Re: ARK4 Final

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From: Gary L L Surrency (gsurrency@juno.com)
Date: Sun Oct 05 1997 - 20:52:01 EDT


The MRF476 is a rather delicate device. However, it also has between 15
to 18 db of power gain. So it is real easy to drive, or overdrive. Since
the breakdown voltage of the various Vcbo, Vceo, and Vebo ratings are all
pretty small, you need to exercise some care when using non-resonant
antennas or tuned feeders. I prefer to use a MFJ-259 antenna analyzer to
pre-set the antenna tuner (that's antooner tenner for Nils) so that the
rig never transmitts into an unknown reactance / resistance.

Having two TAC-1s and one ARK30 at this QTH, has educated me on the do's
and don'ts of MRF476s. If you are subject to mis-treat your MRFs this
way, you may be better off with an NTE235 as a substitute. It has very
nearly the same power gain and drive requirements as the '476, but is
quite a bit more forgiving to mis-matched loads and has higher breakdown
voltage ratings.

In my TAC-1 for 40m, I scuttled the wimpy heat sink provided in the kit,
and mounted the MRF476 on the underside of the PCB with the heat sink tab
thermally fastened to the aluminum bottom plate extrusion by means of a
mica insulator and thermal grease. A nylon 4-40 screw holds the '476
tight against the bottom plate, and can be tightened thru the original
hole in the PCB that was occupied by the original heat sink mounting
screw.

The MRF476 is in exactly the same orientation as when it was mounted
above the PCB, except its leads are bent upward rather than downward.
After drilling the bottom plate and threading it for a 4-40 screw, some
thick thermal grease will hold the mica washer where it needs to stay
before carefully sliding the PCB with the newly mounted '476 into
position.

A little of the length of the nylon 4-40 screw must be filed off, so the
transistor and screw is simultaneously slid into place over the new
mounting location. Holding the PCB and aluminum plate upside down helps
the PCB slide into place without disturbing the mica washer. The package
of the MRF476 *must* be tightly against the bottom of the PCB before
soldering its leads, so there is enough clearance to avoid disturbing the
mica washer / thermal grease assembly.There may not be enough clearance
for a thermal pad instead of a mica washer.

The net result is the entire bottom plate is now a heat sink, and you can
run key down all day a 5+ watts output and never fret about the thermal
condition of the '476 tranny. Of course, high VSWR antenna loads or no
antenna load will still cause the collector voltage to rise high enough
to destroy the '476, so care is still required for those conditions. S&S
provides no protective zener for the collector of the PA, but I've only
lost one '476 myself - and that was due to an un-noticed antenna switch
that was in the wrong position.

Moral: Always use an antenna analyzer (or noise bridge) to pre-tune the
antenna, and double check the antenna switch and connections *before*
transmitting. Your signal will be better, and your PA tranny will be safe
too. :-)

72,

AB7MY
Gary Surrency
Chandler, AZ (Near Phoenix), QRP-L #571, AZ ScQRPions, ARRL VE


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