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Author Topic: Audio intermittently sounds like teakettle or seashell !!  (Read 4271 times)
KG6SII
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Posts: 27




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« on: February 25, 2012, 08:16:01 PM »

This sounds a bit odd, but sometimes the receive audio on my Yaesu FT-747GX HF rig sometimes goes up in pitch over a period of about 10 seconds or so.  The bass goes away, and it sounds like a tea kettle just starting to whistle, or similar to if you hold a conch shell up to your ear.  Then after a few seconds it immediately goes back to normal audio.

Does anyone know what may be causing this?

Thanks & 73's
KG6SII
Glenn

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ZS5WC
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« Reply #1 on: February 25, 2012, 11:46:46 PM »

Hi Glenn,

I had a Yaesu 747GX for years--great rig!.. BUT the annoying Carrier oscillator shift can be a pain.
You see, the carrier oscillator plastic trimmers have a wiper that makes contact with the moving vane of the trimmer, and this wiper becomes a poor contact after a while.

First of all, the trimmers are located on a little board that you can see through the three trimmer holes in the back of the diecast aluminium panel. (I recall they are marked USB / LSB / AM.)
The BEST solution would be to replace these three trimmers with good quality ceramic trimmers, and re-align the oscillators. (I did this to my 747GX and used vertical mount trimmers from an old Yaesu FT-7B parts rig.)
You can re-align by 'EAR' or using a frequency counter.
OR--spray the three trimmers with DEOXIT, turn them round and round till the wiper is clean, and then REMOVE the spray using pure alcohol and / or compressed air.
OR alternately use pure alcohol from the drug store , spray it into the trimmer with a syringe, turn the trimmer around about 50 times, and let alcohol evaporate. Then Align.
(The Contact cleaner acts like a dialectric and needs to be removed.)

Let us know if you solved the problem?.

73 de William
ZS4L / ZS5WC
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KG6SII
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Posts: 27




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« Reply #2 on: February 26, 2012, 10:33:08 PM »

I had a Yaesu 747GX for years--great rig!.. BUT the annoying Carrier oscillator shift can be a pain.
You see, the carrier oscillator plastic trimmers have a wiper that makes contact with the moving vane of the trimmer, and this wiper becomes a poor contact after a while.

73 de William
ZS4L / ZS5WC

William,  wow, thank you very much for the info.  I will definitely check out those trimmers and let you know what I find.  One question, though... I am noticing on PSK31 in Digipan that I am getting reliable responses to my CQ's, but the responses always appear on a different tone/frequency.  I then select the person's tone, but they can't hear me.  I am unable to work PSK31 with anyone for this reason.  Do you think the carrier oscillator shift problem on the FT-747 could be the cause of this?

A thousand thanks,
Glenn
KG6SII
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ZS5WC
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« Reply #3 on: February 27, 2012, 01:41:11 AM »

Hi Glenn,

Unless the oscillators shift during the PSK QSO, I don't think this is the same problem.
The RX / TX should be on the same frequency even if the oscillator shifts.
The signal would just be misaligned through the Crystal filter, hence the funny audio.

Ensure that you have the CLARIFIER switched OFF!..

Let us know how you get on.

Cheers,
William
ZS4L / ZS5WC
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KB1NXE
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« Reply #4 on: February 27, 2012, 07:29:49 AM »

I get this occasionally with my IC-756 Pro III.  It's attributed to the Auto Notch Filter being on and someone tuning up on frequency (or near by).
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N8YX
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Posts: 112




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« Reply #5 on: March 03, 2012, 07:29:09 AM »

I get this occasionally with my IC-756 Pro III.  It's attributed to the Auto Notch Filter being on and someone tuning up on frequency (or near by).
The FT-747 does not have an auto-notch feature.
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KE3WD
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Posts: 5694




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« Reply #6 on: March 03, 2012, 01:41:57 PM »

I suspect OSCILLATION, not alignment problem here. 

Ceramic Filters can become microphonic, sometimes to the point of making hollow sounding audio feedback. 

Easy troubleshoot, tap on any ceramic filters while listening to the receiver on the testbench.  Good ones should not amplify the tap.  Bad ones will act much like a contact microphone.  Wood pencil, eraser end, is my tool of choice for checking ceramic filters for this rather common problem.  Cure is to replace the filter when one has been isolated and found to be microphonic.


73
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ZS5WC
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« Reply #7 on: March 09, 2012, 03:51:48 AM »

 Huh

Hi Glenn,

Any progress on this fault?--just curious..
As the previous poster pointed out--could be a filter as well--(hope not), or perhaps a dry joint around the Xrystals / trimmers.

73
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KG6SII
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Posts: 27




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« Reply #8 on: March 11, 2012, 12:08:56 AM »

Any progress on this fault?--just curious..

William, I have not cracked the case on my radio yet.  It seems to be happening less frequently now.  Perhaps it was more common on certain bands, or I am keeping the thing exercised!  I did try touching the rear carrier-adjust trimmers with a jewlers screwdriver while the phenomenon was happening (I applied some pressure, but did not turn them), and got no audible change for the USB and LSB trimmers, but I DID get an audible response on the CW trimmer.  I was listening to SSB at the time, so it is curious why the CW trimmer was the one affecting it.  I will see if the issue has resolved itself, or if I need to invest some time in opening the unit to troubleshoot.  It is a bit intimidating though.  It's a rats nest of thru-hole components!  I'd almost be happier if the PCA's were more digital!

Thanks & 73,
Glenn
KG6SII
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ZS5WC
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« Reply #9 on: March 11, 2012, 04:26:38 AM »

 Smiley
Hi Glenn,

Don't be daunted with the prospect of opening the rig--just beware those covers can break if you don't apply pressure in the correct spots to release them. Follow the owners' manual.

Regarding the trimmers--you might find that perhaps the CW trimmer is used Together with one of or both of the SSB trimmers--so check out the service manual.
I find repair and restore very gratifying with the older rigs-you'll be shocked to see inside the newest rigs--these SMT devices are barely visible to the naked eye!.. Shocked

That FT-747 is a great little rig-I used mine mobile barefoot with a Texas Bug catcher and could even work DX on 80m all over Africa.

Good luck!
73 de William
ZS4L / ZS5WC
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KG6SII
Member

Posts: 27




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« Reply #10 on: April 08, 2012, 04:08:29 PM »

OK, so I have my FT-747GX open and have tapped on everything with a wooden dowel, and only 1 thing is affected; the CW carrier adjust trimmer cap (for IF passband center offset from carrier).  The PLL trimmers, optional filters, and miscellaneous trimmers all seem fine.

I am about to spray DeoxIT into the CW carrier trim cap, but I don't have the necessary equipment to do a full alignment.  Can I just mark the rotational position of the trimmer, then turn it around a few times to deoxidize it, and return it to about the same spot?  Or is it the trimmer cap so sensitive that it will still require an alignment afterwards?

Thanks much,
Glenn
KG6SII
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KE3WD
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« Reply #11 on: April 08, 2012, 05:18:07 PM »

You will very likely have to perform the calibration routine for that adjustment.

Deoxit and trimmer caps often do not get along, though.  Use of such may permanently detune the capacitor, making adjustment impossible. 

Denatured alcohol is a better choice for this task, and be sure to rotate the adjustment several times to obtain mechanical cleaning action while the alcohol is still wetting the contacting surfaces. 

Marking the cap first may work to get you into the ballpark adjustment area, though, and from there you may be able to fine tune it in a bit easier than without that. 


73
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