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Author Topic: RM 40 20 meter mod needed  (Read 507 times)
KC2KJ
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« on: October 17, 2004, 01:13:33 PM »

I bought an RM electronics linear amolifier model RM-40 at the Sellersville hamfest today. Works ok on 10 meters but output power goes down to 5 watts on 20, from about 15 watts on 10 meters when driven by a Yaseu FT817 (about 1.5 watts). Does anyone know of mod to restore power down to 20 (or 40/80) meters?

Thanks
Mike
kc2kj@mac.com
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K7KBN
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Posts: 2763




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« Reply #1 on: October 17, 2004, 02:05:44 PM »

It's a two step mod.

Step 1:  Toss the RM in the trash.

Step 2:  Buy an amplifier designed to operate in the ham bands.  One with band switching, filtering...little things like that.  Things that CB amplifiers don't have.
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73
Pat K7KBN
CWO4 USNR Ret.
KC2KJ
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« Reply #2 on: October 17, 2004, 03:26:55 PM »

Well, I only paid $5 for it. Would like to use it, perhaps with an output filter if harmonic output is too high. I cant find any data on the transistor. The RN spec sheet says its a MOS RM3. Does anyone have specs on it?
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W8JI
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« Reply #3 on: October 19, 2004, 04:28:27 PM »

The problem is you bought a CB amplifier that was never designed to be linear, or to be used on the Ham bands.

You don't even know what output device it has, and don't have any idea what the signal quality is. Odds are virtually 100% even if the power was up it would have significant splatter. Putting an output filter on it might cure harmonics, but it sure won't cure splatter.  

In all liklihood, you'd almost have to gut the thing and start over. That would require you understanding how solid state PA's work and being able to design one.

What you should really do is get a copy of "Solid State Design for the Radio Amateur" and look at the sections on power amplifiers. You'll learn something useful, have pride in what you did, and have a clean signal if you follow the design suggestions in that book.

I wouldn't toss the garbage out, I'd save some of the parts and build something good on the chassis.

73 Tom  

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KC2KJ
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« Reply #4 on: October 21, 2004, 06:47:22 PM »

To follow up, I measured the amps 2,3,4,5 harmonic. The highest level harmonic on my spectrum analyzer was the third ,about 3 db or so below the fundamental 10 meter frequency as measurred into a dummy load. I suspect a resonant antenna would provide some filtering, but the amount of energy at harmonic frequencies was quite astounding. Glad I only paid (and lost) $5 on this little amp.

Mike
kc2kj
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W9GB
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« Reply #5 on: October 24, 2004, 04:55:03 PM »

I have heard they are made in Italy and someone bought a boatload -- look on eBay they have been there for a number of months.

Just like that great diet pill, did you lose weight?

I would scrap for raw parts -- you may get your money back that way.

gb
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W9GB
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« Reply #6 on: October 24, 2004, 04:58:35 PM »

Here is a good design, by Virgil Stamps K5OOR
http://www.hfprojects.com/projects.htm

Drive requirement: 1-2 watts RF (internal 3dB pad)
Power Output: 35 watts RF output (nominal) 160-10 meters; gain of approximately 15 dB
Spurious products: -40dB or better @ 35 watts
Harmonic content -45dB or better @ 35 watts
Filter Selection: manual 6-position rotary switch; 160, 80, 40, 30-20, 17-15, 12-10 meters.

A six-position Low-Pass Filter Module. The function of the module to attenuate harmonics from the transmitter to be below 40dbC on all HF bands, 160-10 meters. All of the parts including the rotary switch are mounted on a small circuit board.
Notice the number of torroids for filtering each band !

gb
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