TEN TEC OMNI-V Final AMP MRF458 -> 2SC2879 works OK.

Discussion in 'Radio Circuits, Repair & Performance' started by AG6JU, Feb 10, 2012.

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  1. AG6JU

    AG6JU Ham Member

    TEN TEC OMNI-V Final AMP MRF458 burned out several days ago, so I replaced them with 2SC2879, readjusted bias, it worked OK, except at 10 mtr band it put out 75 watts stead of 100 watts. for info just in case somebody have same problem. MRF 458 is difficult to obtain. I did not made any change in circuit, except readjusted Bias to Factory Spec. of TEN TEC for MRF458 which is 500 mA collector current for Both Transistor.
  2. KA5IPF

    KA5IPF Subscriber

    You might try about 200-250MA bias current. The data sheet uses 100ma for a single device. Kenwood uses 250 for a pair.
  3. AG6JU

    AG6JU Ham Member

    thanks, I readjusted to 250 mA for both 2SC2879. Yaesu FT-757GX1 which use same 2SC2879 also specified 225 mA for both. according to datasheet 2SC2879 has more dissipating power capacity and current. hopefully 2SC2879 is more durable. I burn out MRF458 when I put full power into badly matched antenna. I guess TEN TEC use current limiting method , stead of SWR method to protect Final AMP.
  4. KE3WD

    KE3WD Ham Member

    Guessing doesn't work in this game very well.

    The truth is that TenTec specified their own power supply design that incorporated a fast-breaking magnetic circuit breaker on the 13.8V line, set to open when specified current draw was being exceeded.

    When the antenna isn't matched, more current is drawn and the breaker would shut the radio off.

    The manuals for these radios will tell you that, very explicitly, actually.

    When using any other power supply with these wonderful old and good rigs, one should purchase a proper magnetic circuit breaker (AIRPAX, etc. brands) that will cut the power when specified operating current is passed. I forget offhand what that figure is, if you don't have the manuals, a call to TenTec for advice from their technical staff should clear that up, matter of fact they used to offer a circuit breaker specifically for that, likely the same one used in their supply for a replacement part.

    There is no SWR foldback design in this power amp. It relies on that fast breaker instead.

    However, the upside of that is the cleanliness of the PA design.

    Great rig. I used to just love the QSK CW those things can do.

    Failure to incorporate that breaker in the powerline almost always results in blown finals someday.

    Bet you can figure out how I can unequivocally state that last line *grin*...

  5. W9GB

    W9GB Ham Member

    Niko -

    The Toshiba 2SC2879 is a well known RF transistor as well ....
    The difference between the Toshiba 2SC2879 and the 2SC2879A is that the “A” version is RoHS compliant,
    and uses an Aluminum Nitrite insulator (color gray) on the header vs. Berillium Oxide (color white).
    The "A" part will have a RED dot after the "9" in die stamping.

    This Toshiba part sell for $33-$35 each at RF Parts.

    The Motorola MRF-458 was also used in the Ten Tec Omni VI+ HF transceiver.
    Al, W6WQC provided these insights 10 years ago (December 2002) on the TenTec forums:

    The Motorola MRF-454 is indeed a suitable replacement for the MRF-458 transistors as reported originally by W6LX. Both parts use the 211-11 package.
    The MRF-454A and the MRF-458A are respectively identical electrically BUT are in a different package. DO NOT use these "A" parts.

    BTW, M/A-COM continues to produce the MRF-454 !! The M/A-COM MRF-454 RF transistor is a current production and "off-shelf" part.
    Mouser has them for $55 each (cheaper in qty) with over 100 In Stock !!

    Mouser Part #: 937-MRF454

    IN ADDITION, RF PARTS stocks both the Motorola and M/A-COM MRF-454 ($43-$45) and Motorola MRF-458 ($24) parts !!
    The 1983 Motorola data book shows that the MRF-454 is a somewhat more rugged transistor than the MRF-458.
    The MRF-454 is spec'd at 20 amps continuous collector current and a power dissipation of 250 watts while the MRF-458 is spec'd at a collector current
    of 10 amps and a dissipation of 175 watts.

    Both have an identical gain of 12 dB, power out of 80 watts and 50 percent efficiency.

    The MRF454 has higher maximum Vceo and Vcbo ratings.

    The MRF 454 has a higher gain at 30 mhz: 90 watts for 5 watts of drive while the MRF-458 is spec'd at 70 watts for 5 watts of drive.
    The 1994 data book lists the MRF-454 as a direct replacement for the MRF-458.
    Last edited: Feb 11, 2012
  6. AG6JU

    AG6JU Ham Member

    thanks for information, I did not know TEN TEC 's power supply has special type of circuit breaker. I bought my TEN TEC OMNI-V as used in QTH and power supply did not come with it, and I use 100 AH deep cycle marine battery and radio shack 2 A power supply to charge the battery along with 60 watts solar cell as power supply, I have 25 A fuse inline to TEN TEC from Battery, but I guess it may need to be 20 A or probably Fuse does not act fast enough to protect final transister. MRF-454 would may been a better transistor choice than 2SC2879, I got my 2SC2879 on ebay for about $35 each. well knowing those information , I suppose I just have to be careful to tune with lower power into my antenna ( random wire indoor ANT, tuned with MFJ manual tuner MFJ 962D ) , people in QRZ know a lot of technical info, and good place to ask questions. 73 AE6ZW
  7. AG6JU

    AG6JU Ham Member

    there is a actually a few things I don't understand well according to how wide band amplifier such as used in Most of HF radio work,
    #1 Circle #1 on parts of schematic of attached picture , I suppose C-13, C-14, R-16 form some sort of Negative Feed Back to stabilize or lower the gain of higher frequency ?

    #2 Circle #2 C-18 ( 1000 pF ) C-19 ( 560 pF ) along with primary coil winding of collector output transfer, it make tuned circuit ? as LC resonate tune band pass circuit ?

    #3 Circle #3 R-17, R-18 ( 3.9 ohm ) , do they exist to lower power to drive ? since datasheet of MRF458 says input impedance of real parts is about 1 ohm, it seems most of power coming from previous driver stage is consumed in those 2 resisters ( R-17, R-18 ) , am I right ?

    RF power amp  and PA BIAS board question A.jpg
  8. KE3WD

    KE3WD Ham Member

    #1, the circuit is an Integrator. The two caps in parallel equal one .02 value. Integration is a form of negative feedback that does not include any DC component. The caps plus the R of the circuit will allow negative feedback to occur only below a calculated frequency. You might think of this as an active lowpass filter. Stops oscillations from happening at harmonic frequencies above the ones of interest.

    #2 Your own answer is workable here. Again, we see two caps in parallel to achieve the sum of the two values. Here they are being used as a snubber, if you will, bypassing harmonics above a calculated frequency from being introduced into the xformer primary.

    #3, Notice that the two 3.9 ohm zitters are on both sides of the differential. Since they are in parallel, they form a resistance of half the 3.9 ohm value. Placed in series with the Base-Emitter junction of each of the two transistors, well, whenever we see series resistance, the first thing we should think about is "current limiting" although clever designs such as this often incorporate more than one "feature" at the same time.

    These answers are horrendously simplified, I'll leave it to those who love to overcomplicate things in an effort to show off their knowledge to lead you down that primrose path *grin*.


  9. AG6JU

    AG6JU Ham Member

    thanks, I think I sort of understood how that amplifier circuit works, I will experiment more with the circuit , perhaps by changing some value to see what happen.
  10. KE3WD

    KE3WD Ham Member

    Be very careful, matter of fact, I wouldn't recommend that kind of experimentation with your
    TenTec rig's PA amp.

    Disaster can happen in less than 1 milliSecond with a solid state amp.

    Instead, I recommend that you peruse Application Notes on SS RF power amp designs such as can be found at the Communications Concepts website, the Motorola Application Notes are still likely the king of the hill on this subject. Or at least a fine starting place.

  11. W9GB

    W9GB Ham Member

    Helge Granberg, K7ES/OH2ZE (SK, 1996)

    Helge Granberg – the "Father" of high power HF solid state amplifier design.
    Granberg was an RF Engineer with Motorola and was responsible for the seminal Motorola "AN’s" (Application Notes) and "EB"s (Engineering Briefs) on this topic.
    Anyone with an interest in solid state RF amplifiers must read these documents, which can be found here:

    Attention in particular to: AN-758 and EB-104
    All of the solid state amplifiers are based on Granberg's 1960s-1980s work at Motorola.
  12. N8WWM

    N8WWM Ham Member

    I'm wondering if you could use a device that is rated similarly to the stock device power output, but rated at a higher frequency. If so, the 10 meter issues would likely go away. I built a 6 meter amp years ago and used an MRF 245 because I had one on hand and didn't know any better, and it worked peachy. That was a class C amp, but boy did it stay cool working 6 meter FM DX. I think about a third of my e-skip contacts in the 1990's were with that amp.

    73 Doug
    Last edited: Feb 13, 2012
  13. N8WWM

    N8WWM Ham Member

    Glen referred to the AN and EB Moto notes.....WOW, memories. I drooled over the ideas in my first RF Data Manual. Back when Radio Shaft was a decent company they sold that publication for 4.95 (1978). I bought one then and never did anything with it until licensed in 1992. Here's the funny part. The PC board layouts were not to scale and I drew and etched my board for a 2 meter amp as if they were. Using the specs in the manual I tried to get an 80 watt 2 meter amp going with striplines that were probably workable at about 220mhz. Once I actually had a transistor in my hand and looked at the size of the mounting flange versus the size of the hole in the board I took a giant step back. The pages in the Radio Shack version of the manual had to be enlarged to 123% to be correct.

    My Elmer had stopped messing with VHF amps after the 2N5591, and just chuckled at me. He said that 5894 tubes didn't have that problem.
    Last edited: Feb 13, 2012
  14. W9GB

    W9GB Ham Member

    Substituting the MRF-454 for the original MRF-458 used in this TenTec OMNI HF transceiver series would address that specific issue.

  15. AG6JU

    AG6JU Ham Member

    can you tell me , what type of trouble I should be looking when replaced with MRF-454 ? may be adjusting Bias would be different ? might require impedance re-matching ? output impedance ( both real and imaginary ) parts of MRF 454 seems to be lower than MRF 458. ( at least at 30 MHz )

    perhaps , I should experiment with turn ratio of Output transformer ? as well as input transformer. I might able to find better over all match, through out entire frequency band.
  16. W9GB

    W9GB Ham Member

    Well the MRF-458 was only 28 USD from RF Parts, so it was cheaper than the 2SC2879 part you used or the MRF-454.

    You can look at the specification sheets to compare ... many of the characteristics are identical or similiar to MRF-458.
    Whenever you changeout the active component -- you need to measure and propery adjust the bias.

    Last edited: Feb 14, 2012
  17. AG6JU

    AG6JU Ham Member

    yes, I have adjusted BIAS to 250 mA which many other radio set that value uses 2SC2879. the 2SC2879 amplifier works fine, so far. if I would known I can get MRF 458 for $24, I would choose that option, but now I already bought pair of 2SC2879 and installed. Those Motorola note and engineering bulletin are great articles. thanks for many information to help me understand how those circuit works. AE6ZW NIKO
  18. KE3WD

    KE3WD Ham Member

    The little bit of extra RF power output makes little difference in the real...

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