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powerband rfx 75

Discussion in 'CB Radio Modifications' started by my2995dx, Feb 19, 2009.

  1. 1342

    1342 Active Member

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    oh my god there is still intelligent life out here lol.great response thank you!


     

  2. fingers

    fingers Member

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    question on the super tune? i see that you have to add a 1n914 diode in series with a 470 ohm resistor in place of r56 old resistor but is removed for the rfx75 instructions?

    need your input
    thanks
     
  3. jazzsinger

    jazzsinger Bullshit Buster

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    There is no such a thing as power/watts rms.for a purely resistive load AVG power is taken from the product of rms voltage and rms current.

    So technically the P.E.P power is a far more meaningful measurement than any "rms power" measurement,with a pure sine wave the P.E.P power is 2.6 times the avg modulated power at 100% modulation or 4 times the avg carrier power.

    Its not about big numbers its about technical accuracy.

    As freecell has pointed out numerous times before,most semiconductor manufacturers rate their devices in P.E.P so it helps too know what the P.E.P power is if you wan't to avoid melting those rf power transistors.
     
    1 person likes this.
  4. 941Texas

    941Texas Member

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    how much drive from the radio does it take to make it work. i have a radio that does 500 milliwatts max. i wonder if that would work?

    941
     
  5. d1g1man

    d1g1man Member

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    RFX-75 on a Uniden HR-2510

    I suppose it would be possible, but has anyone done it? Or have instructions on how to do it? Thanks!
     
  6. mchaos

    mchaos Member

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    I just put an RFX75 in a cobra 29. I had some slight problems at first, but was able to work them out.

    First I would like to through out an FYI. On the RFX75 page on cbtricks, there are 2 very handy documents that can help with them not working when new. There is a pre-installation guide, which has things to look for that will cause it not to work, that you can fix before install, and a troubleshooting guide.

    I had a RF feedback problem. If I turned the gain up past half, I had issues. If any one has this problem, this is what solved it for me. I made sure to make all wires as short as possible. I grounded the audio transformer to the chassis. I also went around the radio and made sure all the ground points where good.

    Grounding the audio transformer pretty much took care of it. Might have been an issue with this particular cobra 29. Haven't seen anything else around on the net about this issue.

    Also, with tweaking I was able to get just over 150 watts pep out of mine dead keying about 23-24 watts. Its doing just over 60 watts AVG. used a 1 k tone for testing. AMC is not wide open either. I did tweak it using my mod meter but found turning it way down still produced full swing on the meter.

    I found that with out dropping the dead key produced some back swing. Did a bit of testing in the field. Had a buddy of mine key it up, with out any swing mod. It was dead keying 40 or so watts. Once I got far enough away to observe the swing, it was swinging back quite a bit.

    I found that good forward swing was achieved at around 23-25 watts. Could probably go a bit more on the carrier, but that's where I got the best forward swing without sacrificing audio. If I brought it any lower then 20 watts the audio started to suffer and got raspy. more swing, but raspy.

    At about 3/4 on the mic gain produced devilishly loud audio. Right on the cusp of too much. With a modded RK56 its sick.

    after the slight difficulties, its one mean barefoot duck stomping machine.
     
  7. Cityboy

    Cityboy Active Member

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    mchaos the original set of RFX that came out with no pre installation check list and it was hit and miss on figuring out the problems. Your dk is way to high and what voltage are you running it at. Either your meter shows happy watts or something is off because 150w is an unrealistic number for that radio or any other radio. If you run it at 13.8v and set the dk to 12-15w(max) then it will hit around 75-80w. If you installed the Cap and resistor at JP36 then remove it and run it without it, otherwise it will sound like crap. Heck for $40 and getting 4 times the PEP power out of the radio and it will sound crystal clear and awesome. If you continue to have the hum at any point then look to the driver and final in the RFX; there is a quality issue there with the soldering of the pins of the transistors to the board. I know you made the wires as short as possible but that may be a downfall to any repairs that could be made without completely removing it. As long as you tied the wires up and kept them up and away you should have left them alone and kept them factory length. If you had problems like the hum you could have backed out the 2 screws holding it in and touched up the solder points of the transistors with a pinpoint soldering iron and eliminated it possibly.
     
  8. 1342

    1342 Active Member

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    forget about those pep numbers they are meaningless the transistor in that rfx is rated at 75 watts pep but will do it in average.every transistor i have had on the bench did its rated pep power in average power.no transistor ever on my bench did its pep rating but in fact usually will do 2-3x the pep rating.i have seen many meters that were labeled rms or peak that just means avg and pep but labeled different.i have folders full of spec sheets for transistors and if you forget those three little letters (pep)those spec sheets make a lot more sense but still seem a bit off.the average joe does not have a spectrum analyzer on the bench for testing but usually have a good set of meters.i have seen that most meters are pretty accurate at measuring average power but almost all are extremely not accurate at measuring pep. as doc w5lz stated if you like the meter to bounce all over than pep is for you.
     
  9. Shockwave

    Shockwave Sr. Member

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    Is there any valid reason why an accurate power meter should not bounce all over the place with modulated AM? Consider that modulated AM is nothing more then varying the carrier power in sync with the audio frequency and the bouncing watt meter is exactly what you should expect to see. Looking at the envelope on a scope would confirm that the meter should actually be fluctuating much more and much faster. Because meters and our eyes don't respond at audio frequencies, the PEP meter is dampened to hold the last peak applied for a brief second.
     
    1 person likes this.
  10. 1342

    1342 Active Member

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    and just for kicks i took a RMS reading even though it does not mean much???????????????this is back words unless you have a scope you better acknowledge that rms reading and throw that pep reading out the door!
     
  11. Shockwave

    Shockwave Sr. Member

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    The excessive heat and inability to sustain the carrier are key signs of thermal runaway. You are right at the brink of a transistor failure and a long transmission could do it. The idea of replacing the CB finals with Mosfets is good if the bipolars are obsolete but not the best idea if you want 80 watts. Nothing in your CB was designed to cope with the currents involved here from the power cord to the modulator stage.

    Do you have any idea what happens to the impedance the modulator sees when the final stage current increases to make 80 watts? It drops several times lower then the stock impedance and causes premature flat topping with bleedover. It's much easier to add a stage of linear amplification then it is to properly get more watts from a stage that has to be high level modulated.
     
  12. 150w_Cobra

    150w_Cobra Member

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    the forum, but I would like to put my two cents in.

    I know many people don't care for the RFX, and it is true, maybe the RFX is not for everyone. But it has been my experience that these little gems are beyond awesome!!! After installing nearly twenty of them in the past month, I have received nothing but compliments from drivers saying other drivers are "completely blown away at how clean and crisp my radio sounds, not to mention I can actually get out there now."

    I have installed the RFX on several different models from galaxy and cobra. After installation on the galaxy (959 and northstar 330), I have noticed an increase from 30w pep to around 80w pep. Not sure of avg or rms. I say this because everyone uses different standards to measure there cb output. (Still not sure which is best.) But after installation on many cobra 29s, I was completely astonished by the results. On the newer model cobras (just before the lcd versions) I was registering an increase to around 150w with my carrier at 10w. Increase of avg watts to around 45. But I recently installed this on a 1991 model 29. I am getting an avg of 60w. On the pep, I am getting a 8w carrier swinging to 200w!!! I have done nothing to this radio other than install according to directions, and tuning, and I have checked for any previous mods that might have been done before installation. Nothing has been done. I can get out at least 18 miles that I have talked and been able to hear someone else over the noise, but have been told I could be heard over 70 miles away fairly clear with it not being skipped out there. Running the radio to a workman open coil antenna (30 bucks). Have received nothing but compliments on this radio, so I had to put it out there. But please, if there is something else I can do to make this a better radio, let me know. I'm using two different meter, just so everyone will know. One measure avg and pep not sure what brand. Second is a dosy with a freq counter. I will put the old cobra on the dosy tomorrow and give results off meter. I know a dosy measure rms( I think) and the other one I mainly use to check swing and my deadkey, and my avg which I do use quite a bit. I am currently working towards getting a Bird43, but have not been able to purchase one yet. Also have not had a chance to put it on my stepdads oscilloscope, but will sometime this week. Using the combo of meters, I have to say all not one of the radios I have put the RFX on have not been disappointing.

    Now that being said, I am not a professional when it comes to cb's and I am learning all the time. If I say something that is wrong, don't try to kill me. Any and all input, opinions, and insight into cb repair will be greatly appreciated. I have been in the cb field for a very short time, but really want to learn as much as possible. I do not believe in hack jobs, extra super duper macho mod BS, or other cb shop techniques. When you try to pull a quick one on a truck driver and it backfires, so does your career. But that's what you get for making a quick buck and not doing quality work. Treat others radios as your own, and perform quality work, and your name does get out there as a good hand
     
  13. Longshot

    Longshot Member

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    I installed a Powerband RFX-75 in my Mosfet final DX-959 today. After the install I set the carrier to 14w with a 2 watt dead key and it is putting out 100 watts avg with 120 watts PEP. I installed this radio in my Silverado and the first contact was a driver 15+ miles away. He said radio sounds loud and clear and he could not believe I was so far away. Cant wait to try it on SSB. I will get back with results.
     
  14. 9C1Driver

    9C1Driver Sr. Member

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    Hmmm, so did you set the carrier to 14 watts or 2 watts? Carrier and dead key are the same. On SSB you aill see maybe 60-70 watts.
     
  15. CTStallion

    CTStallion Active Member

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    I've installed an RFX-75 (Rev. 2) on an old, single-finaled, SS-3900 and am DELIGHTED with the results.

    In the past, if I wanted slightly more power out of a radio, (without an outboard/external linear amplifier) my only alternative was to install dual finals. This was a time consuming process, and has become increasingly more expensive to find a beta matched pair of 1969's, not to mention all the associated hardware & components, plus the heatsink needed for a Galaxy or Ranger chassis radio. All that to hit anywhere from 35 - 55 watts of output power on SSB. Not to mention that on the AM only Cobra and Uniden chassis rigs, (like Cobra 25's/29's, Uniden 68's/76's) the only 3 options I had were: A - a single 1969/2312 final upgrade or B - the IRF520 or 2030 MOSFET upgrade, which (again) would roughly only yield an approx. 3 - 5 dB gain in measured output power. Or C - an internal 'Stinger Board' that which would require me to remove the internal speaker in order to mount the Stinger, thus requiring me use an external speaker.

    So, I picked up an RFX-75 Rev. 2 (enhanced for better SSB operation with improved biasing) PLUS an RFX-150 over the Christmas holiday season, brought them home, and then had to decide which two rigs to test them in. Believe it or not, (mostly because "they" said it couldn't be done), I mounted the RFX-150 on my Realistic TRC-465 and am COMPLETELY ENAMORED with it. It's now a whole new radio, AND very stealthy. Also, the 2166 driver with the 1969 final in the TRC-465 offers more than enough drive for the RFX-150.
    I was tempted to install the RFX-75 in the RatShack, but I saved it (the 75) for my last remaining single-finaled Galaxy chassis: the old school SS-3900.

    My SS-3900 had already been modded to the hilt with all the typical Galaxy/Ranger EPT board mods and I was already considering adding a 2nd final, heatsink, & associated components to up the power output. However, for much less time, money, parts, and PITA Factor, I was able to install the RFX-75 much easier to this full sized chassis radio than it was to install the RFX-150 on the TRC-465.

    Results with the RFX-75 on a single finaled SS-3900 are as follows (these are conservative measurements, as the RFX-75 instructions indicate that the unit is capable of a 15 watt dead key, but I turned mine down to 10 and that really helps prevent heat buildup, thermal instability/degradation, etc.):

    AM Deadkey: 10 watts; AM Swing watts: 22 watts
    FM Deadkey: 10 watts; FM watts: 10 watts
    CW Keydown: 10 watts; CW watts: 10 watts
    AM Peak watts, 100% modulation, 1000Hz tone: 65 watts
    SSB PEP Output: 65 watts PEP

    Unit is capable of much more than this. However, as I had mentioned, I'm running it a little 'light' which seems to aid in keeping it cool. I can ragchew all I want to and it won't get hot to the touch, merely warm. Cooler actually than my RCI-2950DX heatsink, and the 2950 is only doing 35W SSB PEP, whereas the SS-3900 with the RFX-75 is at 65W SSB PEP.

    I'd have to say that this thing (the RFX-75) is truly a GEM, and solves the heat problems and/or SSB issues previously associated with running a Stinger Board internally.

    Now, to those of you who would opine to use a tiny KLM amp externally instead, I say this: for AM ONLY use, this might be fine. However, keep in mind that more often than not, you will be RF switching, and/or, capacitor delaying (i.e.: on SSB) the keying of your external amplifier, which does NOT lend itself well to either SSB or high speed CW usage. Since you're pulling 8V directly off of the PTT of the transceiver to key/unkey the RFX-75, it really lends itself well to SSB and high speed CW ops IMHO. True however, that you can develop more power with an external amp, the fuse(s) are easier to get to for servicing, and/or your amp might even offer an external receive preamp, things 'lacking' in the RFX-75. In fact, in the unlikely event you blow the fuse in the RFX-75, something I in fact DID DO upon initial key up, (as I had the output power of the SS-3900 turned up way too high initially and developed in excess of 100W output on AM momentarily before the fuse blew), you will be required to take the cover off of your radio to regain access to the RFX-75 fuse assembly.

    Now, the RFX-150 on the other hand, is a whole different animal. Because I was mounting it on a small radio, I lost my PA and External Speaker jacks, as the mounting plate for the RFX-150 is now covering them over. I installed my RFX-150 in such a way as the rig gets its power directly from the RFX-150, and so I only have to use the one set of heavy power cables coming from the 150 to power up the whole she-bang. I also mounted a subminiature SPST switch on the 8V (blue) wire energizing the 150 on the side of the TRC-465 just in front of the (already) side mounted mike jack. This permits running barefoot, or energizing the RFX-150 for high powered operations.

    RFX-150 results on a tuned TRC-465 are as follows:

    AM DK: 30 watts; AM Swing Watts: 50 watts.
    AM Peak Watts, 100% modulation, 1000Hz tone: 130 watts.
    SSB PEP Output: 135W PEP

    On both radios, AMC and ALC circuits were left in tact.

    Also of note, on both radios (the way I installed the devices), the RFX units are always 'ON', as they are always receiving 13.8 VDC down the line, whether the radios are on or off. This is a consideration you must take into account IF you install them directly off of your batteries. In this instance, you might want to place a heavy duty switch in your 13.8 VDC line. In my situation, the power source in the mobile (RFX-150 on the TRC-465) is only energized when the ignition is on.... so no problem with draining the car battery. In the shack, the SS-3900 is connected to a power supply that runs off of the AC mains that I switch OFF when not in use, thus unpowering the 13V supply to the RFX-75.


    My next test will probably be to put an RFX-75 on a Cobra 29 or 25, and you can be sure that I'll also adjust the output of the Cobra/input of the RFX such that the RFX-75 does NOT exceed 75 PEAK watts of output power when modulated. This should provide a decent amount of longevity without any thermal issues. Quite frankly, with RF being what it is, the receiving station will see absolutely NO DIFFERENCE on their S meter if you're running 60 watts, versus 90 watts of output power. Not only that, but you exceed 75 watts on an RFX-75 with a 12-13VDC power source, and you'll blow that fuse in it eventually (like I did on initial transmit test.)

    As far as SSB operations are concerned, I never had a 1st generation RFX unit to put on the scope (Wawasee Catalyzer), but I DID run SSB tests on BOTH the RFX-75 Rev. 2 and the RFX-150 and can attest to the fact that IF you moderate the SSB output power they provide a nice clean waveform. However, if you run them maxed out, they WILL FLAT TOP on you, so be warned in advance. I even heard rumors of square wave patterns, and/or insufficient SSB drive issues on the first gen 75's, but have no first hand experience with same.

    I'd conclude that the RFX-150 is perhaps best geared towards the DUAL-FINAL and/or SSB user, whereas the RFX-75 is maybe designed for the single final radio user, chiefly because IT becomes THE final amplification stage(s) of your radio. Also, to address a question posted earlier in the thread about "WHAT IF" my radio only delivers 500mW in my driver, I'd say that the RFX-75 would be your go-to choice in this instance, and certainly NOT the RFX-150.

    For ME, the 'toughest' part of the installs was drilling holes in perfectly good radios, ESPECIALLY the 3/8" (minimum) you'll need to route all the wires of the RFX-75. Likewise, the mount of the RFX-150 on a small chassis offers its own set of complex issues, so be warned.

    Good luck & happy experimenting. Looking fwd. to learning about additional experiences and outcomes with these two nifty enhancements.
     

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