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MMIC's replacement for blown downconverter transistors

At 04:25 PM 7/8/02 -0800, Edward R. Cole wrote:
>You know I've read these queries on here and wondered why a mmic would not
>be a good replacement device in these converters?  In particular thinking
>of the MAR-4 or -6 which operate just fine at VHF and require no RF ckts.
>Just 50-ohm input and output.  You can bias them to chose the gain you
>want, as well.  I'm using two MAR-6 IF-amps at 144 MHz in my home-built
>1420 MHz radio telelscope.  I get 20 dB gain using 12v supply (and
>appropriate resistors).  Mine are built as separate units in die-cast boxes
>I bought from Downeast Microwave (for you North American hams).  In fact, I
>bought EVERYTHING needed for $35/each!  Looks like the small boxes they use
>for their preamps.  The mmics are something like $5/each.
>I do not use the Transystem AIDC3733, so confess not knowing much about
>them (other than I read here).  Maybe the mmics are not appropriate...maybe
>one of our experts wishes to reply?
>dos centavos de AL7EB


Theoretically the use of a MMIC as a replacement for the IF amplifier 
transistors ought to work quite well. The only slightly tricky part would 
be conjuring a new matching circuit between the output of the mixer and the 
MMIC so as not to add noise to the overall system.

  I have successfully built lots of pre-amps for 144 and 435 over the years 
using MMIC's. The other tricky thing about MMIC's in my experience is 
making sure you shield the inputs and outputs from each other to reduce the 
potential for oscillation and noise.

If I hadn't come across a cheap source of replacement's ($1 each), I was 
definitely considering "converting" my 31732 to use a MMIC in the IF block. 
Financially it doesn't make much sense, but the educational and self 
satisfaction value are priceless..(.for everything else there's mastercard 8^)


"A man who picks a cat up by the tail learns a lesson he can learn no other 
way" .... Mark Twain

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