. Would this one work? It’s adjustable from 0-70db
Single Knob 50 Ohm Manual Attenuator 0-70dB x 10dB Steps DC-2.2GHz Model 50R-029 - JFW Industries
Single Knob 50 Ohm Manual Attenuator 0-70dB x 10dB Steps DC-2.2GHz Model 50R-029 - JFW Industries
prcguy
Member
That attenuator is only rated for 2 watts and its in very coarse 10dB steps. How much power does the radio put out that you want to feed into the Master II PA?
WX5812
Member
I’m found to be driving it with a Motorola Maxtrac. I’ve seen online somewhere that you can bring that thing down to 1 watt please correct me if I am mistaken
prcguy
Member
I would find out what the Maxtrac can be turned down to and still operate reliably, then you can calculate what value of attenuation you need. At 1 watt a 6dB attenuator would give you 250mW into the PA. At 2 watts a 10dB attenuator would give you 200mW. At 20 watts a 20dB attenuator would also give you 200mW but of course it would have to handle 20 watts continuous.
I think it would be better to run the radio closer to 5 watts for reliability, then use a 10dB attenuator rated for at least 5 watts to get you to 500mW, then you can use a low power 3dB attenuator and fine tune the radio power to drive the PA.
I think it would be better to run the radio closer to 5 watts for reliability, then use a 10dB attenuator rated for at least 5 watts to get you to 500mW, then you can use a low power 3dB attenuator and fine tune the radio power to drive the PA.
radioman2001
Member
There are 2 RCA jacks the one on the left is into the IPA the one on the right should go to the RX. You can bypass the whole IPA and go in to the PA at I think 5-10 watts? Easy to find out transmit and see how much power is driving the PA. That looks like a 40watt unit.
Bringing the power out down on an Maxtrac is tricky, if you have a 45 watt unit anything below 20 watts is going to give you grief, if you have a 25 watt unit you can go to about 5-7 watts.
Bringing the power out down on an Maxtrac is tricky, if you have a 45 watt unit anything below 20 watts is going to give you grief, if you have a 25 watt unit you can go to about 5-7 watts.
WX5812
Member
I’ll just play with it and see what happens. Just trying to use things that I already have and am having fun doing so..... so it already has an output to go to antenna. Can you clairify which port I need to hook the radio coax too?
radioman2001
Member
The PA output after the antenna switch is on the front of the radio chassis by the control cable. In put to the IPA is as specified above the left RCA connector.
WX5812
Member
Ok so if I go directly into the Pa through the port in the left (opposite side of antenna port) I might can go into it with 5-7 watts?
What continuous output power can this thing handle without damage for 5-7 minutes with fans on it at all times.
What continuous output power can this thing handle without damage for 5-7 minutes with fans on it at all times.
WX5812
Member
When you say the maxtrac will give me grief below 20 watts what do you mean?
Maxtracs and their friends will turn way down in power, but they seem to keep drawing the same current
and I cant tell you how clean the output is in that state
they get just as hot just as quick
not to mention the way the firmware drops the power back as the seconds tick by
and I cant tell you how clean the output is in that state
they get just as hot just as quick
not to mention the way the firmware drops the power back as the seconds tick by
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