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the MRF173 80W 28V module with 150W transistor can i ?

 
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the MRF173 80W 28V module with 150W transistor can i ?
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bayleaf
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Post the MRF173 80W 28V module with 150W transistor can i ? Reply with quote
as above i have one of the above modules used it for some time , sadly the MRF173 failed , now i had to hand a SD2931-10 150w DEVICE , same case as MRF but rated at 150w at 175 mhz , now i fitted this into module works but gets hot , even when running on the 28v psu that we used for the MRF 173 i measured the gate voltage it read 3.3v seems to run at about 1.5v gate voltage and not get too hot but with 1 w drive and 28v supply there is only 5w out put so what should gate voltage be ? i also note this device can accept 5w drive would higher drive ,higher supply volts and less gate volts cure my problem ? , i notice the 150w PCS module is almost identical but uses a SD 2941-10 what would be the differences ?
sorry for all the questions i just dont want to blow this SD2931-10
Thu Sep 04, 2008 8:58 pm View user's profile Send private message
pcs
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Post Reply with quote
Can you please show us some photos of how your setup looks
and how you connected coaxial cable, heatsink and fans.

I have seen some horror images from users of these
modules, I hope you connected everything correctly.

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Marko - PCS Electronics
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Thu Sep 04, 2008 9:06 pm View user's profile Send private message Visit poster's website
bayleaf
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Post Reply with quote
yes i will try to take photo but cant tonight , but setup is same as used when we had the module running with the MRF173 that ran sweet as a nut did not break into a sweat while it was drawing 4amps at 28v , with a 1w drive and cooling fan direct onto heatsink , assuming all that is correct is this transistor able to be used in this module what should gate voltage be ? and can we drive it with 4 w ? the drive board is a *Ads not allowed* kit board that you can switch between 1 and 4 w ,
Thu Sep 04, 2008 10:22 pm View user's profile Send private message
pcs
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First, do you realize palet amps are not supposed to be used like that?

You have to do a minimum of these things to any palet amplifier:
1. Add heatsink that is a lot bigger than the pallet
2. Add fan (ok you did this)
3. Add output low pass filter for harmonics

The next problem I often see is soldering coax wire. For
some reason many don't grasp the concept of keeping all
wires short at radio frequencies. This means soldering coax
to the pallet in a way that minimizes inductivity of the leads.

I have recently received some photos of installed palet amp,
installation was done by "telecommunications engineer
with many years of experience- his own words". I regret not
keeping it, it is a very good example for all the things you should
never do. There was no heatsink. To make matters worse,
the palet was not even screwed to the aluminum enclosure,
instead it was suspended in air on the top of a long screw.
There was no fan, coaxial cable had practically inches of
excessive lead length. Just method of wiring the coax probably
caused a loss of 20-40Wof power. Of course the final died
and guess why the palet overheated.


So why are palets produced like that? Palets are mostly used
by system builders, they are typically combined into bigger
amps. For example, 4x300W into 1KW amp, all four placed
on one big aluminum block.
To combine palets like that you have to keep phase of individual
amplifiers as close as possible. Extra filters would cause big
phase differences and make combining impossible.
So the palets are first combined together and than there
is just one filter to clean the signal at the end.

Regarding your case, another transistor probably means
your bias voltage is wrong, possibly causing excessive power
draw. Also, some transistors are less efficient or may not
work as efficiently in the circuit designed for another
type of transistor. Another problem are potential spurious
emissions caused by using another final. It would be good
to check that amp with spectrum analyzer.

Hope this helps you and others looking at palet amps.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Thu Sep 04, 2008 10:38 pm View user's profile Send private message Visit poster's website
bayleaf
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Post Reply with quote
ok thanks for your reply , just a quick update , had to laugh at the discription you gave of that set up , thankfully that DOES NOT resemble mine , i have used big heatsink big fan infact two fans one blowing throgh heatsink one blowing air down wards onto top of board , used short patch cables between driver , amp and yes the lowpass filter rated at 150w, back to the question of why mine was getting hot well it seems to get LESS HOT when driven with 4 watts than with 1 watt at present it is running with gate voltage set at 1.91 v , o/p is around 30watts , temp is luke warm ,still only supplying it with 28v as we have not yet made a 50v PSU must check if the electrolitic caps are rated high enough as well, i have not got the board in front of me to check as it is on site in use but im sure you would know if any components would need uprating , i am in the electronics industry as an engineer but not normally into RFengineering i am only doing this for a mate as a favour that is mistake number one ,
the spec sheet for that transistor says absulute max 6v gate voltage , there is no way this would take that in this board so is most likly down to cicuit design as you said , sady i dont have a spare thousand or so quid to purchase a spectrum analyser although i would love to have one , thanks for help any other helpfull hints are welcome
Sat Sep 06, 2008 11:13 pm View user's profile Send private message
pcs
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Post Reply with quote
Keep an eye on quiescent current, that is current with zero drive.

You should be able to find a typical value in datasheet and set
your amp to that value.

You're probably too deep into linear mode, causing overheating
even when there is no drive.

You could also be oscillating at some frequency until sufficient drive
is present (4W).

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Sun Sep 07, 2008 12:03 am View user's profile Send private message Visit poster's website
bayleaf
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Post Reply with quote
yes the quiescent current was just over 100ma with no drive and supply at 28v , which i thought was about what i would of thought it should be ,so if it is osclating it is only when some low level drive is applied , thats where the SA would come in handy but for a one off repair its not viable , think im going leave it running for couple of weeks on 28v while i build the 50v PSU , graduly turn up the volts and monitor temp and gate while i do it , but just to be sure think will underun it at around 40v rather than have it flat out , the MRF173 did not seem to like 24/7 operation at 80w as that died after a few weeks , having said that the SWR was about 2:1 spec for MRF said it could handel much worse , i did advise my mate to improve SWR which i helped him to do , rather than haul aerial down to adjust that we moved operating frequency its now a 1:1 SWR , i did some experements with SWR with LPF after and b4 the SWR meter and with LPF after the meter it does make SWR go up should that not have any affect on SWR ? as with it inline it will no longer be 1:1 SWR like it would be with no LPF or am i being just a bit too picky here ?
as i said i am not an RF engineer but would it not make sence to put LPF b4 amp to cut out any harmonics b4 they could be amplifed ? and hope the amp does not osc at some weried frequncy
Sun Sep 07, 2008 9:21 am View user's profile Send private message
pcs
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Post Reply with quote
Only use LPF after the final amp, no need to put it in before that.

LPF can affect SWR readings a bit since it may not be aligned
completely or designed correctly; it can change impedance.
If the effect is small I would not worry about it.

_________________
Best regards,
Marko - PCS Electronics
--------------------------------------------------------
Turn your PC into a FM radio station!
http://www.pcs-electronics.com
fax +386 4 2316 128
Sun Sep 07, 2008 10:43 am View user's profile Send private message Visit poster's website
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