Antique 2 x MRF475 Linear Amplifier

[443]

Found this in my junk box today. This has been floating around in my junk box since like 1992. I forgot I even had it. I think I originally had the entire casing and heatsink that went with it.

Looks like a 2 x MRF475 linear amplifier with thermal tracking (via diode under board see bottom side pic) bias circuit. Though a resistor or Choke for the Bias Circuit appears to be missing.

What I would LIKE to do with it, is forever, I wanted to build a small low power compact amp that would perform well from at least 80-10 meters to go behind my Softrock RX/TX, which has only 1 watt max output. So I'd need a high gain amp. I built a single MRF455 amp and biased it all to hell at .7V and it works OK, but this looks like it would work better for my needs. Original Motorola Datasheet on this MRF475 (they both test good BTW on handy old Micronta neon bulb transistor tester) suggest that it will do around 15W with 1W input (per transistor) and also suggests that the gain is quite high in common emitter config down to 2.0Mhz.

Anyone have a complete one of these or a schematic of the original? Would I be way better off building it as a 2 x IRF520 unit?

Pics:

 

[443]

Also just a note. I am sure it had sil-pads under the MRF475's, so the tab is not grounded, it is hot, is the collector, the board has vias in the screw holes that hold down the MRF475's. The original builder simply cut the center leg off the transistor, and used the tab as the collector.

 

[kopcicle]

The closest I get is ...

Ignore the driver section for the most part and see the output of the tip31 as the voltage derived from d2 .

Obviously the input and output transformers are missing. I'd mess with it as a useful bench tool when I needed a bit more from , well , anything.
As a daily driver of any kind, not so much. The world has passed it by.

 

[443]

Ignore the driver section for the most part and see the output of the tip31 as the voltage derived from d2 .

Obviously the input and output transformers are missing. I'd mess with it as a useful bench tool when I needed a bit more from , well , anything.
As a daily driver of any kind, not so much. The world has passed it by.

Thanks, yeah the transformers are around here somewhere. If not easy to replace them. For my use, I just need something to bump that 1W output up to about 10-14W PEP for my Softrock RX/TX. I mostly use it for digital mode work. My single MRF455 amp I have been using with it work great 20M and up, but below that I can't seem to get much output from it. I'd like to use this little MRF475 board as a driver amp for a 2 x SD1446 or 2 X MRF454 board in same housing/casing to pack around and use with the Softrock. The MRF455 amp wouldn't be so great for that because in order to get the gain up where I wanted it, biased it high, and it puts out on 20M/17M/15M quite a bit more power, too much for a driver in the config I want it in, like 20-50W out on those bands, but falls flat below 20M. It may be the transformer material, I haven't tried changing them or testing them, they were just pulled straight out the junkbox, no idea what material they are. I'd like to completely tear down that amp and use the case/heatsink, etc for something else anyway.

 

[kopcicle]

 

[443]

I have an old 2 X board like that, but it's from Maxtronics I think from like 1993, never used it. But it's also got provision for relay, and everything else as well. Who makes the board pictured above? I've seen them used in a lot of Ham projects, but never bothered to remember where they got them from.

 

[443]

 

[kopcicle]

but never bothered to remember where they got them from.

http://www.communication-concepts.com/an762/

 

[nomadradio]

Could this be the board for Lou Franklin's linear?

Looks similar to the pic on this page, but not identical.

http://cbcintl.com/tripower.htm

73

 

[443]

Could this be the board for Lou Franklin's linear?

Looks similar to the pic on this page, but not identical.

http://cbcintl.com/tripower.htm

73

Yep that's it, slightly newer revision.

 

[443]

http://www.communication-concepts.com/an762/

Cool, I see they still selling them. PCB is $14.00, transformers already done are $15.70. The metal can transistor/IC thing they use for the bias circuit is a metal can version of the 723 Regulator? Never seen one LOL.