5300 100W Amp - How it Works

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kj5kb
Posts: 29
Joined: Sun Nov 08, 2009 10:33 pm

5300 100W Amp - How it Works

Post by kj5kb » Tue Mar 02, 2010 2:47 pm

Interesting...

The 100W radio starts off as basically a 50W radio, MRF247 (75W rated) final.
The 50W goes out of the main chassis to the amp chassis.
The 50W goes through a 2-way 3dB hybrid power splitter/combiner, now you have 2 x 25W outputs
25W from each side of the power splitter goes to 2 x 6dB gain 100W transistors SD1477....so now you have 2 x 100W outputs.
the 2 x 100W outputs are combined with another 3dB hybrid to make a 100W output.
The 100W goes BACK into the main chassis for T/R switching....then back out the same as in a 50W unit.

So we're making 250W of RF just to get 100W. What a waste. No wonder the 100W ones take a gajillion amps!

Looks like it would be pretty easy to 'downconvert' a 100W to a 50W, too.

So anyone have a clue why it was designed this way?

Coupla thoughts:
-50W radio was already designed...easy to do the splitter/combiner "thing" from the existing 50W output rather than redesigning it ?
-Soft fail...one final dies, you still have ~50W ?

kj5kb
Posts: 29
Joined: Sun Nov 08, 2009 10:33 pm

Re: 5300 100W Amp - How it Works

Post by kj5kb » Wed Mar 03, 2010 1:41 am

More thoughts...

Assuming a 100W CAN be converted to a 50W...it would save approx 15 AMPS on transmit as well as a lot of space/weight.

The PA is 200W...200W / 14volts = 14.285A. Nothing is 100% efficient, so figure it takes at least 15+ amps just to run the 100W PA.

kj5kb
Posts: 29
Joined: Sun Nov 08, 2009 10:33 pm

Extra wire?

Post by kj5kb » Sat Mar 06, 2010 10:00 am

In addition to Power and RF in/out, there is 1 wire near the center of the 100W PA that goes back to the main chassis.

SWR monitor? Bias?

Anyone have a manual for the 100W handy?

I'm curious...should I decide to attempt a downgrade of a 100W radio...can i just disconnect and cap it?

willbartlett
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Location: Boston, MA

Re: 5300 100W Amp - How it Works

Post by willbartlett » Tue Mar 23, 2010 11:10 pm

Done this a half dozen times.

Remove both semi-rigid lines inside the main radio body, and remove the bias/switching line (late, tired, don't remember which it is) and then jump across the spot on the RF out trace with solderwick, or a piece of brass shim stock correctly sized. Then you need to decide whether you want to shorten the rf out cable, or just leave it long. there are some screws holding the extra PA to the main deck. Remove them and the radio gets half as long.

No need to reprogram for power, it'll just go as it is. might be a little low at the bottom end as the power calibration was done for power after the 6db stage, and it's not linear in gain over frequency.

Will

kj5kb
Posts: 29
Joined: Sun Nov 08, 2009 10:33 pm

Re: 5300 100W Amp - How it Works

Post by kj5kb » Thu Mar 25, 2010 2:07 pm

Thanks Will.

Wasn't sure if the extra wire could be removed without angering the main unit.

I have no need for more than 50W anyway, and the lower power consumption would be well worth it.

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