[ICOM] IC-756PRO2 tx audio issue

[Stephen Budensiek]

Hello,
   One thought.   I'm wondring if you're having the famous driver
transistor failure problem.   Mine would do that, the drive would drop
slightly because one of the two driver transistors was shorting out,
killing the bias voltage (running it in class C with the one remaining
driver).   It eventually toasted itself, took the other driver
transistor out, burned open a resistor in the bias circuit on the
drivers.   If it is, and you keep driving it at 100w output, it will
toast both the drivers, because the one is way over worked producing
enough drive for 100w output (can't quite do it), then it will fail
also, have 0 watt output.

73's de Stephen, K0SD in TX

**************************************

On Sun, Sep 12, 2010 at 4:02 PM, Eric Dobrowansky K2WD <k2wd at comcast.net> wrote:
>        Good Afternoon, I have an IC-756PRO2 which is experiencing audio problems on SSB. The radio will work fine at times. Then, all of a sudden, the transmit audio will go into distortion, getting worse the longer I transmit. RF feedback external to the rig has been ruled out. The power supply and dc power cable have also been ruled out. Same for the antenna. I have tried multiple supplies and cords. Same on a dummy load or antenna. The radio is being run into an IFR COM120B (internal dummy load), and monitored on another HF receiver. The test is performed by using one of the recorded TX msg memories. The msg recording itself is crystal clear, and sounds clear in the radio's monitor function even when the tx distortion issue is present. This also rules out a microphone issue. The problem also occurs no matter what power level the rig is set to, and occurs the same whether the pwr control is set for 10w or 100w. If I repeatedly play the recorded message, it will sound crystal clea
>  r at first, then slowly get to the point where it sounds like a poorly driven or biased solid state transistor amplifier (remember how those cb linears sound on ssb?). The problem will dissappear for a day, then come back for a day. Then return again at a later time. Very fustrating! I am guessing something is beginning to fail on the PA module. Once in a while, if I key the rig in CW or RTTY, I will see the power output fluctuate down from 100w to around 90w, then back to 100w. Anyone see such behavior before? Eric K2CB         Good Afternoon, I have an IC-756PRO2 which is experiencing audio problems on SSB. The radio will work fine at times. Then, all of a sudden, the transmit audio will go into distortion, getting worse the longer I transmit. RF feedback external to the rig has been ruled out. The power supply and dc power cable have also been ruled out. Same for the antenna. I have tried multiple supplies and cords. Same on a dummy load or antenna. The radio is being run into a
>  n IFR COM120B (internal dummy load), and monitored on another HF receiver. The test is performed by using one of the recorded TX msg memories. The msg recording itself is crystal clear, and sounds clear in the radio's monitor function even when the tx distortion issue is present. This also rules out a microphone issue. The problem also occurs no matter what power level the rig is set to, and occurs the same whether the pwr control is set for 10w or 100w. If I repeatedly play the recorded message, it will sound crystal clear at first, then slowly get to the point where it sounds like a poorly driven or biased solid state transistor amplifier (remember how those cb linears sound on ssb?). The problem will dissappear for a day, then come back for a day. Then return again at a later time. Very fustrating! I am guessing something is beginning to fail on the PA module. Once in a while, if I key the rig in CW or RTTY, I will see the power output fluctuate down from 100w to around 9
>  0w, then back to 100w. Anyone see such behavior before? Eric K2CB
> Good Afternoon,
>
> I have an IC-756PRO2 which is experiencing audio problems on SSB. The radio
> will work fine at times. Then, all of a sudden, the transmit audio will go into
> distortion, getting worse the longer I transmit.
>
> RF feedback external to the rig has been ruled out. The power supply and dc
> power cable have also been ruled out. Same for the antenna. I have tried
> multiple supplies and cords. Same on a dummy load or antenna.
>
> The radio is being run into an IFR COM120B (internal dummy load), and monitored
> on another HF receiver.
>
> The test is performed by using one of the recorded TX msg memories. The msg
> recording itself is crystal clear, and sounds clear in the radio's monitor
> function even when the tx distortion issue is present. This also rules out a
> microphone issue.
>
> The problem also occurs no matter what power level the rig is set to, and occurs
> the same whether the pwr control is set for 10w or 100w.
>
> If I repeatedly play the recorded message, it will sound crystal clear at first,
> then slowly get to the point where it sounds like a poorly driven or biased
> solid state transistor amplifier (remember how those cb linears sound on ssb?).
>
> The problem will dissappear for a day, then come back for a day. Then return
> again at a later time. Very frustrating!
>
> I am guessing something is beginning to fail on the PA module. Once in a while,
> if I key the rig in CW or RTTY, I will see the power output fluctuate down from
> 100w to around 90w, then back to 100w.
>
> Anyone see such behavior before?
>
> Eric
> K2CB
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