Notes I found, maybe on rigpix database, for the ICOM IC-720A -iw9hgs Two problems: 1.No good RX until PTT was pressed a couple of times. 2.Attenuator was useless, 50dB signal drop. I found out that relays need to be cleaned since no DC courrent was flowing over them. I unsoldered R5 and R9 from ground and connected them to +DC, on SWR unit. Now relay auto cleans with DC, and 13dB attenuator works fine and stays ON on lower bands. S-meter readings are too generous when att is OFF. I also had to adjust frequency reference and USB capacitor, plus LSB and CW and coils. RX is good and TX is adjustable from 1w to 100w if compressor is ON. Only 2.3 KHz TX audio was too narrow for SSBquality required nowadays, so I added the 1.8KHz, ref123, INRAD filter. The old one stays there only for narrow CW now. Both TX and RX quality have improved. IF pass band tunning is much easier to use. Compressor makes TX audio stronger but very "thin" wich is OK for DX. When OFF, TX audio sounds so much natural!! I still have to check processor adjustment.. Let's see what I can get. Any advice over there? Fan works everytime you transmit and was a bit noisy. So I have replaced the old fan with a new one, and much more silent one!! My 720A's rotary relay switch was never making contact with the contact pads. It'd click 20 or so times, passing the appropriate contact several times, and finally give up, stopping randomly. The detent ring on the switch was loose, so the switch contact would always land between two contact pads! When I epoxied the detent ring to the shaft (where it was loose) it worked, and now the contact lands perfectly on the pad, and has worked since. It never clicks more than 6 times (7 position switch - if it clicks 7 times, it'd end up right where it started!) The IC-720 must have been one of the very first general-coverage transceivers that Icom developed, having a first IF frequency in the VHF range. The radio is in fact a double-conversion superheterodyne radio, although in some publications it is called a quad-conversion. The reason is that the third and fourth IF are only used for the variable IF shift, where the fourth IF frequency is identical to the second; a well-known variable bandwidth concept (NOT just an IF shift!) invented by Collins, where two sharp filters "run into each other" The radio came well-equipped with filters from the get-go: a solid (physically very large!) roofing filter in the first IF at 39.73 MHz, two crystal filters (AM and SSB) in the second IF (9.0115 MHz) and a crystal filter in the third IF (10.75) used for the variable bandwidth feature. Optionally, an additional AM filter can be added for the third IF, and a 500 Hz CW filter for the second IF. I was lucky enough to have the CW filter already installed. From a construction standpoint, I really like this one. It has aluminum alloy die-cast panels on the sides, back and front. The top- and bottom covers are made from very thick sheet metal, much thicker than what the competitors used. The whole radio is painted in a dark-green metallic tint, which looks almost military. The over-all styling makes the military impression even stronger. From the inside, everything is laid-out very well, it makes for a very clean impression. The buildup is modular; practically every PCB board is in its own tin can. The same thick sheet metal is used as a frame for the modules. The interconnections are all done with high quality connectors. The PA is completely integrated in the large, diecast heatsink on the back and is equipped with a very quiet fan. The power supply is external, the radio runs of 13.8 volts DC at 20 amps peak which is kind of the norm for 100W solid-state radios for this frequency, now and then. In receive mode it draws about an amp. The controls are all laid-out logically and feel solid, including the push-buttons. The main dial has a superb, adjustable flywheel action, one of the best I have ever experienced. The large, heavy knob feels like it tuns on ball-bearings and is completely free of any mechanical play, much better done than later Icom radios like the 746 and 756 series. The radio uses a rotary relay as a microprocessor-controlled bandfilter selector for the transmitter filters. I have never seen this before in any ham radio, although I am very familiar with it for commercial and military radios. The stepper is quite noisy but it seems to work well. Many other IC-720 users have reported problems with this stepper relay but I haven’t found any, which might be because my radio was used only lightly by the previous owner. If you have seen the QST report for this radio you will find that the two-tone dynamic range was tested to be over 90 dB for this radio, a significant result for a radio almost 25 years old. Now for the bad news. If you have to service this radio, you have a problem. The inside module cases are mounted on top of each other and very difficult to reach. And almost impossible to work on in exposed condition without an extender wiring harness. The most difficult to reach module is the PLL unit. And this is where my radio actually had a problem; it lost PLL lock in a 100 kHz band section which repeated itself every 1 MHz. This point in the direction of a VCO having drifted out of its control range in one of the PLL sub loops (most general coverage transceivers of the pre-DDS era had this intricate system of 3 PLL loops in the frequency synthesizer) I managed to expose the PLL module by carefully moving the two modules (RF and IF) out of the way. By doing so, it was impossible to maintain RF connection to the other modules, but this is not such a problem as long as the synthesizer still gets the control signals from the CPU, as well as the supply voltage, then it can function on its own but without the receiver being active. I actually felt like a surgeon applying a heart-lung machine to the patient… The synthesizer concept used in this radio is different from traditional designs. It is a very novel concept where two VCOs run practically parallel, where the control voltage for the first VCO (this VCO is part of a traditional wide-band PLL) is used as a coarse control for the second (and main) VCO (which is the one that generates the actual first LO for the receiver). The second sub-loop (the one that generates the 100 Hz steps) then is used as a reference for the main VCO (and PLL) such that the actual PLL control for the main VCO has a voltage control range of only 200 kHz or so. In this way one gets the best of two worlds: Lower phase noise that traditionally can only be achieved with “narrow” PLLs, while still allowing for the full 30 MHz range for the main VCO as a whole. But as so often, highly innovative designs can (will) suffer reliability problems in the early phases. That’s the price one pays for this concept as well. It requires a very good tracking mechanism between the two parallel VCOs; if they drift too far apart (drift as defined in free-running condition) it will drive the 100 kHz sub-loop against the Vcc rail. This is what had happened in my radio. Matters got further complicated because the manual has a very disturbing printing error in the alignment instructions; if you follow the instructions to the letter you will not get it to work because of that. All this forced me to first thoroughly understand the circuit before I went for the adjustment, which is difficult to do when you only have the diagrams without the description (I only have the owners manual, not the service manual) The fact that Icom had waxed all the ferrite cores solid in the oscillator coils was another problem, I first had to meticulously clean al that, without breaking the fragile cores. FYI, wax cleans up nicely by using petroleum-based paint thinner and Q-tips. Do NOT use lacquer thinner instead, it will dissolve the polystyrene coil formers! As a sidenote, do you know how to secure ferrite cores in their coil-formers without using wax? Cut open your wife’s elastic hair-bands (when she is not home, hi) and salvage the thin strands of elastic rubber. Then, stick a strand in the coil-former first, and THEN insert the ferrite core. It will keep the core rigidly in its place while still being adjustable. A trick I learned a long time ago from having tinkered with old German tube broadcast radios when I was a youngster! Anyway, it all was worth the trouble. I now have a like-new IC-720a which works (and looks) like a champ – Like I have said on many occasions, I love the way those early-eighties radios look, they don’t make ‘em like that any more! If you need the complete user- manual for this radio, including the high-resolution scans of the diagrams and board layouts, email me. I have scanned (and restored) the drawings to the point where they are better than the original. Drop me a line and I will email you a copy, through www.yousendit.com (the file is 100 MB large!) - ac5xp@arrl.net Back in the old days I used it on RTTY with true FSK via the accessory connector in the rear. Anyone remember the Microlog AIR-1 with the Commodore 64? No DSP here but there is effective passband tuning and the receiver is general coverage to boot. autore modifica rotary switch: udomahr@t-online.de I fitted all the possible upgrades when Icom were clearing them out in 1990 - FM unit, FL63A 250Hz CW filter, AM PBT Filter. I did the AMTOR timing upgrades, disappointed as they messed up other modes so I put it back to original state. it is a solid performer and it works very well with additional components that provide whatever other capability you might want like DSP, and Sound Card Interface I was just given an IC 720a by a local ham. He didn't want to repair an intermittent display. Icom suggested cleaning the stepper relay. This rig also had no rx when attenuator in. RIT didn't work. I took apart the RL1 in swr board, cleaned the upper contacts with a small strip of paper and a few drops of deoxi it...also cleaned the whole mechanism. Cured the freq problem. Next, found -10v not going to RIT circuit from broken copper land on PLL board. Sprayed/reseated the many wiring harness's and this thing performs excellent. RX hears a .1 sig on 28mc with no problem!! Two mods to the rig are recomended. (1)Full trasmit coverage. (2)inductor and resistor added for the TR relay mod To the point. The stepping relay. It is noisy as hell, and it works. Scott at MTS has told me that he hasn't replaced one yet. The later models, like the one I own, came with a factory modification that provide a positive voltage to the relay which keeps the contacts clean. Amazing that people get all excited about the stepping relay. This is exactly what auto-tuners use, and nobody seems to mind. If you are a CW man, make sure it has the optional filter installed. I have seen them (the filters) for sale on eBay, but they are somewhat hard to come buy. A better bet is an Inrad filter, which is better than the factory original, and is still being produced. This is one of the first radios that ICOM got right---including the dual VFOs that work. p.s. The radio works best with the ICOM SM-5 desk mic. Icom IC-720A (IC 720 A IC720A) extended xmit Transmitter is unlocked by snipping the light blue wire that's at the very end of the top hatch cover to the left middle side of the transceiver. Newsgroups: rec.radio.amateur.equipment From: dfeldman@teal.csn.org (Dave Feldman) Subject: Modifying IC-720A to get rid of rotary relay Message-ID: Sender: news@csn.org (news) Nntp-Posting-Host: teal.csn.org Organization: Colorado SuperNet, Inc. Distribution: usa Date: Sun, 26 Sep 1993 22:02:34 GMT Lines: 35 Icom IC-720A (IC 720 A IC720A) Modifying IC-720A to get rid of rotary relay I have just finished modifying an old IC-720A HF transceiver to replace the rotary relay filter control with a set of reed relays and a small digital circuit to select the appropriate relay and provide feedback to the existing CPU <#> in the radio so it thinks it's still on the rotary relay. The cost for parts was about $25; the cost of my time was slightly more than that, but it was "fun". The modification is so far successful, and the radio is much quieter in band switching as well as being more reliable (the old rotary relay was impossible to get mechanically aligned correctly, leading to poor connectivity between the rx/tx and the antenna, as well as many missed stepping pulses). Summary: Use the step-pulse line to drive a small SPDT relay; use the SPDT relay to drive an R/S flip-flop (built from two nand gates) to provide a clean step signal (the existing step signal has some kind of very high speed <#> clock train superimposed on it and therefore was not usable directly). Feed the pulse train into a CMOS decade counter type 4017. Take the appropriate decade outputs (active high) and recreate the switch-position feedback signals using a small diode matrix ala the original design. Also use the decade outputs through 33K resistors to turn on 2N3904 NPN transistors, which are used to select the appropriate (1 of 7) DPST reed relays. The CMOS chips (the nand gates and the decade counter) can be had at Radio Shack and must be powered from 9V (or so) because the radio does not use 5V logic (of course...) I mounted the reed relays inside the original low-pass filter subassembly, and the rest of the circuitry on a small perf-board near the SWR module (connected by ribbon cable). This is not a detailed design description but if you are interested I will provide more details. This project did ssalvage an otherwise-worthless IC-720A, but I don't think I'd do it again (rather time consuming, but perhaps you can benefit from my experience and save some time). Inquiries to dgf@netcom.com 73 Dave WB0GAZ most radios will operate within a 20-30mhz range of the original frequency. you may have better performance at the top or the bottom ends of the tuneable range. 1. remove power and antenna 2. remove screws open case. 3. locate main board and cut ligh blue wire, pin 1 of plug J-10 4. reset the microprocessor (see owners manual) 5. reassemble the radio.