Yaesu FT-7B Mobile/Base HF Transceiver and
YC-7 Frequency Display
Are you thinking about doing some mobile or portable operating? If so, you should take a look at the FT-7B, Yaesu's new hf transceiver. If not, you should look at it anyway, because it's also suitable for fixed service. An improved version of the FT-7, intended for the market generated by the Japanese no-code license, the B model features 100-W PEP input on SSB, AM and CW. The transceiver covers 80 through 10 meters. The bandswitch has four 10-meter positions, each covering a 500-kHz segment. A crystal for 28.5-29.0 MHz is standard equipment; the others are optional. Other features of the FT-7B include a noise blanker, an rf attenuator, a crystal calibrator, concentric rf and af gain controls, a clarifier (RIT) and one fixed channel per band (crystals not supplied).
Circuitry and Performance
The unit is completely solid-state. It contains 54 transistors, 78 diodes and 8 lCs. The usual premixing arrangement is employed to produce an i-f centered on 9 MHz. A 6-pole crystal filter having a shape factor of 1.67 (6 dB/60 dB) establishes the selectivity under all conditions except a-m transmitting. In the receive mode, a monolithic filter precedes the noise blanking gate to provide "roofing" against strong signals outside the crystal filter passband.
The VFO operates at 5 MHz and uses a bipolar transistor. The transceiver performs within its stability specification, but I would expect better performance from an FET. At room temperature the unit stabilized after one hour of operation, during which time it drifted 1 kilohertz. This performance is acceptable for home station environments, but the mobile operator trying to have a QSO during his half-hour drive to work may have trouble on a cold morning. The tuning mechanism operates smoothly and features anti-backlash gears.
The noise blanker is worth looking into because it appears to lack the ills characteristic of other units. This circuit contains seven transistors, six of which are FETs. One of these is used in an 8545-kHz crystal oscillator that establishes a 455-kHz i-f for the blanker. The significant feature of the FT-7B noise blanker is that it doesn't appear to degrade the receiver's dynamic range. Yaesu achieved this improvement at the expense of a slightly higher blanking threshold. Noise pulses must be somewhat more offensive than usual before they are blanked.
In the cw mode, a two-pole RC active audio filter follows the product detector. This filter has a 6-dB bandwidth of 80 Hz, but as would be expected from the simple design, the skirts aren't very steep. The receiver age bandwidth is 2.4 kHz in all modes, so the audio filter isn't the lifesaver it could be.
It would be nice to have cw selectivity at the i-f, but I don't know of a simple way to get it. On 80 meters, the sense (usb/lsb) of the signal is inverted, causing it to be located in a different part of the i-f passband than on the other bands. The cw offset frequency is 800 Hz on 40 through 10 meters and 1200 Hz on 80. If you peak the received signal on the nose of the audio filter, you can't transceive on frequency in the 80-meter band unless you shift the clarifier 400 Hz. The clarifier has plus or minus 3 kHz of range, which was enough to allow me to contact a DX station who was operating on split frequencies.
On ssb, the receiver audio sounds clean and the age action is smooth. A diode envelope detector is used in the a-m mode. The 2.4-kHz filter is used in both voice modes, so only one sideband reaches the a-m detector, resulting in a poorer signal-to-noisc ratio in the detector. Additional i-f and af amplification is used with the a-m detector in an apparent attempt to equalize the a-m and ssb sensitivities, but the result is a somewhat noisier receiver on am.
The manual states that the transceiver is spurious-free. With the antenna input terminated by a 50-ohm resistor, I found internally generated responses at 14.001, 21.201 and 28.801 MHz. All of these spurs were weak (below 1 fiV equivalent antenna input), but I found the one at 14.001 MHz to be offensive, the futility of competing on that frequency with 50 watts into a vertical antenna notwithstanding.
The overall performance of the receiver is good. We didn't perform the Hayward dynamic-range measurements because the cw-selectivity characteristics aren't the same as those of other transceivers. The numbers derived from the tests wouldn't be directly comparable to previously published results. However, the real test for receivers in New-ington is how closely you can tune to W1AW while copying weak signals. My house is one mile from W1AW. So long as I tuned W1AW out of the i-f passband, I couldn't tell it was on the air. This was without the attenuator activated.
The transmitter works well on ssb. Using the hand-held microphone supplied with the transceiver, I received a good audio-quality report from N1FB during a 10-meter ground-wave contact. He reported high voice recognizability, even though my signal was too weak to move his S-meter (our stations are 10 miles apart and our antennas are cross-polarized). When I switched to a-m, he couldn't hear me at all. Suspecting a malfunction, I made some a-m measurements, and found the FT-7B to be working perfectly well. Why couldn't N1FB hear me? A-m simply isn't as efficient as ssb for weak-signal work. The FT-7B has considerable circuitry devoted exclusively to the a-m function. Rather than merely unbalancing the modulator and transmitting an "a-m compatible" signal through the filter, Yaesu chose to modulate the control gate of a dual-gate MOSFET amplifier following the cw carrier oscillator. The crystal filter is bypassed, resulting in a genuine dsb a-m emission. As can be seen in the a-m envelope photograph, Fig. 3, the modulation linearity is adequate for voice work. The waveform is similar to that obtained with screen modulation (remember screen modulation?). When I applied nearly 100 percent sinusoidal modulation, the average power increased from 12 watts to about 16 watts. A separate a-m ale circuit prevents the final amplifier from being overdriven on positive modulation peaks, but it's still possible to generate plenty of splatter from the negative excursions. If the mic gain and drive controls are adjusted as prescribed by the manual, the unit modulates cleanly.
I'm a cw operator, so I looked forward to using the FT-7B on the mode where its 50 watts of output would be most effective. I made several contacts using the hand key, and all of the receiving operators said the rig sounded good. Then I called a station who was sending faster (about 25 wpm), and he asked me to reduce the weighting on my keyer. This puzzled me, because I was sending with a bug! The next day I arranged tests with W1VD and N1FB, who recorded my signal. When he played the tape for me, I couldn't copy it! The dual-trace oscilloscope photo of the keying signal and the resultant rf envelope, Fig. 4, documents the problem. After the keying pulse has ended, the rf output continues for at least 20 milliseconds before it even begins to decay. I tore into the circuit and didn't stop until the unit produced the waveform shown in Fig. 5. My modification is radical, but it allows independent control of the attack and decay slopes. The details of the modification appear in the "Hints and Kinks" section of this issue. Realizing that my approach was somewhat of an overkill, I asked Yaesu for a simpler solution. The engineers reported that R1015 and C1012 have been changed to 47 kfi and 0.33 jjF in current production models. I restored the circuit to its original configuration and changed the two components. The third keying photo, Fig. 6, shows the results of Yaesu's fix. The performance is superior to that of the original circuit, but is somewhat sensitive to temperature variations and component tolerances. Yaesu also suggested changing C1012 to 0.047 ^F and placing it between collector and base of the keying transistor in a Miller integrator fashion. If you plan to operate cw with the FT-7B, listen to it critically on a local ham's receiver.
A phase-shift oscillator generates the cw sidetone. The nearly pure sine wave is a pleasant departure from the raucous notes produced by the multivibrators in some other rigs. The sidetone output is rectified and used to activate the T-R relay for "semi-break-in" cw. The relay hang time is adjustable.
The YC-7B Frequency Display
Mobile operators must be able to determine their frequency quickly, with no more than a glance away from the road. The YC-7B remote digital display fills this need. The unit is an optional accessory that plugs into a rear-panel socket of the FT-7B. Stick-on Velcro strips allow the display to be mounted anywhere within reach of the umbilical cable.
The YC-7B counts the final mixer injection frequency. Preset commands from the FT-7B ensure proper carrier frequency readout on all modes. On 80 meters, an 18-MHz crystal oscillator heterodynes the LO signal to the proper range for the counter. The time-base frequency is 655.36 kHz. No special temperature compensation is used, but the overall stability should be at least an order of magnitude better than that of the FT-7B VFO. The readout resolution is 100 Hz, but the instrument counts down to 10 Hz, with a 0.1-second gate time. This unit does not add any spurious responses to the receiver.
Most of the FT-7B circuitry is assembled on a dozen phenolic pc cards which plug into three mother boards. The card sockets are individual gold-plated spring pins soldered into the mother boards. The mobile operator needn't worry about the reliability of the sockets — the cards are held firmly in place by the top cover. Two wired-in pc boards and the VFO and PA modules complete the electronics. The VFO and PA are shielded, of course. Most of the tuned circuits are on the mother boards, so you can repeatedly remove and reinstall the plug-in cards without upsetting the alignment. The PA heat sink protrudes from the rear panel. The sink is adequate for voice and cw duty cycles. The a-m rating applies to RTTY and SSTV service. Two screws secure a flat plate to the heat sink fins. A small fan could be mounted to this plate very conveniently.
Aesthetics and Impressions
The unit certainly is compact. That's not surprising, considering the cars it was designed to be installed in. At a time when the styling of Amateur Radio equipment is diverging toward the "military" and "hi-fi/furniture" looks, the FT-7B represents a refreshing alternative to these extremes. The cabinet is painted a businesslike metallic blue that won't look out-of-place in your car or on your kitchen table. The four-color dial and meter are highly visible, yet not at all garish. For fixed service, the analog dial is easy to read, and with its 1-kHz resolution and good linearity, you really don't need the optional digital readout. It's handy, though, for precise clarifier tuning and keeping track of the VFO. All of the controls are conveniently located.
I experienced a small amount of TVI while operating the rig into a dummy load on the same table with my plastic-encased television set. You may have to scrape some paint off the mating metallic surfaces of the FT-7B enclosure if you live in a weak TV signal area.
A QST advertisement for the FT-7B reads: "Enough power to drive those linears!" The manual makes no mention of using the transceiver with an external amplifier, but if you dig into the schematic diagram, you'll find that the ale line and the 13.8-volt transmit line (to control a relay) are brought out to the power connector. There's an unused set of contacts on the T-R relay, but they aren't accessible from outside the transceiver.
The attention Yaesu paid to the a-m mode is perplexing. If the intent was to make the transceiver compatible with converted CB rigs, a better solution is to install BFOs in the CB rigs. If you want to participate in the second genesis of a-m, you'll never compete with those plate-modulated Valiants and DX-lOOs! I would much prefer to see the a-m mode scrapped in favor of some advanced ssb/cw features, such as sharp i-f selectivity, full break-in, VOX and even (bite my tongue) speech processing.
Tinkerers will love this rig, for one can remove most of the cards without unsoldering any wires. If you like, you can fabricate a completely new set of cards. Serious experimenters will undoubtedly conceive numerous worthwhile modifications. With a little ingenuity, a remote VFO could be plugged into one of the
fixed-channel crystal sockets. Another possible improvement would be a VFO drift correction circuit using feedback from the YC-7B. If you apply the correction voltage to the wiper of the dial calibration potentiometer, you won't have to violate the VFO compartment.
The FT-7B offers something for everybody. You can have plenty of fun with it just like it is. And if you're ambitious, you can turn it into a truly deluxe station. The equipment is covered by a three-month limited warranty. — George Woodward, W1RN
Copyright © 1980 by the American Radio Relay League, Inc.