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Author Topic: 2SC2879s exact drop in replacments for MRF-454?  (Read 11863 times)
KU7I
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Posts: 305




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« on: May 12, 2015, 12:44:50 PM »

Planning ahead in the event the MRF 454s bite the dust in the MA1000B. An older e-ham thread indicated the 2SC2879s can be used in lieu of the original MRF454. Does anyone know if this is true or verified? Is there any bias circuitry that would have to be evaluated and possibly changed or are these true exact replacements for the MRF 454? Thanks in advance. Lane Ku7i Newport RI
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N7BMW
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« Reply #1 on: May 12, 2015, 12:57:21 PM »

I have a TenTec Hercules II that uses '454s.  When they were discontinued TenTec went to 2SC2879's but I believe they made bias circuitry changes to use the '2SCs.  TenTec has good telephone support and they might answer that question for you.
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KD5YAQ
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« Reply #2 on: May 12, 2015, 01:38:11 PM »

They ARE NOT an exact replacement. the main issue before even going into the gain differences which I am sure is what most folks will pay attention to is actually the input and output impedance being different which will require a few circuit changes. Next is the amount of bias applied which will directly affect the linearity and idle current. Last, of course, is the gain, and the different amount of excitation / drive required.
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KU7I
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« Reply #3 on: May 12, 2015, 02:01:30 PM »

Looks like RF Parts is selling the MRF454:

http://www.rfparts.com/mrf454-ma.html

Lane
Ku7i
Newport RI
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NM3G
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Posts: 75




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« Reply #4 on: May 13, 2015, 04:19:42 AM »

The MRF454 is still in production; DigiKey shows 43 in stock at $46.05 each (qty 1), and $44.875 (qty 10). In addition, M/A-Com has 2070 in stock. RF Parts is also a good source (my first choice).

DigiKey, Richardson RFPD, First Source Inc, Microwave Components and Mouser Electronics are all listed as distributors.

(Source: http://www.macom.com, enter MRF454 into the "Start Searching ... " window)

73,

Rick
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KD5YAQ
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« Reply #5 on: May 14, 2015, 10:12:51 AM »

the original MOTOROLA MRF454 has not been in production for some time. The same with the Toshiba 2SC2879. There are replacements or equivalents (not to be confused with subbing one part for another which the original poster mentioned) but the replacement MRF454 and the 2SC2879 (made by DEI) are also not "exact" replacements but they will work. Features on die, like emitter ballasting, and the way the junctions are designed are different along with the amount of junctions. Remember we are taught that a transistor is a transistor. This is not true. Some of these RF output devices are actually MANY transistors on one die to make up one transistor. So with that said, they are and can be made different. For instance, the original Toshiba 2SC2879 was much more rugged than the replacements available today. It could handle over voltage and also SWR at different phase angles where as the newer ones cannot without going "poof". Also the IMD performance for a given amount of output is different as is the HFE (gain) and the idle current requirements for linear operation. Now, once again, you can make the circuit work with them as a replacement. But you do need to tweak the input and output impedance a tad, and also make sure that idle current is correct so that you are operating the transistor in a linear and desired fashion but at the same time not pushing it towards class A which creates a lot of heat and in some cases instability and oscillation.
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KM3F
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« Reply #6 on: May 14, 2015, 11:29:37 AM »

For the term exact replacement between the two is no.
The 2sc is higher gain and some parameters are a bit different.
Example:
I used a matched pair in a 2m amplifier as bolt in.
The two differences for that application was Idle current and negative feed back component values needed to be changed to prevent self oscillation.
Otherwise they work ok in this application.
Just be awhere if anything is not working correct and see what it takes to correct it.
Good luck.
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KD5YAQ
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« Reply #7 on: May 14, 2015, 08:09:04 PM »

You used 2SC2979s in a 2M amplifier?Huh!!
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KM3F
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« Reply #8 on: May 15, 2015, 11:19:08 AM »

Yes sir.
Good luck.
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KD5YAQ
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« Reply #9 on: May 15, 2015, 01:26:57 PM »

Alright...how when they are rated up to 30Mhz MAX! I could see wven trying to pus hthem to 35-50Mhz, but 144? No way! They would have no gain at all and not only that but the internal structure would never work. We can't be talking about the same device here. Even an SD1446 is rated upwards of 54Mhz and will work on HF as well as 6, but much higher and it becomes unstable.
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KK5DR
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« Reply #10 on: May 15, 2015, 02:58:47 PM »

It should be pretty easy. The only thing that will have to be done is to set bias on each pair.
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KM3F
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Posts: 909




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« Reply #11 on: May 15, 2015, 09:46:56 PM »

You have to look at the specs for them.
They are rated to nearly 200 mhz. so 144 is within working range.
The Kenwood TS 480 uses them all the way to 6m.
As I mentioned I put them in an old Mirage 2m amp that is still working today.
Gotta check your references.
Good luck.
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W3RSW
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« Reply #12 on: May 16, 2015, 05:57:23 AM »

2sc2879's not 2979's causing some confusion?
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Rick, W3RSW
KD5YAQ
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« Reply #13 on: May 17, 2015, 03:30:06 PM »

We are discussing the Toshiba 2SC2879, which is an HF bipolar RF power transistor. Rated up to 30Mhz.
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W3RSW
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Posts: 606




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« Reply #14 on: May 18, 2015, 05:54:08 AM »

Quote
KD5YAQ
Member   
RE: 2SC2879s exact drop in replacments for MRF-454?
« Reply #7 on: May 14, 2015, 08:09:04 PM »
Reply with quoteQuote
You used 2SC2979s in a 2M amplifier?Huh!!

Then this was a typo?
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Rick, W3RSW
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