Another Amp to look at

We received some stock of the new KL503 broadband (well a little) mosfet amplifier this week. 8 little irf520’s in a near row I thought. I was wrong, there were 9.

It has one driving 8. Disaster. max input is only about 10 watts PEP. how can we drive this with our 40 watt radios. Bugger bugger bugger. I thought I was buying 2ea kl203’s in parallel and instead I ended up with this!!! I don’t own a 12w single final CB. What were RM thinking of when they made this? Maybe they wanted to hold the output down to protect the 8mosfets in the final from overdrive by sacrificing the driver?

It looks like a kl500 except it is blue. Works OK. it put out 300watts and that was about it. but that is still pretty good for the price. Drawing 24-25 amps on SSB to do that. Nice enough and cheap enough but I wish they had done away with that driver section. When will they come out with a cheap 40-50W input amp?

Plug is nice and easy to use…..

About David Donaldson

64 and Live in Jakarta, Indonesia. like to ride my bikes.
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28 Responses to Another Amp to look at

  1. dd182003 says:

    Well tonight I thought I would run through the amp and see what coverage it had on HF. This is one weird amplifier….. Bottom line is it can work down to 3mhz!!!
    But it does have it’s issues.

    The RM Italy website states:
    “KL 503
    1-6 W Input power
    250 W Output Power
    20 – 30 MHz all mode
    26 dB Antenna preamplifier”

    This all makes good sense and lines up with what we also found with our earlier tests on 10m. The amp will actually do a little more than 250w which is good…Good to see RM under stating the capability of their amplifiers.

    I didn’t want to break anything so I decided to test with 5w rtty signal and hooked up a radio with SWR meter, the amp, swr/power meter and dummy load.
    Started at 29mhz and worked my way down using number one (low) on the power selector switch.
    For 5w in we saw almost 50w out. Turning the power selector switch around to level 6 or High saw 220w out on the meter.
    I was actually looking for SWR at the radio but saw none.
    I kept dropping the frequency and at 22mhz the output had climbed to 60w. Still no SWR ….Hmm Kept reducing the frequency and by 17mhz we had 100w and still no sign of any input SWR.
    14mhz showed 110w out and no SWR with 5w drive.
    12mhz showed 160w out with no swr and 9mhz gave 200w out for the same 5watt rtty signal in. Still with selector on low. 9mhz had 200w out on level one. Time to see what this amp can do. Tested with selector in positions 2 to 6 with same input and the output remained constant. 200w. The only change was that the input SWR rose on level 6.
    8,7 and six mhz showed similar results. 5mhz seems to do something to this amp! 450w on the meter with no input swr (1:1) and 5w drive.
    I told you it was weird. 4mhz had 150 out and 3mhz had 300.

    Conclusion: The amp can be run down to 3mhz as long as you keep it in low and the drive at 5w. For those with FT817’s and want the cheapest way to increase the power this would have to be it. Is it clean? Who knows somebody else needs to look at that but there are some strange things going on in that input circuit and without that 5w input driver transistor limitation this might be a pretty good amp. it does seem that something strange is going on in the input section with those weird high readings in low and the inability of the power selector circuit to alter the output at different frequencies. The circuit seems similar to any number of broadband amps we have seen with the toroid layout and splitter combiner circuits but in this case they have arranged four transistors to a side instead of the normal 2 bipolar transistor in the more conventional push pull amp circuits. Certainly saves money on toroids when you build like that!
    Next step is to try it out on air and see what it sounds like

    • Dave in CT says:

      Nice job…
      This is the first place I could find with real pictures of this amp. A friend smoke a few of the irf520’s but it still worked a little. I think it has a burned resister or 2 as well. I wish I could find a schematic but I will enlarge your pictures to see if I can read the color bands. I found the irf520 for 45 cents each online but don’t know the shipping cost yet.
      I figure after 9 of these and a couple resisters, if it doesn’t work I’m not out much for trying..
      Merry Christmas!!

      • dd182003 says:

        The IRF520 will work but it will not develop quite as much power as the ones RM use which I have since heard are: MS1307. I have no idea where you can buy these mosfets but they are out there.

      • rfguy297 says:

        I called the International Rectifier Sales headqurters in the US. They do not manufacture a MS1307 N-channel mosfet, PERIOD. These devices (now showing on EBAY) are being manufactured using the IR marking somewhere in Asia (guess that means they are IR fakes-not surprisingly). Note that all sales for them originate in Asia! Not one reputable US semiconductor et dealer offers them as International Rectifier product. (Allied, Newark, Mouser, etc) Also, looking through the International Rectifier website, they only show manufacturing facilities in the US. (But I find this unusual in the current economy) Regardless, the Asian made parts do seem to work for cheap CB amplifiers, so consider. It would however be nice to know the “REAL” manufacturer and have a full specification sheet on these mosfets. And yes, they are not IRF-520’s.

      • dave says:

        Wow…I have 9 of the IRF520 coming in. I sure hope they work. Thanks for the tips RFGuy297!

        Have you used the IRF520 in place of the MS1307 in this amp?

      • dd182003 says:

        I have not but I have seen the results of one who did and it worked but output was about 25% less than the original RM3 mosfets.
        You can buy the originals direct from RM Italy as well.
        But rest assured the 520’s will work

      • dave says:

        My friend doesn’t want it back so if I can get it going with a little less wattage who cares right? It will be a beat up box anyway. I just need to find out what value a resistor is. It looks like most of them are 10ohm 3 + 5 watt flameproofs..
        Thanks for the replies.

      • dd182003 says:

        post a pic of the damage and I can post one with the exact detail for you. The boards are in the cupboard still.

      • dave says:

        I already bought the real cheap .45 cent ir520’s and they should be here today.
        When I get home tonight from work I will post the pics of the burnt up amp. I don’t want to order them from Italy and pay more for everything. I figure it was given to me and for $10.00 I will give it a shot but I don’t want to spend anymore. If it doesn’t go well for me someone else can have a go. 🙂

      • Dave says:

        It was a 3 watt 10 ohm resistor.
        Well guys sorry but I bought 9 of the ir520’s before I took the board out. I was able to take out a couple of the transistors but one was smoked really bad and fell apart. It burned into the board and destroyed the traces around it. I don’t have any of the trace repair copper tape and I’m not going to waste anymore time on it. If anyone else is interested in it. they can have at it. I will give it away for postage if anyone wants it.

      • Dave_in_CT says:

        I forgot to post that this blown amp has been sent to the first guy that asked for it.
        I have received several emails about it..
        Sorry Guy’s I forgot to post it gone..

  2. rfguy297 says:

    Did you actually verify that all of the Mosfets were in fact IRF-520′s? I want to order some spares for this amp, but have been un-able to get any info from RM on what is actually inside. They seem to be “hiding” technical details.

    IRF-520′s are very inexpensive, as I’m sure you already know.

    (Apologize for posting this in the wrong “Busy, Busy, Busy” post)

    • dd182003 says:

      No RM won’t admit what they are bt there isn’t anything else they can be! If you write to RM italy they will sell you the rm3 for a price but I am 99% positive they are IRF520. Just make sure you buy enough to get 8 matched ones and the 9th singlr one is probably a 520 as well.

    • dd182003 says:

      go ahead and use the irf520’s !

  3. greg burbage says:

    i recently purchased a 2011 kia soul. i wanted to put my galaxy dx959 {modified} my texas star 500 and my 102″ whip in it. with all the hassel of fishing coax and power wires, and the small amount of room, i decided to use a magnum 1012 10-12 meter hanheld with the 12 volt cig connector with the pl 259 antenna connector. i beefed up the wattage as not to distort the tx and i use it to drive the new kl503p. i use a 102 whip with spring and groundplane kit on a mag set up. the radio and amp matched to a tee. it created a super portable set up for full time mobile use or rent a car situations, or pack it up and take it on vaca. i get a good 200 pep am!!! im currently trying to wire a turbo echo mic and a astatic non amplified noise canceler.. i sacrificed some but i can still dx and talk. im impressed with the 1012 hanheld. its a magnum 257 in a small package. 73’s sandbur

  4. Pingback: 250 watt difference btwn power supply&battery? - Page 2

  5. rfguy297 says:

    No, I have not yet had a need to replace any of the RM originals….I keep input power at 4 watts, and the amplifier seems OK with this. No over heating problem, and good output power. I just like to have some spare’s on hand.

    Now then, here is another twist on this subject of the RM/aftermarket replacement mosfets. In an e-mail chat with a friend in the Netherlands, he voiced the opinion that local consensus seems to be that the MS1307 devices being offered on EBAY are in fact IRF-520’s that have been RE-PRINTED by the Chinese to say “IR and MS1307” In this way they can get away with saying they are genuine IR manufactured product. My friend also stated that some have purchased these Chinese remarked replacement parts on EBAY, and also found them to perform sub-standard to the original RM mosfets in power gain.

    Good luck. The mystery seems to continue. Only RM knows for sure, and they won’t be giving you any details.

  6. Very nicely said.I discovered your publish from Yahoo and enjoyed reading it. Have you been writing for long?Just the other day I recently set up a blog myself and its been a enjoyable process. I’ve met some new people since then but it is tough sometimes! Anyway, many thanks for your post!

  7. tony says:

    I would love to have the plans to build a a 7530 mosfet amp. Build my first two mosfet then I can cascade upward. Interested in the 50 voter as well. Show me

  8. Mike Daniels says:

    The kL-503 hd are on the way to United States it’s the 503 in high drive form to where you can run 40-50 watts into them

    • dd182003 says:

      Thats about time. Hope they removed the driver and didnt just pad the input with resistors.

      • Mike Daniels says:

        Not sure I ordered on last week it’s on it’s way I saw a picture online can’t tell the difference at all looks exactly the same so maybe they did what you hope they didn’t do

  9. Mike Daniels says:

    I guess after 8 years they listen they have the had version out now just made it to the states kl503hd 35w Input for about 175.00 just ordered one on its way we’ll so how it is, honestly I liked the kl503 I previously had not one problem about 325w on am with a 3 watt input , it’s a nice looking amp also , it’s a good design cosmetically when you strap a radio to it .

  10. Be interesting to see what they have done to the 503hd. The 203 was such a sweet sounding 100w+ amp for for the price I always wondered why they didnt combine 4 of them for an easy 500w at 40w input.

  11. Need to see a photo of the circuit board

  12. Check out the kl703. 4ea kl203 in one box. 16 little mosfets! Hey Suckers..I got me a 16 pill amp here…gonna blow you retards away!

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