From: Paul Harden (email@example.com)
I'm attaching some comments on the MFJ rigs I prepared a couple of years
ago. I'm reposting it due to the recent inquiries about the MFJ rigs.
72, Paul NA5N
MFJ 90xx SERIES 5W QRP TRANSCEIVERS
The MFJ rig works as advertised with room to spare. This would hold true
for any QRP rig using the same general circuit. The NE602/MC1350/NE602
scheme is an excellent 3-chip receiver with a sensitivity that rivals big
rigs (~.2uV/-120dBm region). It favors well to many of the recent kits on
the market, with the advantage of purchasing a ready-to-go rig, and one
feature often missed ... they're built like a tank in a stout enclosure
that can well tolerate some ruff, banging around use.
The XMTR portion is about the same ... works great for its simplicity with
good harmonic supression on the output. It is hard to misadjust the
transmitter to exceed FCC specs for the -30dB down harmonic supression,
EXCEPT FOR OVERDRIVING the transmitter which will guarantee out-of-spec
emissions. The 2N5109 Q8 driver and MRF-476 Q9 PA final operate pretty close
to their limits. This should be no problem for CW, but would likely be fatal
for RTTY/packet at full power. This would be true for any rig running 4-5
watts from a single MRF-476.
1. L3 VFO CAL is very touchy! 1/2 turn=50 kHz on 40M, ~100 kHz on 30M.
2. Tweaking the RF-IF cans by sound off a weak signal (to keep the AGC from
kicking in) is a very good way to optimize the IF's for best gain and noise
figure. Recommend using an Amtor/RTTY sig for a more constant signal
3. T1, the first IF can after the U1 NE602, has the most profound effect on
images. If you're having images, birdies or excessive noise floor,
adjust T1 for peak, then back off a bit to reduce noise. This will
scarcely effect IF gain, but images and noise drop 10dB in <1/4 turn.
4. BFO adjustment C67 is also very touchy (although only effects detected
CW tone, not the VFO frequency). Use insulated screw driver.
5. In the lab tests, note the AGC voltage vs. the gain reduction produced.
In my opinion, the gain reduction is too high for weak signal reception
(typical QRP work!). Try setting AGC V. (TP2) to 3.6 or even less with
no signal for enhanced weak signal gain. If you're using the CW filter,
this will also give a better sounding sidetone. Play with it for your
taste. There is nothing sacred about the AGC voltage. It is strictly
for operator convenience. A lower setting will make it a bit prone to
strong signal overloading, however. One quick poor-man's method to
properly set the AGC voltage is to adjust the AGC gain pot for a
pleasing sounding sidetone. When the sidetone is loud and raspy, you
are driving the receiver into gain compression; when it sounds pure,
the AGC gain reduction is properly set for strong signals. This does
not effect the overall gain to weak signals.
6. One of the mods I've added to several MFJ's is a front panel mini-
toggle switch for AGC FAST-SLOW. The AGC FET has a 1M from the gate
to ground; the switch in AGC FAST places a 5.1K across this 1M ohm
resistor to greatly speed up the discharge time of the AGC cap. This
works great in high static situations, where static impulses seems to
nearly mute the MFJ due to the long time constant. The AGC SLOW position
in the MFJ "as-it-comes" (no mod to the time constant).
1. VFO DRIVE R41. MFJ manual says don't exceed drive past the output power
plateu or else spurious emissions may occur. An UNDERSTATEMENT. It gets
so ugly on a spectrum analyzer, I haven't the words to describe it. You
CAN judge by the sound of the sidetone ... when it starts to hum a bit,
your 2nd and 3rd harmonics are coming up. When sidetone gets raspy, you
are generating hash from DC to light and the MRF-476 is in VHF oscillation.
The point where the sidetone just leaves that pure sound is maximum power
with the cleanest output and the most reduced harmonics. In other words,
set the drive control for max output just as the MFJ manual describes.
2. Peaking the TX1 and TX2 cans (L6 and L7) is very important. These are
the filters coming out of the TX VFO Mixer NE602, before the Q7 buffer.
Adjust for peak output on RF meter. While peak on L7 may seem a bit
sluggish, exact peak has lowest harmonic power and provides proper signal
level and bias to drive Q8. Don't hesitate to peak these TX cans.
Lowest harmonic content occurs when these are peaked.
3. When peaking TX1 and TX2, your output power can also fall off as you
move away from the frequency where you peaked them. If you desire a
more constant output power vs. frequency, stagger tune TX1 for peak
power towards the bottom 1/3rd of the band and TX2 towards the upper
1/3rd of the band. Output power may be reduced 0.5W this way, but the
overall output power will be flatter across the tuning range.
4. TX OFFSET C66 is very touchy. This is what determines your TRANSMIT
frequency by mixing the VFO from the RX with the TX offset freq. Obey
the proper sequence in the MFJ manual for the order in adjusting the
VFO CAL, TX OFFSET and BFO for proper TX-RX offsets. HINT: If you can
get the TX offset for exactly 700 Hz shift, then adjusting the BFO for
the 700 Hz sidetone is easily done just by sound. Then, when you answer
a CQ, tune in the station for the same tone as your XMIT offset, and you
will be right on his frequency.
CLOSING THOUGHTS/TECH NOTES
1. Some of the earlier versions of the MFJ's were prone to frequency drift,
up to 1KHz in the space of a QSO. This was dramatically improved by MFJ
by incorporating silver mica caps in the VFO circuit. If you have an
older rig with the dipped mica caps in the VFO, call MFJ ... they will
send you the more temperature stable caps for free, or for shipping.
This has not been a big reported problem in the past couple of years
since MFJ made this change.
2. I destroyed the Q7 2N5486 FET transmitter buffer (for reasons I won't
elaborate!). The old junk-box standard, MPF-102 FET, worked just fine
and would probably be a suitable replacement for any of the 2N5486's.
3. The Omniron relay used for the T-R switching is rated for 100,000
actuations. This means if you set minimum delay for almost QSK operation,
you could burn out that relay by exceeding its actuation limit. It is
available from Digi-Key for about $5.
4. If you take your MFJ apart ... make sure when you bolt down the MRF-476
PA, you get it good and tight for maximum heat transfer. It does get
hot. Also, don't loose the mica insulator (no part of Q9 should be
touching ground). Q8 driver 2N5109 also can get quite warm with active
use. My 9040 had a heat sink on Q8 and ran cool, but my 9030 did not and
ran hot to the touch. Added a heat sink for precaution.
Hope the information and comments help. Of course, your mileage may differ.
These are fun rigs, work well, and easy to modify, play with and learn some
electronics. Always interested in your comments or measurements.
72, Paul Harden, NA5N
firstname.lastname@example.org or NA5N@rt66.com
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