1c. Re: output mods for 350xl
    Posted by: "Jim Strohm" n6otq@yahoo.com n6otq
    Date: Tue Mar 27, 2007 1:08 pm ((PDT))


--- Arthur Trampler <atrampler@mchsi.com> wrote:

> > Jim,
> > 
> > Try here:
> > 
> > http://www.eham.net/reviews/detail/961
> > 
> > I think this will give you what you need.

Thank you, Art.

For the rest of us, here's the text description of the
mod.  That link has some more interesting info, and is
worth a look.  

========

The replacement RF output transistors are a matched
quad of 2SC2290: http://www.rfparts.com/2sc.html

< moderator note: today's price for MQ set is
USD$87.80, which is not excessive IMHO >

Eliminate 300pf base-emitter caps on each PA
transistor and add 1000pf silver mica directly across
secondary of each PA input transformer (some rigs used
1200pf caps on PA input transformer secondaries in
place of 300 pf caps on PA transistors - still go to
1000PF). Place PA temp. compensation diodes (there are
2) directly on top of center PA transistors using
silicon heatsink compound for good thermal bond. Add
1uf leaded chip cap from PA bias feed to ground on PA
board. Add 1uf leaded chip cap across the temperature
comp. diode lead to PA board ground where it leaves
the PA board. Set keyed PA current to 2 amps with no
RF drive. Add negative feedback to each PA transistor
base to collector using 1uf leaded chip cap in series
with 3 paralleled 120 ohm wirewound resistors. This
adds just the right amount of low frequency negative
feedback to properly stabilize the PA. Set the ALC
trimmer cap. (on back of rig) on 10M with 50 ohm dummy
load connected directly to rig. Proper setting will
result in ALC limiting output to 100W with front panel
ALC control set fully clockwise. 12 o'clock setting
will result in ALC coming in at 150 watts out on all
bands. Be sure to run proper ALC setting so you don't
damage grids of any linear amp connected to rig.

The proper plug and play power supply output
transistors are a quad of 2N5686. I purchased 5 for
$15- new on ebay. Set supply to 13.5V out.

<moderator note: data sheet at 
<http://www.onsemi.com/pub/Collateral/2N5684-D.PDF>
This is a 50A, 80V super-duper 2N3055 and as long as
you take note of that Mighty Mouse current rating, you
can substitute.  Disti prices each on these exact
replacements are nearly as high as the 2SC2290 finals,
which IS excessive IMHO>

The power supply power transformer secondary has 3
equal paralleled windings. Split them up and feed them
to 3 separate 35 amp 600PIV bridge rectifiers on a
small heatsink mounted inside the power supply case.
Add a very small low noise muffin fan inside the power
supply and position it to draw outside air in through
the stock side vent slots and blow air across the
rectifiers and the power transformer. Change power
supply wires to #6 stranded wire. 

These changes will result in the PA outputing 180
watts on 160 and 10M and 300 watts 80 through 20M. You
can also add WARC bands to the rig with 3 crystals
tied through the factory 10 position aux. band
switching. You can figure the crystal specs from the
manual. You cannot add 60M.

<moderator note: the 350 will tune 5 MHz on aux
crystals.  So if you added an external crystal
selector you could add the necessary frequency control
required for 60M, but you would also have to observe
the 50W ERP limit.  Since a 350 can easily be tweaked
into 300W out, I would say that attempting to operate
on 60M is strongly discouraged with this rig. 
Remember, 60M is also USB and has a narrow audio
bandwidth limit.  One reason that Atlas radios sound
so great is that they have a slightly wider transmit
audio than is legal for 60, and you would have to
filter this back. It would be easier and probably
cheaper to buy a modern semi-QRP rig that is 60M
ready.  Save your 350 as a classic.>

More on 60M:
<http://www.arrl.org/FandES/field/regulations/faq-60.html>

I'll try to get this info moved into the files section
in the next few days, but no promises.