IC7100 FM TX issues on all bands: power varies with modulation.

Discussion in 'Radio Circuits, Repair & Performance' started by IZ4UFQ, Feb 10, 2019.

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  1. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    I have bought an used IC7100, not modified (no SSB power mod), latest firmware installed (Release E5 from 2016).

    I was testing the radio for the notorious SSB TX power issue, which is in fact non-existent, my Bird 4391 wattmeter shows a proper 100W PEP in HF by using the PEP power measurement function.

    While I was at it, I tested the power in all bands and all modes, and noticed an anomaly: power goes up with modulation even when it should not (that is, in FM mode).

    For example, in UHF FM at full power (30W nominal) I get 30 W if I stay silent, but I get 35W if I whistle in the mike.

    This happens also in VHF FM, power goes from 47 to 57 W when modulating, and it happens in HF too (10 meters FM).

    If I set RF power to a lower value (20% for example) the effect is even more evident: in HF (10 meters FM) I get 25W if I do not speak in the mike, and more than 60 (my Bird shows overflow with a 50W slug) if I whistle in the mike.

    What's happening? Is my IC7100 modulating in FM with some spurious AM modulation?

    I have tried resetting the radio (from the menu) but the issue remains.
     
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    You need to look at it on a 'scope. It will show if the power out is not stable in FM mode.
     
  3. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    Since I also see the supply current increase in a significant way when the ouput power seems to increase I presume that the ouput power is really increasing.

    It should be interesting to check if another 7100 behaves in the same way, but I don't have another one available.
     
  4. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    Just for info, I have discussed the issue in an italian language forum, too.

    To sum it up, it seems that other Icom radio with DSP modulator show a similar behaviour.

    Icom ic 821H - analog FM modulator
    Icom ic 756 pro3 DSP modulator
    Icom ic 7600 DSP modulator

    821H (144.5 MHz) unmodulated carrier 40W, modulated (1KHz - dev. 5KHz) 40,3w
    PRO3 (50 MHZ) unmodulated carrier 50W, modulated (1KHz - dev. 5KHz) 63W
    7600 (50 MHZ) unmodulated carrier 50W, modulated (1KHz - dev. 5KHz) 58W

    I still have not found another IC7100 for comparison.

    Here is the link to the italian forum discussion:

    http://www.arifidenza.it/Forum/topic.asp?rand=7055475&whichpage=1&TOPIC_ID=344538&#686226
     
  5. GM8IOA

    GM8IOA Ham Member QRZ Page

    Hi .I have an ic7100 and have had a similar problem. Originally I has the low power problem and modified the set as per the YouTube video. It worked fine but
    after having done the mod I found that my Power on VHF and UHF was changing with the mic audio.
    I know you said that you do not have the low power problem and your set is not modified. I found that by removing the modification that the problem disappeared on the VHF/UHF FM mode.
    I looked at the circuit diagram and what the YouTube modification was doing and deduced that it was interfering with the ALC on all bands and all modes.
    In order to get round the problem I installed a small relay to only put the Modification on the HF bands and not on the VHF/UHF bands. This did help.
    I then obtained a document for the service manual for the IC7100 which contains the alignment procedure for the set. I went through the procedure and found that the problem then disappeared completely. You do need some test equipment to do the service procedure and it is a risky thing to do but if you backup your set to an SD card and do exactly what the document says it is fine.
    Just for interest, before I did this I connected an audio signal generator to the mic and checked the HF SSB power output. I found that with a pure 1khz tone, the set produced the full power of 100 watts but if I added a second tone to the mic I/p along with the original 1Khz tone the power dropped to around 50 Watts. I than did the same test after the YouTube mod for low power on SSB and the full power was seen with two tones on the mic input.
    My advice is for you to check the IC 7100 for the modification yourself and not rely on the sellers statement.
    Another point of interest is that there seems to be two circuit variations of the ALC and I have the circuits for both. due to the surface components used in the IC7100 I have avoided any attempt to change my version of the ALC to the alternative circuit as my eye site is not to good. My impression is that depending on the version of the ALC circuit as to whether the set has a power problem or not without the Modification.
    If you want a copy of the Two circuits and the service manual let me know.

    I hope that this may help
    Phil G8IOA
     
  6. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    Phil, thanks a lot for your answer.

    I have personally checked my IC7100 and no hardware mods have been applied. I know about two possible mods: one adds a capacitor to the ALC feedback delay, and the other shorts a transistor that should change ALC behaviour in AM only, making the change permanent in every mode.

    Both of that mods are not present in my IC7100. I have checked it myself.

    And in fact I can confirm that in SSB the PEP power is 100W, but the average power is about 30W, which is consistent with an unmodified IC7100. This has been measured using human voice, and not a tone generator for AF input.

    I have not tried to realign the radio (via software, using the procedure described in the service manual) because I don't have all the instruments needed.

    Could you please tell me what firmware version is installed in your radio? I have the latest, that Icom says has been modified to increase SSB power (without the mod, of course). Maybe they made some mistake and it's the latest firmware that causes this issue.

    Thanks

    Fabio IZ4UFQ
     

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