From owner-qrp-l@Lehigh.EDU Tue Dec 2 13:01:49 1997 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id NAA21207 for ; Tue, 2 Dec 1997 13:01:47 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <34982-34220>; Tue, 2 Dec 1997 12:57:13 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <34832-62632>; Tue, 2 Dec 1997 12:56:54 -0500 Received: from zia.aoc.NRAO.EDU (zia.aoc.nrao.edu [146.88.1.4]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id MAA185726 for ; Tue, 2 Dec 1997 12:56:19 -0500 Received: (from pharden@localhost) by zia.aoc.NRAO.EDU (8.8.7/8.8.7) id KAA25219 for qrp-l@lehigh.edu; Tue, 2 Dec 1997 10:55:54 -0700 (MST) Message-Id: <199712021755.KAA25219@zia.aoc.NRAO.EDU> Date: Tue, 2 Dec 1997 10:55:54 -0700 (MST) Reply-To: pharden@aoc.nrao.edu Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: Paul Harden To: "Low Power Amateur Radio Discussion" Subject: MFJ Review X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO 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 OVERALL OPINION. 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. RECEIVER SECTION 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 level. 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). TRANSMITTER SECTION 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 pharden@nrao.edu or NA5N@rt66.com