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AMATEUR RADIO By GARRY CRATT, VK2YBX The old vs. the new - we compare a vintage shortwave receiver with today's technology Ever wondered how a Collins communications receiver from the 1950s compares with a modern synthesised counterpart? Read on the answer might just surprise you. Recently, we had the opportunity to compare one of the old-time favourite receivers, made in the mid 1950s, with the standard monitor receiver in our lab, a Yaesu FRG-7700. The results of this comparison highlight the outstanding design and construction techniques used in post-war equipment, and show that these units can still represent good value for money today. Most amateurs have heard of the brand name "Collins". In the 1950s, the name was synonomous with rugged high-quality communications equipment. And the most famous of all the Collins equipment was the "S line" series of transceivers. Earlier models included the "75A" series of amateur band only receivers, and prior to that, the "51" series of receivers, which surfaced on the surplus market in the late 1960s. Unearthing a relic The particular receiver we "discovered" is the 51J4, the last in a series of five receivers, manufactured in the early 1950s. This prize was unearthed during a visit to a connoiseur's shack, where it had • I\ The Yaesu FRG-7700 is a modern shortwave receiver featuring phase-locked loop tuning, digital frequency readout and memory circuits. It has better sensitivity than the Collins 51J4 but is eclipsed in some areas. 82 SILICON CHIP been buried for the previous 10 years. We decided that it would be interesting to test the receiver to determine what advances, if any, had been made in receiver design and construction in the last 40 years. The first and most obvious observation to be made about the 51J4 is its weight and size! The unit is massive, measuring 585mm across the front panel, 265mm high and 380mm from front to rear. It's total weight is around 20kg. The unit contains 19 valves or, as the manufacturer would call them, "tubes" and the total power consumption is 85 watts. By contrast, the FRG-7700 measures 334 x 129 x 225mm (W x H x D) and weighs just 6kg. So, as far as size and weight are concerned, considerable improvements have been made to receivers since 1950! The specifications provided in the manufacturer's handbook are quite respectable, even by today's standards. One notable specification is frequency stability - the dial calibration is guaranteed to be within 300Hz at room temperature and the unit has an operating temperature range of - 20°C to + 60°C. The sensitivity is quoted as 5µ,V for 10dB S/N but our unit measured 2µ, V for the same ratio (0.5µ, V for the FRG-7700). The complete block diagram for the Collins receiver is shown .in Fig.1. The biggest difference between the two receivers is the complexity of the Yaesu PLL generator, frequency counter, and memory circuits. The Yaesu receiver has a first IF of 48MHz and a second IF of The Collins 51J4 was built like a battleship. It covers from 540kHz to 30.5MHz in 30 1MHz bands, features mechanical tuning and contains 19 valves. The unit weighs about 20kg and has a total power consumption of about 85 watts. 455kHz. Synthesised local oscillators are used to ensure good frequency stability, although the end result is only as good as the Collins unit! The Collins receiver Basically, the 51J4 is a superheterodyne unit covering 540kHz to 30.5MHz. This tuning range is covered in 30 1MHz bands. The receiver also contains AGC circuits, a noise limiter, and a lO0kHz crystal calibrator. The first IF is either 2.5MHz or 3.5MHz and depending upon which band is being received, single, double, or triple conversion may be used. To enable each 1MHz band to be tuned, a highly stabilised permeability tuned oscillator is used to heterodyne against the first IF. This oscillator is fed from its own DC regulator to ensure stability. The second IF is at 500kHz and the receiver is equipped with both .NOT[: BANDS 4-30 EV EN BANDS-INJ. fREQ=B ANO I-JO +2 ODD BANDS-INJ . l='REQ: BAND NO+~ Fig.1: block diagram of the 51J4. It features permeability tuning, mechanical filters & either single, double or triple conversion. mechanical and crystal filters at that frequency . Mechanical filters Mechanical filters are electromechanical bandpass devices whose dimensions are about one quarter the size of a cigarette packet. As shown in Fig.2, a bandpass filter consists of an input transducer, a resonant mechanical section consisting of a number of metal discs, and an output transducer. The frequency characteristics of the mechanical section give almost rectangular selectivity curves as shown in Fig. 3. An electrical signal applied to the input terminals is converted into a mechanical vibration at the input transducer by means of a process called ''magnetostriction''. This mechanical vibration travels through the resonant mechanical section to the output transducer, where it is converted again by the magnetostriction process to an electrical signal, which appears at the output terminals. In order to provide the most efficient electromechanical coupling, a small magnet in the mounting above each transducer applies a magnetic bias to the nickel transducer core. The electrical impulses then add to or subtract from this magnetic bias, causing a vibration of the filter elements which corresponds to the AUGUST 1990 83 5 I 10 I 15 I 20 I ~ 25 dB I COUPLING ROO \ MAGNETOSTRICTIVE - -------1 DRIVING ROD 30 I 35 I 40 I 45 I COIL 50 I ELECTRICAL SIGNAL (INPUT OR OUTPUT) ELECTRICAL SIGNAL (INPUT OR OUTPUT) Fig.2: inside a mechanical filter. The input signal is converted to mechanical vibration by magnetostriction and travels via a number or resonant discs to the output transducer. exciting signal. There is no mechanical motion except for the imperceptible vibration of the metal discs. Magnetostrictively driven mechanical filters have several advantages over electrical equivalents. In the region of 100-500kHz, the mechanical elements are extremely small and so a mechanical filter with better selectivity than the best of conventional IF systems can be enclosed in a package smaller than a crystal filter. In addition , the frequency response characteristics of the mechanical filter are permanent no alignment is necessary or possi- ble. Unfortunately, cost and advances in modern crystal filter performance mean that mechanical filters are rarely seen in these days. Another major drawback is that mechanical filters are limited in frequency to 500kHz or so, while quite reasonable crystal filters can be made for frequencies up to 30MHz or so. However, considering the age of such devices, their performance is unbelievable. Phasing control Another most useful feature in the 51J4 is the "phasing" control. This allows control of a variable notch filter within the IF response The circuit techniques may be ancient but they still add up to excellent performance. They sure don't make 'em like this anymore! 84 SILICO N CHIP kHz Fig.3: typical response curve of a mechanical filter. The result was excellent selectivity. curve, allowing unwanted heterodyne s from adjacent channel signals to be notched out. This feature is found only rarely in shortwave receivers and is also seldom used in amateur receivers. One of the tests we used to determine the ability of a receiver to process strong signals is called the "blocking test". For this test, we injected a signal on the desired frequency, and then injected a second interfering signal 3kHz away. The amount of interfering signal required to degrade the SINAD reading of the desired frequency by 3dB determines the receiver's ability to withstand a "blocking" signal. Fig.4 shows our test layout. Most modern receivers have rather poor performance in this area. The "fix" on these receivers, [including the FRG-7700) is to provide an attenuator which can be adjusted by the operator to minimise the effect of strong signal overload (and also receiver sensitivity). In a well designed receiver like the Collins 51J4, no attenuator is necessary due to the excellent design of the RF, IF and AGC stages. In fact, our unit exhibited a 30dB improvement over the FRG7700 in this critical area. There was one area where our vintage receiver lacked in performance and that was SSB reception. This is quite important for amateur service these days but perhaps not so in the 1950's. Fortunately, the subject of improving SSB performance was covered by the American magazine Ham Radio in their February 1978 issue. Basically, this issue detailed the modifications required to change the BFO to a product detector, and to change the AGC attack and decay time constants to allow correct reception of SSB. Some audio stage modifications are also detailed. After we implemented these recommended modifications, the "DESIRED FREQUENCY" SIGNALGENERATOR SINAD METER COLLINS 51J4 RECEIVER SIGNAL COMBINER "INTERFERING" SIGNAL GENERATOR Fig.4: the test layout used to determine a receiver's ability to withstand a "blocking" signal. The 51J4 was considerable better than the FRG-7700 in this important area. C206 5pF TD ------...11--<)DETECTDA V110A T106 ,-------------7 I !JI I,{;[ I 1 II ___ ._..... I I I I B+ I BFD PITCH I I C218 .01 12 L - - - _ _ _ _ _ _ _ _ _ _I .,. Fig.5(a): this is the original 51J4 BFO circuit. The tube is turned off by switch S112 which short circuits the screen voltage to ground. .05 TD PINS 6,7 V110A AM o--- ~-- ,, S1128 (' 5pF1 TD PIN 3 XV112A VIA C209 ----- I I TD R154 (VOLUME) I I I I I I n------xo_1.., " ' ~ _______ ~,:/_ _ __ ovI1- SJV.,. 01 5 J 1N270 TD PIN 3-""41t--\l\Mr--l-.,_-1---.----------<--- TD R171 V110B AND XV111A C205B 1-1.5 + 1M 0.1 l 100VJ Fig.5(b): the revised circuit substitutes a 6BE6 tube for the original 6BA6 to provide a product detector. The AGC attack and decay time constants are also modified as shown to allow SSB reception. receiver performed equally as well as the FRG-7700 on SSB signals. Fig.5 shows some of the necessary circuit changes. In summary, considering the age of this receiver and the techniques used, its performance is amazing. Although the sensitivity is somewhat worse than modern receivers [and this can be overcome with a good HF antenna), it outperforms modern receivers in many other respects. It's also worth noting that the price of a Collins 51J4 on the secondhand market is similar to the price of modern pre-loved Japanese receivers. If size and digital frequency readout are not of major importance and a few valves can be kept as spares, the Collins 51J4 receiver should continue to provide good performance for many years to oome. ~ A UGUST 1990 85