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The ICOM IC-275All-Mode 2 Meter Transceiver

BY DAVE INGRAM', K4TWJ

Fig. 1- The ICOM 1G-275 all-mode 2 meter transceiver. Unit is quite compact and ulti­matelydeluxe.

E volutions and expansions are a nat­ura l part of our amateur radio world, andthe unit featured in this CO review is aprime example of tnat situation. Our mi­gration toward upper spectrum bandscontinues to gain momentum, with 2 me­ters being the center hub of activity. FMand repeater operations' 'kicked off" thisevolution, then the use of SSB, globespanning/Phase III OSCAR satellites. andinterlinked Packet operations set theband flourishing with action 24 hours aday. The more this trend continues, themore appealing our VHF bands become,and the greater significance all-modetransceivers such as ICOM's new IC275hold for today's amateurs.

Measuring 3~"H x 9~"W x 10"0,the 1C.275 is size-identical to ICOM 'spop­ular lc.735 HF transceiver. Also, likethe Ic.735. there's a massive heat sinkwithin the cabinet's rear area . Thecabinet and front panel are flat black,with chrome switches and a brightness­adjustable amber display adding an air ofsophistication. The main tuning knob isdrag-adjustable . perfectly balanced, andfitted with a rubberized grip for a pro­fessional feel. The unit's bottom left rowof front controls fit flush with the cabinet,but they spr ing out for easy adjustmentwhen needed.

There are actually two variations ofthis transceive r. The 1C.275Ais a 25 wattoutput unit with a built-in AC supply. The1C.275H is a 100 watt output unit that re­quires an external 20 amp, 13 volt DCsupply such as ICOM's PS55. Both Ic.275versions include a 6-pin rear connectorlike those on the 1G-730, 735. 745, 751,271 , etc., for DC power. This "plug com­patibility" is quite handy for "instant up­grading" or " switch hitting" in the mobilesetup. One day you can operate HF, thenext day VHF. etc . Nice!

It's difficult to pick the 1C.275's mostoutstanding aspect. but its frequencycoverage is a good starting point. The unit

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24 • CO • July 1987

receives 138to 174 MHz, which meansyou can tune all of 2 meters, the public­service frequencies, mobile phones.pagers, utilities, some weather satellites.plus nationwide NOAA weather stationsin the 162to 163 MHz range. Visualize us­ing this rig in your car during an emergen­cy or while traveling. You could call forhelp, shift frequencies. and monitor am­bulance, police , fire. wrecker, etc . The1G-275's transmit range of 140.010 150.0MHz should also appeal to MARS andCAP enthusiasts interested in deluxe per­formance. If you 're considering comput­er control. there's a serial R5-232C con­nector on the IC-275's rear panel. It usesa 1200 baud data rate and allows controlof frequency. mode, VFO (A or B) andmemory selection via a home computer .Additional IC-275 specifications are in­cluded in Table l.

Special Features Galore!The transceiver's dual VFOs are com­

plemented by 99 tunable memories thatstore frequency, transmitter offset, andPltones. Memories are selected by an in­dented action "Memo" knob. The main

tuning knob swings any memory or eitherVFO across the 1G-275's full frequencyrange . Remote tuning is also providedwith the unit's mike "UP/DOWN" but­tons . The overall results are similar to 99VFOs that remember favorite spots wheninitially selected , plus two "qenerat-our­pose" VFOs . Another attraction is the" call" channel. Program it with your fa­vorite frequency, offset, and PL tone,press the button .and bingo.you're there .The unit also includes 32 Pl tones, andtheir actual subaudibte frequency (in Hz)can be displayed on the dial's readout.Standard repeater offsets of ± 600 kHzare selected by pressing the "DUP" but­ton. Any odd offset is available by press­ing the "SET" button (offset then appearsin display). rotating the main tuning knob,then pressing "DUP" again to store thepreferred offset for that VFO or memory.

A fascinating variety of scanning meth­ods are included in the 1C.275. It will scanthe full (138to 174 MHz)spectrum,or youcan program limits into special memoriesP1 and P2. It will also scan memories(non-programmed memories are skipped),or scan by a selected mode. Constantlybusy memories can also be scan-locked

Say You Saw It In CO

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and moved th rough 01 0 and on to FL2 orFL4 , depending on the selected mode.

Take a second look at the noise gate'scircuitry. Sampled noise pulses gothrough two amplifers before detection(and deriving its own AGC), then they'redetected, amplif ied, and used to " gateoff ' signa l flow from FL1 to 010 duringthe precise t ime of a noise pulse .

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Table /- General specifications of ICOM's IC-275/ransceiv8rs.

_ RF ou tput _

. Modu illion ~tem

. Ma. imu m '-encv drI"' tion

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_Carrier .,ppra1 ion

• Fre<!uency eoY"~

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. Sen, itivity

• Recei~ . ynem

• R_iva model

_ lnteo-med iat e lrequenc'"

• Selectivity

_Spu, iou, r"po"", ,ejection

• Audio outpu t impedance

. Aud io output pow.r

_ ArT var iable ,ange

Fig. 2- Overviewot the IC-2 7S's main d isplay indica tes readout of frequency memory,VFO, mode, scan, etc.

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fier , 07. This 3SK121 is a GaAsFET, andits high gain and low noise figure reallymake the IC-275 a hot performer . Alterthe next BPF there 's a high dvnarmc­range balanced mixer consisting of two2SK125's (01, 02). These gems provideexceptionally low lntermcd in the 1C-275.The resultant 10.75 MHz~onverted sig­na l is then passed to FL1 , noise gated,

out via a front switch. A newly designedhigh-speed Direct Digital Synthesizerand PLL unit operate as a double-PLL sys­tem to produce super-fast scanning andTfR operation. The tatter aspect is alsobeneficial in Packet radio operations.

Pressing the front-panel " DATA" but­ton disables the 1C-275's front mike inputwh ile allowing rear DIN connector inputfor Packet operations and providing a 5m ill isecond PLL lockup time-a cleverand convenient idea .

The 1C-275's performance " trumpcard" is in its sse and CW department.Unique features here inc lude PassbandTuning , IF Notch, speech processor,transmit and receive aud io tone controls(they function on all modes). noise blank­er, semi o r fu ll CW break-in , continuouslyvariable RF output control that funct ionson all modes and independe nt of the m ikegain , and a mult ifunct ion meter that in­cludes FM " cente r tuning" and SWRbridge operation.

Circuitry OverviewAs I expla ined in my September and

October 1985 CO " World of Ideas" col­umns, the most effective and unbiasedway of evaluating any unit's " innards" isthrough a brief block-diagram study . TherC-275's block d iagram is thus shown infig. 3. The rig may initially appear com­plex , so let's simplify it. We 'll first pointout that the receiver's "front end " andt ransmitter's " fi nal" stages have beenbroadbanded so exact Irequencvrmem­ory select ion , stability , T/R timing , etc.,are determined by a si ngle " local oscilla­tor" signal from the PLL unit (bottom mid­dle of diagram). After that process, othe rsignal-handling stages in the 1C-275 arefixed in frequency . Ove rall , the m ult i­stage PLL unit can be visu alized as aVCO that 's cont rolled by the CPU in itsadjacent "Iogiclfront unit " (bottom tettof diagram).

Frequency-tuning the 1C-275 involvesrotating the main knobfmai n sensor,wh ich chops an LED beam via its flywheeland sends beam interruption signals to adecoder and dial pulse counter . Thecounter's info rmation is used along withthe CPU 's RAM/ROM-sto red data to es­tablish the PLL's output frequency. If thatdescription was too complex, merelythink of the logiclfront unit as the " tuner"for the PLL unit , and the PLL unit as a lo­ca l oscillator for the receiver and trans­mitter. Its output moves from the 127.25BPF (middle bottom area) to 08, theLOamp.

Now let's trace the 1C-275's receivepath (a pocket magnif ier is he lpful in c ir­cui t study). First notice that activating thefront-panel " PREAMP" switch applies 13vo lts to the antenna connector for power­ing the optional mast-mounted preamp.Now follow incoming signals from the an­tenna through the LPF, into the atteoua­tor , through the BPF, and tothe RF empu-

Say You Saw It In CO July 1987 • CO • 25

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Fig. 3- Block diagram of the ICOM IC·275. Discussion in text.

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26 • co • Ju ly 1987 Say You Saw It In CO

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non. and the 2 meter antenna was con­nected. After dialing a loca l repeater, Ipressed the "OUP" button and bingo­600 kHz offset. I then clicked out the re­cessed mike gain . set it via off-air reports ,and the tun began. Using the other VFO,I took a spin through the commercialbands to monitor a few mobile 'phonesand suburban police (they still use VHF inthis area, although metro police havemoved to 460 MHz). If you haven't tr iedthat monitoring during the day, you 'vemissed some good chuckles. The JG-27S'sremote tuning via mike up/down buttonsis great for mobiling.

The 1G-275 quickly became a favoritelitt le gem at the home OTH. especially forOSCAR satellite operat ions. The pass­band tuning works like a champ. Its cen­ter position gives full sse bandwidth,whi le tuning it to either side shifts IF re­sponse and nar rows the passband to re­duce interference and noise . ORM isn't abig problem on 2 meters SSB (yet). but abandwidth-opt imized IF always improvessignal-to-noise rat ios and peaks receiverresponse. I also use the IF notch to fur­ther reduce noise, and the result is agreat VHF OX or contest rig. If you'rereally serious in these areas , YOU'll wa ntto add the rig's optional AG-2S mast­mounted preamp.

Considering the 1G-275's fast TlR timealso makes it attractive for 2 meter moon­bounce activity. A stack of four Cushcraft"Boomer" antennas, some low-loss co­ax, and a healthy linear amplifier shouldswing that activity in fine style. Severalmoonbounce tests are conducted eachyear in which BIG setups with large an­tennas offer opportun it ies lor basic­equipped amateurs to exper ience th isunique mode. Their signals can usuallybe copied (and often worked)with a goodmu ltimode rig and ts-erement Yagi.

Being act ive on Packet (and writing abook on Packet radio). I naturally con­nected the 1G-27S to my computer setup.Its action was aga in smooth and fast . Nohassles with TXDELAY or RETRY counts;just set the TNC's output level and enjoy.

Several other lG-275 operating fea­tures warrant ment ion. The built-in SWRbridge is great . and it beats " digging out"a separate unit for antenna checks. Nine­ty-nine memories are grand . You can use15 for utilities, 15 for publ ic services, 15for SSB, 20forOSCAR. 4 for weather, etc .The lG-275 and a mating 2 meter antennaare small and light enough to go any­where you go-mobiling, vacationing,anything . It's a double handful of the" good life ."

The 1G-275 is supported by a full line ofcomplementing accessories. They includethe AG-25 preamp, UT-36 voice synthe­sizer, UT-34 tone squelch , CT-16 satellitemtertacerrlq tracking unit , and Fl-83 250Hz CW filter. For more information, con­tact ICOM America, tnc., 2380 11 6thStreet, SW., Bellevue. WA 98004 . ~

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tmues " straight ahead" to 040, the c rys­tat oscillator's FMing modulator . That re­sultant signal then " makes its way" up to013 and follows the previously outlinedRF path 10 the antenna . I would like tocover severa l of the diagram's additionalpo ints , but this review might become toolengthy or boring . Drop me a letter withyour opinion. Do you prefer o r dislike our" technical rap"? Should it be longer orshorter?

On The AirAlthough the 1G-275 is quite elaborate ,

it's surprisingly easy to operate. The firsttime I used it was wh ile mobiling home(why wait?), and I was chatting on localrepeaters while listening to NOAA wea­ther forecasts within two minutes of turn­on. Installation was a snap; my low band1G-735 was moved to the car's rear seat ,the 1G-275 was slipped into its front oosl-

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Signals from 01 0 can take one of threepaths according to mode. The ssa routecontinues through FL2. the IF notch, SSBfilter with its twin mixers (that establishespassband tuning). 011 and 012, the de­tector , sque lch, and on to the speaker.Going back to 01 0, FM signals go down,through 1C6, through Fl4, the squelch ,and on to the speaker.

Tracing the transmit path begins at themike (look in diagram's center), contin­ues through 034, 035, and on to 039 and040.At that po int the SSB path "makes aleft turn" (follow dotted tine) and movesinto the "SSB Mode" mixerlbalancemodulator . The DSB signal then movesthrough FL2 (exit one sideband, to pro­duce SS8), 01 3. 05{06 (it's then up-con­verted from a 10.75 MHz to a Pt.L-netero­dyned 2 meIer frequency), on tnrouqh0 3,04, IC1. and to the antenna. Return­ing to 039 and 040, FM-mode audio can-

28 • CQ • July 1987 Say You Saw IIln CQ