Product Review A comparison between ICOM IC-7800 and IC-781 1. Background I had used nearly the entire IC-756 family from IC-756 (original), IC756Pro2 to IC756pro3 and eventually bought my IC-7800 in early 2006. I have been very satisfied with this new ICOM DSP flagship transceiver since then. Upon joining the Yahoo groups for the IC-7800 (ic7800@yahoogroups.com) and IC-781 (IcomIC-781@yahoogroups.com) , I notice that there have been interesting discussions on various pros and cons between these two high end transceivers. Some of these comments are from learned and experienced RF engineers. I think the only way for me to really know the true picture is to buy an IC-781 and do the comparison myself. I consider this exercise should be educational to me as I am not a radio professional. 2. Transceivers used in the test IC-7800 My IC-7800 was bought in early 2006 and is the latest version with 3 kHz roofing filter. The firmware was updated to ver. 2.10. It has been functioning flawlessly since the first day in my radio shack. IC-781 I deliberately did not buy a second-hand IC-781 from US for this project but I bought an IC-780 (Japanese version of IC-781) instead. The mains voltage in Japan is 100V, which is the designed operation voltage for the IC-781. Therefore, there is less chance of having the well-known heat problems in the REG unit of the PSU for the Japanese version. The IC-781 and IC-780 are electrically identical. I bought my IC-780 in Osaka, Japan. The rig was re-aligned by ICOM Japan before sending to Hong Kong in early April 2007. I also have official information on readjusting the output power from 100 watts to 150 watts, and on the general TX frequency coverage modification for the IC-780. In other words, the IC-780 in this test is identical 1
to any IC-781 except the front-panel nameplate. Furthermore, this IC-780 comes with all stocked filters. Unlike my previous owned IC-756 (original) http://www.qsl.net/icom/ic756/fl44a.html , no NATO IF filter modification http://www.qsl.net/icom/ic781/ic781filter.html was done to the IC-780. In the rest of the text, all mention of the IC-781 refers to this IC-780 purchased in Osaka. 3. Limitations of the Test This is not a laboratory test, because I do not have the sort of high-end test equipment encountered in the ARRL Lab or other test entities. It is only an A-B comparison test in my QTH using the same antenna and my ears. In Hong Kong, most of us live in high-rise apartments in urban areas. Full-size antennas or even Yagis are out of the questions. My QTH is on a very high floor of a high-rise multi-storey residential apartment. It is about 150 metres above sea level and has a spectacular view towards Victoria Harbour. I am using monoband ham sticks manufactured by www.hamstick.com All the antennas are mounted horizontally, pointing towards Victoria Harbour. 4. Test Areas 4.1 Receiver noise floor (i.e. quietness) According to the specifications of both transceivers, the sensitivity at 14 MHz is 0.16V. However, IC-781 has only one PREAMP but IC-7800 has PREAMP 1 and PREAMP 2. By using a simple XG2 reference signal generator from Elecraft www.elecraft.com , under PREAMP off situations, the IC-781 is more sensitive than the IC-7800 and shows a higher reading on the S-meter for a given input signal level from the XG2. On the other hand, if both transceivers are set up with PREAMP turned on, the sensitivity is more or less the same and shows similar S-meter readings 2
with the signal supplied by the XG2. I do not have any advanced equipment to measure the noise floor, but I tested the radios under the following setup: 4.1.1 Antenna input terminated with 50 ohms dummy load The transceivers were at their maximum sensitivity and with PREAMP turned on. The AF gain was turned all the way up (fully clockwise). The DSP NR in the IC-7800 was off. The internal noise of the radio, as heard in my headphones, was definitely less for the IC-7800. The finding was the same no matter whether I used a pair of Sennheiser Hi-Fi headphones f or Heil communications headphones (www.heilsound.com). With DSP NR in the IC-7800 turned on, the difference in the above is enormous. 4.1.2 On-the-air test The band condition was not all that brilliant during the test, and there were many weak signals. Both radios were set up with PREAMP OFF , where the IC-7800 is supposed to be less sensitive (see 2nd paragraph of 4.1 above). However, there was nothing I could hear on the IC-781 and not on the IC-7800. In other words, the IC-7800 receiver is clearly quieter than that of the IC-781, and hence has better ability to dig out the weak signals. If the IC-7800 DSP NR is turned on, the ability will be further enhanced. 4.2 Selectivity and survival among local KW stations Hong Kong is a small city and well known for its populstion density. There are a few KW stations near my QTH, within line-of-sight distance. By using the same IF filter bandwidth in both radios and the roofing filter of the IC-7800 set to 15 kHz, I noticed that the IC-7800 could work weak stations much closer to those local big guns. I can conclude that the selectivity of IC-7800 is far better than that of the IC-781, and its IF-DSP filters are much tighter than the analogue filters in the IC-781. 3

If DSP NR is ON and a tighter roofing filter (e.g. 3 kHz) is used, the IC-7800 is an even better performer. If you need to hunt weak DX signals among big guns, IC-7800 is your combat fighter. 4.3 Manual Notch The notch functions of both IC-781 and IC-7800 are effective, but the IC-7800 has a much deeper notch. When notching the same carrier signal, the AF Gain on the IC-7800 has to be turned all the way up to give the same audio output as in the IC-781. For example, when the AF Gain in the IC-781 is at 9 oclock, the AF Gain in IC-7800 has to be turned fully clockwise to produce the same audio level in my headphones when notching out the same carrier signal. 4.4 Spectrum scope Having mentioned so many of the IC-7800s strengths, there is something interesting in this area. No doubt, the colorful TFT display and the ability to view a 250 kHz span in IC-7800 is attractive. However, the grass level of the background noise / signal shown in the IC-7800 spectrum scope is indeed higher. In other words, it will be easier to find a signal among the grass on the IC-781 band scope than on the IC-7800. The desired signal to grass ratio in the IC-781 band scope is in fact higher. In this regard, I would wonder whether the display scales in both band scopes are the same. Is the band scope in the IC-781 a linear scale? Or is the band scope in the IC-7800 a logarithmic scale? If my assumption is correct, this may be corrected by a firmware update in IC-7800. Editors note: The vertical scales of both scopes are logarithmic (nominally 10 dB/div. The IC-781 spectrum scope is analogue, whilst the IC-7800 scope is DSP-based. It is possible that the higher grass level on the IC-7800 scope is caused by noise generated in the ADC associated with the scope DSP. The IC-756Pro3 scope is also analogue, and can see a -131 dBm signal at the antenna socket. The IC-756Pro3 scopes grass level is some 10 dB lower than that of the IC-7800. 4
4.5 Audio quality and the pleasure of listening While the IC-7800 has all the cutting edges and capabilities required for weak-signal hunting, there are audible DSP artifacts in both SSB and CW reception. There is nothing irritating in listening to the IC-7800 - but simply a bit of listening fatigue. On the other hand, the audio from the IC-781 is more pleasant to listen to. When using the IC-781 for SWL and listening to shortwave broadcasts, the audio is sweet and musical. My two young sons, who have much sharper ears, also concur with my observation. Does this imply a similar argument between LP records and CDs? 4.6 AGC System The AGC systems in both radios are effective but the operator will have more flexibility in changing the AGC settings on the IC-7800. I did not observe any irritating AGC pumping due to sudden signal bursts (transients) on the IC-7800. 4.7 Twin PBT I find the Twin PBT on the IC-781 to be less tight than that of the IC-7800. Perhaps, this is due to the difference between analogue and DSP filters. One should bear in mind that the stock 455 kHz IF filter in the IC-781 is the FL-96 (2.8 kHz). As the stock 9 MHz IF filter is the FL-80 (2.4 kHz), this bandwidth mismatch tends to make Twin PBT somewhat sloppy. 5. Conclusion In terms of weak-signal hunting, selectivity and survival among the local big guns, the IC-7800 is clearly ahead of the IC-781. However, if you are only interested in working the S9 stations and fond of rag-chewing or even some SWL, the IC-781 offers very pleasant audio. Furthermore, the IC-781 is constructed by using through-hole components which are user-repairable to some degree. For the IC-7800, my hands are up in the air, and the only way to have the radio serviced is via a trip to the ICOM service centre.

6. Acknowledgements In writing this review, I read Matt Erickson KK5DRs similar review in his webpage. At the same time, my friend Adam VA7OJ/AB4OJ has kindly edited my report. 73 de Johnny Siu VR2XMC

Icom IC781 ALC Power Spike 13 Nov 2002 IC-781 ALC Info
(also see follow-up K1VF UPDATE, Below) From: Vince, K1VF
I also noticed a severe power spike on transmit at the leading edge with serial # 03156 (late production purchased 11/20/2000) versus early production serial #01285, which performed flawlessly. Any amplifier using 3CX800A7s, and equipped with a properly working Grid Trip system, simply will not allow operation with such an exciter fault. It will (or should) trip/fault immediately. Close inspection showed virtually no changes in the IF unit PC board version and schematic between the two date code units, but the later unit experienced unusable transmit power spikes upon start of PTT in all modes. The problem turned out to be the following: 1. C47 and C48 were reversed on the later unit. 2. R108 was populated as 4.7K versus 100 ohms. 3. R112 (4.7K) and R109 (4.7K) were both populated as 1K. In all five cases, the early unit was populated per the schematic in the Service Manual, and the new unit, after changes to agree with the schematic, performed flawlessly. In summary, final values for these components per the schematic should be: C47 = 2.2F, be sure to observe polarity C48 = 0.47F, be sure to observe polarity R108 = 100 ohms R109 = 4.7K R112 = 4.7K These changes (to originally designed) values result in identical, flawless performance of the ALC system. K1VF 13 Nov, 2002 (Proceed to UPDATED post, which follows)

05 Apr 2003

From: Vince, K1VF Icom 781 ALC Power Spike UPDATED w/SUMMARY UPDATED: April 5, 2003
Although the above changes (from my initial post of 13 Nov, 2002) improved the ALC behavior, reducing a 200+ watt spike, there was still a persistent 1-2msec spike of 10-15% at the leading edge of SSB and CW operation, at all combinations of power and drive level settings. Although this was not enough of a problem to bother the grid trip protection of 3CX800-type amplifiers, it did create problems when attempting to use ALC generated by an external amplifier to the ALC input of the 781. The external linear amplifier ALC would provide a sharp negative-going spike (1-2msec following the input power spike), that in turn would saturate the 781 ALC, reducing its output to zero for the 2-second time constant, before it recovered. The following changes (including an amended change to R109 from my initial post) cured the problem completely, and power on SSB (tone injected) and CW now rises cleanly from 0-100% with ZERO overshoot. 1. The -9.0Vdc supply to the ALC system was low, at -7.5Vdc. The problem was traced to the PI Unit; R8 was changed from 5.6K to 4.7K, 1/2W. (Alternatively, you can place a 30K in parallel on the back side of the board). 2. The ALC timing transistor, Q35, was not saturating on fast attacks, meaning that it was not able to charge timing capacitor C47 fast enough to properly control the leading-edge attack time. Given the typical DC Gain spec of Q35, R109 must be reduced from 4.7K to 1K to allow this transistor to fully saturate. 3. Diode D15 was changed to a high-speed Schottky diode, type 1N5711. Note: This diode was either missing or intentionally depopulated on the later, serial 3156 unit; however it is needed for the leading edge attack, and provides an additional current source to charge C47 directly via the primary FWD-PWR difference op-amp IC4-p7.
These changes were made to both units (Serial #01285, #03156) with identical results. They provide clean rise times (from 0-100% in 3msec CW) output with no overshoot or ringing at all combinations of Drive and PWR levels.
In summary, the following components are either incorrectly installed, or should be changed as follows: IF Unit PC Board: C47: C48: R108: R109: R112: D15: 2.2 F 0.47 F 100 ohms 1K 4.7K 1N5711
PI Unit PC Board: R8: 4.7K

K1VF 05 Apr, 2003

Reader Comments and Feedback to K1VF IC781 ALC Circuit Fix
Date: From: Subj: 14 Nov 2003 VE2DC (Mel) VE2DC mod superseded by K1VF mod
I have changed my 781 ALC circuit to the improved circuit as suggested by K1VF and there is an small and worthwhile improvement in ALC performance. The slight remaining leading edge overshoot has been eliminated. Therefore: my original fix has been superseded!!! Thanks Vince.

Date: From: Subj:

11 Aug 2003 Fred VE3FG This mod fixes what Icom Canada could not
This modification really works!! Thanks to K1VF's ALC circuit modification, I finally have an IC781 that has a properly functioning ALC circuit. Output from the IC781 and IC4KL now responds as it should when the 781 is keyed using PTT in the SSB mode. There is no longer and occasional momentary delay. Since day one my 781, intermittently, behaved just as K1VF described. "would saturate the 781 ALC, reducing its output to zero for the 2 second time constant, before it recovered". K1VF reported that he measured the -9.0 Vdc supply voltage to the ALC system in his 781 and found it to be low at -7.5 Vdc. After measuring, I discovered the same voltage readings in my 781. Changing R8 as per K1VF's modification, I obtained the correct -9.0 Vdc reading. Many thanks to K1VF for providing this excellent modification. VE3FG Fred

Date: From: Subject:

29 Feb 2004 Steve, K0SF K1VF Update to ALC Fix
With the new formatting of www.mods.dk, K1VF's 2003 update to the IC-781 ALC problem is missing. As I had applied K1VF's fix to my IC-781 (SN 2739) and found it to completely eliminate the overshoot, I want to list it here again in its entirety: The following components are either incorrectly installed, or should be changed in accordance with K1VF's recommendation. IF Board: C47: 2.2 F C48:.47 F R108: 100 ohms R109: 1K R112: 4.7K D15: 1N5711 PI Board: R8: 4.7K The IF board changes required about 2 hours, the PI board about 1 hour. It is an evening project. I used a 1N4148 diode for D15; since it's reverse bias transit time was nearly that of the 1N5711 Schottky. Also, I left R109 alone. The factory had installed an 820-ohm resistor in this position. Note that the user will also need to check the -9V supply coming from the PI board. Before modification, my -9V line was low, -7.4 VDC. K1VF recommends changing R8 on the PI board to 4.7K to achieve -9V. When I changed R8 to 4.7K, my -9V line lifted to -9.4 VDC, but I left it that way. A 5.1K might be a better choice for R8. The circuit is Beta dependent, and does not use true voltage referencing.
Before modification, my QRO HF-2500DX amplifier's grid overcurrent reset would trip constantly. The problem was extremely annoying, making it almost impossible to run the amplifier in a contest. I have not had to reset the QRO once since the modification. I will try to get a digitizing scope from work and take screen dumps of the modulation envelopes from IC-781's with and without the fix to demonstrate the difference. I have 2 more IC781s that I will be modifying for friends; I should have an opportunity to do this. Thanks to Vince, K1VF for really digging into this elusive problem and coming up with the "silver bullet" fix. Steve, K0SF
KG4LRU Rating: 5/5 Date: Apr 9, 2005 16:32 Subject: IC781 ALC fix for good Time owned: months I too have been suffering with the dreaded ALC spike I have several 781s; 1402, 2209, 2700. Two of these had the components reversed and or missing, this helped but did not fix the radio. My radio has not been in for alignment and was needed voltages were off so I packed it up and sent to Burghardts in SD. Dave in service there is a great 781 Tech; he was impressed by Mels work and I am very grateful to K1VF for all his work put into to the research on the mod. I run the Acom 2000a and now can set it at 600 watts and it will not budge over 600 watts no matter how loud I shout. 73 to all you can safely invest just a little more money in your 781 to make it perfect. If you have a 781 with the dreaded ALC spike, it can be tamed. Try this mod; it works! Proper alignment, and this mod, put the 781 back into the contest fray. 73 Phil