KB2LJJ Radio Mods Database
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1. icom ic 756pro htm 4 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database m om cw moth s 3 taggare User comment Making 756P continuous Xmit 1 6 29 999 MHz Mods desired to use 756P as Xverter on 24 MHz which is out of its stock Tx range I followed the suggested steps for making 756P continuous Xmit from 1 6 29 999 Bottom cover removed by 10 screws 6 on bottom and 4 on sides no need to remove feet CPU box is connected to mother board via two multi conductor connectors Easy to gently rock it back and forth pops right off Diodes on my Ver 2 were instantly identifiable Heated one end used very small blade knife to lift desoldered end Left diodes connected and hanging by other end Total time required approx 10 minutes Piece of cake No observable reduction in operation upon firing it up User comment Extended Xmit including 30 60MHz From K3CW On my 756PRO I found that leaving diode 3753 in place and all other locations from 3751 to 3756 empty enables transmit continuously from 1 6MHz to 60MHz The mod suggested above enables transmit from 1 6 to 30 but transmitting is still disabled from 30 to SOMHz I think it s an older 756PRO serial 1090 Note 1 Experiment at your own risk I am 2 Use a dummy load not an antenna User comment IC 765 Pro Freq Mod From NL7HQ Derek Edmondson http www kb21jj com data icom ic 756pro htm 5 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database After experim2. F Power Amplifier in the range of 5 100 to 5 999MHz This LPF is in the circuit from 2MHz to 6MHz However it has a 1dB roll off point at about 5 0MHz increasing to more than 3 6dB at about 5 35MHz In my preliminary testing on 5 3MHz it appears that the Low Pass Filter causes 50 to 75 Watts of the output power to be converted to heat within the PRO s circuitry RF Power Amplifier transistors RF output transformer switching relays and 2 6MHz LPF section We are now looking at the design of a modification for the PA LPF to enable safe operation of the IC 756PRO2 and PRO on 5 3MHz Here is a schematic of the PRO s 2 6MHz LPF before modification LOW PASS FILTER SCHEMATIC CLICK HERE There are 2 solutions to this problem 1 Modification changing the existing capacitors inductors values of the LPF along with the possible addition of another inductor or trap 2 Modification to switch the existing 6MHz to 8MHz LPF in the circuit on 5MHz instead of the 2MHz to 6MHz LPF Again Please do not contact the author of this comment about this mod Everything that is known at this time about it has been posted already Thank you ICOM IC 756PRO CW Filter Shape Selection Author Greg Ordy ordy seed solutions com by Greg Ordy Introduction The Undocumented BPF Indicator Two Ways to Specify the Filter Width Do It Yourself The Difference Introduction The ICOM 756PRO documentation is silent when it comes to describing the CW fi
3. KB2LJJ Radio Mods Database Home Radio Mods Database and Manuals Modifications for the Icom IC 756PRO Expand tx freqeuncy for the IC756 PRO ICOM IC 756PRO CW Filter Shape Selection Eliminate an adjacent channel noise problem QST mod for IC 756PRO 50 Mhz 100 Watts Adjustment 50 Mhz 100 Watts adjustement Open the bottom lid of the radio Looking on Main board for the tiny potentiometer at the bottom side of the transceiver There are only numbers marked on the board You have to look in the service manual to find out what number is correct R 509 is the one to adjust the 6m 100W The R 509 is located on the Main Unit R 509 is the proper adjustment point but you need to make sure you follow the proper procedure per the service manual http www kb21jj com data icom ic 756pro htm 1 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database MAIN unit Bottom view of the transceiver L261 TX peak adjustment R509 50 MHz band output power adjustment R507 HF bands output power adjustment R545 Ic APC adjustment By Kevin McClinton W7JRL Icom Technical Support Representative Expand tx freqeuncy for the IC756 PRO Author Hans PA3HGT pa3hgt amsat org You need a very fine solder iron to remove the 3 diodes from the main board You have to remove the bottom cover of the 756PRO When you removed the cover and you have the backside of the 756 PRO facing you you can locate the dsp box on the left
4. enting with several of the diodes D3751 thru D 3756 this is what I found on my 756 Pro I S N 01850 Removal of 3751 3752 3753 gives TX 1 6 MHz to 30 MHz plus normal 50 MHz to 54 MHz Removal of 3756 3754 and 3755 were already gone from factory killed the TX on 50 MHz to 54 MHz Reinstalling 3756 restored 50 MHz to 54 MHz Reinstalling 3753 opened TX from 100 kHz to 60 MHz continuous However be very careful from 30 MHz to 50 MHz as the block diagram shows no Low Pass Filter support there Also the Band Pass Filter range for 30 MHz to 50 MHz is abnormally broad when compared to adjacent band pass filter blocks 20 MHz wide compared to between 2 MHz and 8 MHz Regards User comment 100kHz 60MHz TX Mod From Bonnie KQ6XA Now reporting success for 1600kHz to 60MHz Continuous Transmit This includes the 5MHz Band of course Model IC 756 PRO original Serial Number Serial Number approximately 2650 Version Number 2 on original packing box for USA market Procedure Removal of diode 3751 and 3752 only After successful modification the following notes apply Note 1 Diodes 3753 and 3756 remain in place Note 2 Diodes 3751 3752 3754 3755 are absent The following notes apply to operating range Note 3 TX power drops out below 1600kHz due to band filter Note 4 Transmitting below 1600kHz not recommended Note 5 Transmitting is possible continuously from 1 6MHz to 60MHz Note 6 Transmitting is not recommended f
5. lter shape factor and how it can be selected ICOM tried to improve the situation with a brief mention of this topic in its Advanced Operation Guide for the 756PRO but it does not clearly describe how to select the filter shape I will try to do a little better on this page I would like to thank Rob Peebles W8LX for bringing these capabilities of the radio to my attention The 756PRO receive audio coming out of a digital signal processing DSP intermediate frequency IF stage is much http www kb21jj com data icom ic 756pro htm 7 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database different in character than other purely analog receivers I have captured some of my impressions on another page On CW the filter response created by the DSP is extremely sharp That is signals outside of the pass band are substantially attenuated In the analog domain sharp filters can lead to ringing and other artifacts Digital filters can have similar problems In addition it has been claimed that the 756PRO receiver highlights flaws key clicks in the transmitted signal In other words it hears the clicks better than other receivers These various factors lead to CW receiver performance which some operators do not like One suggestion to improve performance has been to alter the CW filter shape Perhaps if the filter was not as sharp the artifacts would not be as pronounced Apparently a special DSP ROM software is available in Japan but inf
6. or all widths of 500 Hz or less The second way to adjust the IF filter width is to use the Twin PBT knobs These concentric knobs can be adjusted to narrow the filter width Here is the entire point of this web page When the IF width is adjusted with the Twin PBT knobs the BPF indicator will not come on even for widths equal to and under 500 Hz Obviously you must start with a width greater than 500 Hz so that the PBT indicator is off to begin with My own opinion is that this is a bug that really is a feature My guess is that ICOM engineers wanted the sharp CW filter to be on for all widths of 500 Hz or less When using the BW button it works like that But when the Twin PBT controls are used the check for crossing the 500 Hz boundary is not made and the filter does not change shape Do It Yourself http www kb21jj com data icom ic 756pro htm 8 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database The best way to evaluate the two different filter shape factors is to A B compare them for a constant filter width Here s my suggestion for doing that Pick a band like 40 meters that usually has a number of CW signals and some background noise especially at night Set the 756PRO to CW mode and enter the filter set mode by pressing the Filter button for 2 seconds Select the first filter and press the DEF button to return the filter to the factory default width of 1 2 KHz Since this width is greater than 500 Hz the BPF indicato
7. ormation on this modification has been difficult to obtain and in any case changing the DSP software is claimed to be a major project in terms of taking apart the radio to access the relevant parts It turns out that for filter widths of 500 Hz and less it is possible to select one of two filter shape factors One is the sharp CW filter and the other appears to be the SSB shape factor which is not as sharp Calling one the CW shape factor and the other the SSB shape factor is my own terminology Rob Peebles W8LX estimated the sharp CW shape factor to be 1 5 and the SSB shape factor to be 2 0 This page describes how to select the two different shape factors in the CW mode it cannot be selected in SSB mode The Undocumented BPF Indicator The top line of the main display contains a yellow indicator labeled BPF This indicator is not mentioned in the documentation 756PRO user s manual It appears immediately to the right of the pass band width indicator page 10 of my manual It is my belief and claim that this indicator shows the IF filter shape factor state When the indicator is off the display is dark the SSB shape factor is in use When the BPF characters are visible the CW sharp shape factor is being used Early 756PRO users discovered the BPF indicator soon after the radio was introduced It was clear that it was tied to the CW filter width As best as I can tell you must be in CW mode in order to be able to use the sharper filte
8. r When the CW filter width is set to 500 Hz or less via the BW F 1 button in IF filter selection mode the BPF indicator and mode automatically turns on Until I talked to Rob Peebles W8LX I assumed that all CW filters 500 Hz or less in width would have the BPF indicator on This turns out to be untrue It is possible to have CW filters at and under 500 Hz without having the BPF indicator on Since the BPF indicator is not on you will be using the SSB shape factor even if the filter width is reduced to the minimum value of 50 Hz Two Ways to Specify the Filter Width The current filter width and shift offset are displayed on the top line of the main display The BW field indicates the filter bandwidth and the SFT field indicates the filter shift I believe that these two fields along with the BPF indicator completely capture the IF filter state of the radio There is no more data to view or change There are two different ways to specify the filter width Until this issue came up I assumed that both ways would always arrive at identical results This is not true One way changes the BPF indicator and the other does not Again this discussion applies to CW mode In SSB mode the BPF indicator will never turn on The first way to adjust the IF filter width is to use the BW F 1 button in conjunction with the main VFO knob You must enter the IF filter set mode in order to use this way When this way is used the BPF indicator will turn on f
9. r will be off Turn the outer Twin PBT knob to the right 10 clicks Turn the inner Twin PBT knob to the left 10 clicks This should result in a 200 Hz filter Since the filter was narrowed with the Twin PBT knobs the BPF indicator should still be off You now have a 200 Hz wide filter with the SSB shape factor Select the second filter and press the DEF button to return the filter to the factory default width of 500 Hz Since the filter width is 500 Hz or less the BPF indicator will be on Press the BW F 1 button and while holding the button rotate the main VFO knob to the left until the width reduces to 200 Hz You now have a 200 Hz wide filter with the CW shape factor You now have two filters with identical widths but with the two different shape factors Tune around the band and use the Filter button to select the different filters Remember when evaluating filters check the BPF indicator state When BPF is on you are using the sharp CW filter shape When the indicator is off you are using the broader SSB filter shape The Difference So is this a distinction with a difference While the difference is not large my own subjective opinion is that it significant The SSB filter shape sounds much more like traditional filters in an analog IF radio The CW signal edges are not as sharp and grating The sound is a bit softer Even the background noise of the band changes I could imagine that it would be much less tiring on the ears to lis
10. rom 30 000MHz to 37 000MHz because of potential for harmonic emissions If transmitting in this range is required it may be possible to use an external transmit low pass filter with a roll off at 40MHz to suppress harmonics at 60MHz and above Note 7 For test purposes transmitting of a very low power signal is possible between 100kHz and 1599kHz This may be useful as a short duration signal source however if continuous transmit in this range is required more testing must be done to verify that the transceiver will not be damaged Disclaimer Amateur Radio Operators have a long tradition of modifying radio equipment Many people use amateur transceivers as test equipment This message is for educational purposes only All the information that is known at this time has been provided in this comment Please do not contact the author about this comment on the modification The bottom line It s your radio Do with it what you like Don t tell me about it User comment SMHz Operation Power Amp Low Pass Filter From Bonnie KQ6XA http www kb21jj com data icom ic 756pro htm 6 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database Update 5MHz Channel USA UK operation of IC 756 PRO and IC 756 PRO 2 Caution 5 3MHz operation is not advised until a further modification to the Transmitter Power Amplifier s Low Pass Filter Presently the PRO has a Low Pass Filter after the RF Power Output Amplifier that presents a high SWR to the PRO s R
11. side the shiny box You have lift the dsp box with care out of the main board After removing the dsp you can see a row of diodes with some spaces You can see on the right of the row the number 3751 that diode is already missing hi You have to remove 3 small diodes located on the left side of number 3751 diodes 3752 3753 and 3754 when you have a version 03 Icom IC756 PRO When you have a version 02 remove diode 3751 3752 and 3753 diode row IC756 PRO version 403 before modification after version 3 mod after version 2 mod diode installed diode not installed resistor http www kb21jj com data icom ic 756pro htm 2 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database Trouble to find the diode row It s about 1 2 inch located under the CPU If you can t find out the version of your 756 PRO 02 or 03 look at the topside of the box see white sticker If there s something going wrong I ve no responsibility This info is from a Icom infosheet 73 van Hans PA3HGT uit Yerseke PA3HGT 9ONICED WVL BEL EU Here is two pictures from astra 8 6 MAIN UNIT TOP VIEW H FEE FEH AEE ETT TEEPE ELLLLLLLI a Ue 5 on Location of removing diodes 8 http www kb21jj com data icom ic 756pro htm 3 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database e 8 9 8 RARE Remove D3751 D3752 D3753 http www kb21jj com data
12. ten to the SSB filter shape for extended periods In contest situations however with crowded bands the CW filter shape will probably be more desirable Your mileage may vary With appropriate test equipment it would be possible to make a more accurate estimate of the shapes of the filters Although I don t have that equipment I do have the program Digipan which is PSK31 software with an integrated audio spectrum analyzer Here is a Digipan screen capture showing three filter choices http www kb21jj com data icom ic 756pro htm 9 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database 1000 I 0 Hz BPF agile 7 nBPF 200 Hz BPF http www kb21jj com data icom ic 756pro htm 10 di 12 23 08 2009 22 38 09 Hz 10 KB2LJJ Radio Mods Database Hz 10 http www kb21jj com data icom ic 756pro htm 11 di 12 23 08 2009 22 38 09 KB2LJJ Radio Mods Database Observe the polarities on both capacitors the negative side of C316 is towards the front panel There s space to mount the new cap using double sided tape next to the memory back up battery This will improve the bypassing on the 8V rail and should cure the problem 0 4 Mount here with ldouble sided tape User comment 756 pro adjacent channel noise problem From Jim Jarvis N2EA This is a good modification It removes the Noise Blanker drive signal from the main audio in the 756pro As the first person to make this modification and one of the au
13. thors of the QST article I would have expected pubs credit for use of my our copyrighted work This is a good service however and K1KP and I would have granted permission and even provided a better picture than the one you chose that option is still open to you Contact me 73 N2EA http www kb21jj com data icom ic 756pro htm 12 di 12 23 08 2009 22 38 09
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