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
     
    AA2EC likes this.
  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
     
  7. KD7F

    KD7F XML Subscriber QRZ Page

    Fabio my IC7100 is not modified and I also don't have a problem with low power. I don't use the microphone that came with the radio. I purchased mine in mid 2015.

    Regarding the output level changing with FM modulation, I also see the effect. I suspect their algorithm is limiting the bandwidth following modulation which will cause some incidental AM. A peak reading meter will see a larger variation than an average reading meter due to the envelope wave shape I see.

    My firmware hasn't been updated I don't believe. Looking at the version info I have the following:
    CPU: M 1.11; S1.00; C1.10. DSP: I 1.03 D 1.00
    I don't recall updating it.

    Commenting on Phil's observation that going from one tone to two tones dropped the power from 100w to 50w without the mod, that is exactly what should happen with the average power if the peak power remains at 100w. If both tones are at the same level, the peak two tone input waveform will be twice the amplitude of the individual tones, thus the average output level caused by each tone will be 6dB below the peak output or one quarter of the peak power. The total average output power in the two output tones would then be half of the peak power. When looking at the single tone, the average and peak powers are equal since the output envelope is constant. I have an LP700 and have verified this behavior on my unmodified 7100. The single and two tone peak power was measured as 97w. The single tone average was 97w and the two tone average was 49w. Of course compression must be turned off while making these measurements.

    The capacitor and transistor bridging ALC mods I have seen on the Internet prevent proper ALC operation which results in excessive output levels and overshoots. They definitely increase levels seen on a power meter. It would be better to reduce the gain in the ALC amplifier by changing the value of the 330k feedback resistor if a problem with overactive ALC existed. I suspect the supplied microphone is causing the low power issue seen. Mine exhibited strong pops with breath entering the sealed chamber they have the mic element in. I think they made the hole on the front of the mic so small in an attempt to reduce the pops but it reduces the sensitivity at the same time so the pop to audio ratio still remains large. The ALC mods seem to work because they slow down the ALC response so it can't respond to the pops, but it can no longer respond to other peaks either. I don't use the supplied MIC but I wonder if increasing the size of the opening on the front of the mic and filling the cavity in front of the element with cotton might help.

    Jim, KD7F
     
  8. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    Jim, thanks a lot for your contribution to this thread. I'm glad to see that my conclusions are supported by your tests, too. I have never tried using a different mic, basically because this is meant (to me) to be a more or less portable radio, and using a desktop mic is not an option. Your idea of a mic mod is interesting, but it seems that no one else has tried it. Still if you google "ic7100 mic mod" you will find other people have applied electrical mods to the mic (and not to the ALC). Some other people have used the mic from the 7300, and some have used the one from the IC706, to have better TX audio quality. Also, it seems that some 7100s come with the HM-151 (the one with full keyboard that was sold also with the 7000) while other 7100s (like mine) come with the HM-198.

    Since I seldom use the 7100, I have not tried working on the TX audio quality.

    I'm actually somehow unhappy about this radio. While it's not a bad radio, it could have been better than this.
     
  9. KD7F

    KD7F XML Subscriber QRZ Page

    I have an IC706 which has been experiencing failures due to intermittent through hole vias on two of its PC boards. I have been finding and fixing them but I fear it will be a never ending source of problems (bad run of boards?). I bought the 706 new in 2003 and the first failure occurred in 2008. The most recent just happened last week. I ordered a second 7100 which is actually arriving today to replace the 706, which I use in a secondary operating position in my living room. I will then have two HM198s so I am going to modify one and compare the two from audio response and output level perspectives. If the modification makes a big difference I will let you know the details. I will also compare the 706 MIC for another data point.

    I like the 7100. It is a step up from the 706 feature wise. It is just too bad ICOM paired it with the HM198 MIC. I actually saw a post somewhere where someone was saying the HM198 was better than the HM151 from an audio perspective, so perhaps ICOM has changed its design since 2015. I will open the new one up and compare it to my 2015 version.

    Jim, KD7F
     
  10. IZ4UFQ

    IZ4UFQ Ham Member QRZ Page

    I'm looking forward to see your test results. Consider also this mod I got from mods.dk...
     

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