[NLRS] question of transverters vs radios

[Gerald]

On Wed, 2006-01-11 at 03:39 -0600, Phil Lefever wrote:
> 
> At 03:48 PM 1/9/2006, you wrote:
> >    Here's  my  situation.   I  have a 706mkIIG with no special filters or
> >    features, a FT736R with just 144 and 440 and no special features and a
> >    1.2GHz  transverter  by  MM.   Basically  I  have 6, 2,  and .7 meters
> >    covered with ssb and 23 cm if I ever get it hooked up.  I was thinking
> >    of buying a 222 MHz module for my 736r at a cost of $375 from ebay.  A
> >    similar  transverter  from  DEM  would  be ~$400 but require alot more
> >    effort to interface it.
> 
> Mike,
> 
> Personally I own a DEMI 222 transverter and I have used a 736R on 220 for
> a couple contests.
> 
> The particular 736 I used didn't perform well, I suppose there could have
> been something amiss with this unit. My issue was hearing spurious signals
> through the band at times. The W0ZQ beacon, which is REALLY strong at my
> house, could be heard all over! The transverter has never had this problem.
> 
> As John pointed out, you could buy the transverter kit and interface for
> less then the $375 price or the rare-as-hens-teeth 736 module. Yes that is
> in kit form, but like John I found the assembly to be quite easy and there
> was almost no tuning. It seems that many of us seem to be shy about building
> kits these days, from time to time we all need to remember which end of a
> soldering iron to hold ;) Interfacing the transverter isn't nearly as hard
> as you might think too. Once you interface one transverter its easy to
> just switch the IF to add more silver boxes too.
> 
> With the age of the 736 and the availability of modules decreasing I'd
> likely work to find a more current solution myself.
> 
> 73
> 
> Phil, KB0NES

The 736 on 6m, 2m, and 220 has a fundamental design flaw. The balanced
mixer is terminated with a crystal filter instead of a broad band
termination. According to Phil Maas the mixer guru, that can cause 30 dB
poorer intermod and up to about 10 dB added conversion loss. With only a
single double gate mosfet before that mixer the radio can't stand that
added conversion loss. My mod which needs better documentation and
before and after intermod and sensitivity checks inserts a MAV-11
between the double balanced mixer and the crystal filter to get in a
broad band termination. The 11 dB added gain from the MAV-11 and the
improved termination seems to make the receiver come alive. Now there's
far less difference between MDS and AGC threshold (I measured that at
about 30 dB before modification) but I'm not sure about the intermod. I
still have to run a double cavity filter to keep out channel 13 TV,
especially with the antenna pointed south at the 2000' foot tower 20
miles away. Without two good low phase noise signal generators I won't
be able to run conventional intermod tests, but I can check for blocking
range with a manual noise figure set up and one good signal generator
which I have and I can check that at various spacings which is my plan
someday when I get enough bench cleared to hold NF setup and the 736.
Right now a handheld would slide off the pile onto the floor so there's
work to do accessing the workbench.

I posted a long harangue several years ago on WSVHF about the bad design
of the 736 receiver front ends. Yaesu knew better, the earlier FT-767 2m
module is designed right (and performs quite well according to reports
I've seen).
-- 
73, Jerry, K0CQ
Dr. Gerald N. Johnson, electrical engineer
All content copyright, Dr. Gerald N. Johnson