ICOM 746pro RX Audio / Filtering Problem

Discussion in 'General Technical Questions and Answers' started by KC3LNB, Jan 27, 2021.

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  1. KC3LNB

    KC3LNB Ham Member QRZ Page

    Hi folks,

    I’m a fairly new ham. Only a tech, and not too technically savvy. I’m reaching out to see if anyone has had a similar issue with an ICOM 746pro that I’m experiencing now. Or, may have ideas as to the problem/fix. I mostly work CW on HF 80, 40, 15 and occasionally 10m I do occasional work SSB and digital on 10m & 6m (when bands are open). Anyway, here is what I’m experiencing:

    · Intermittantly but more increasingly (abt 50% of the time) my RX audio in CW turns to a “buzzing” sound. So, instead of crisp dits/dah’s… I get buzz, buzz.

    · When the buzz starts, if I tune up the band to SSB (just listening on HF, I’m only a tech) the voices sound buzzy, mushy, like Charlie Brown’s teacher.

    · It comes and goes. Mostly for long periods (say 30 min or so) it starts/continues or goes away.

    · This seems to only occur on 80 & 40m when I’m operating. When it happens, I’ll tune to 160 & 20 just to check if it happens on all bands at the same time. Here is what I’ve found. If it happens on 80m, it occurs simultaneously on 40m, and a quick QSY to 160 will find it happening… but NEVER on 20m. I’ve never had the problem on 15m, 10m or 6m.

    · I’ve tried adjusting filters, NR, Notch, RIT, AGC etc. Nothing affects it.

    · I’ve run the rig on only battery power to see if it’s the power supply (MFJ 30am switching Ps)

    · When the “buzzing” occurs, the rig does not seem to have selectivity at all. I get signals from 10 to 15khz on either side of the base frequency. When the buzzing stops, selectivity and crispness of dits/dah’s go back to normal.

    · I borrowed a TS-440s from my club and did a side-by-side test. Same freq. Same antenna (controlled by a manual switch). TS-440s, no problems at all. 746pro, buzzing comes and goes. I’ve documented this experiment on videos and can share.

    I switch bands and antennas quite frequently and have made mistakes over the last year or so. Example, I have 6 antennas and sometimes mismatch antenna to intended frequency. Most my antennas are tuned to specific freqs, but if I mismatch I end up having the rig’s auto tuner try to tune a non-resident antenna. I have read this could be a problem. Especially if I did NOT turn back the RF output power.

    Anyway, I would appreciate any thoughts or opinions. Its hard with COVID to visit experienced hams to have them take a look at the rig. Not sure where to turn so I thought I’d post this in QRZ and see what the responses of experienced folks may be.

    Scott, KC3LNB
     

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