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Author Topic: problem in ssb talk path - any help appreciated  (Read 701 times)
FOXBAT426
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Posts: 274




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« on: November 10, 2010, 08:53:42 AM »

hi, i have a strange problem with my ft-890. after turning on the radio i can transmit normally in ssb mode (usb or lsb) for about 10 minutes or so. great voice peaks, full 100 watts etc etc.

After about 10 minutes or so the transmit signal drops down to about 10 watts or so with no voice peaks on usb and dead on lsb. no matter how loud i talk into the mic, whistle etc (using a dummy load) the meter shows only 10 watts out on usb and nothing on lsb.

 AM and CW modes continue to work fine.

it seems to be thermal related. after letting the radio cool off for a period - it starts to work again for 10 minutes or so and then starts acting up all over. i've reset the processor and it doesn't seem to matter.

any ideas would be appreciated.
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WB2WIK
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« Reply #1 on: November 10, 2010, 09:00:41 AM »

A hair dryer with an extension cone made of mylar or something that won't melt (even cardboard might work if the dryer doesn't get super hot) to pinpoint a hot air stream to a small area, combined with a can of circuit cooler (freon spray) work wonders in finding a component (or solder joint, or connection, or almost anything) that changes with temperature.

Operate with the covers removed and with the radio "cool," use the hot air stream from the focused hair dryer to make small areas of the rig warmer, to see if you can get it to quit.

If so, then immediately used focused circuit cooling spray (these cans come with a thin nozzle, like WD-40 has, to focus the coolant on a very small area) to make that part cold again and see if it springs immediately back to life.

This is kind of "buck shooting" a problem, but it often works and reveals a bad part or connection.
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KE4JOY
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Posts: 1384




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« Reply #2 on: November 10, 2010, 09:17:23 AM »

You dident mention .. is this a new symptom?

Did it use to work okay?

Carrier oscilator tube okay?
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FOXBAT426
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Posts: 274




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« Reply #3 on: November 10, 2010, 09:41:58 AM »

its a solid state rig - no tubes.
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WB6BYU
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Posts: 13574




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« Reply #4 on: November 10, 2010, 10:08:14 AM »

What does the output audio sound like when that happens?

It sounds to me as though the BFO frequency is shifting somehow, which would mean that the output
has a limited frequency range on USB (and far enough out of the passband on LSB that nothing gets
through.)  In an analog rig I'd suggest an unstable BFO crystal, but that would affect CW as well
(though you may still hear signals at higher pitches.)

Try turning down the modulation and measuring the carrier frequency on a receiver (you might have
to unbalance the modulator to get enough signal to operate a frequency counter.)  See if it changes
when the problem manifests itself.
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KB4QAA
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Posts: 2488




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« Reply #5 on: November 10, 2010, 10:53:14 AM »

I think you are over modulating or over driving the poor thing.

Are you staying within the ALC limits?

Try turning off any processor.

Reduce your power to something like 70%  (no reason to run a rig at 100% most times give it a little margin)

Check your Mic Gain turn it down to keep at the bottom of the ALC scale.

Does that model have a fan?  Is it working?  Your radio isn't sitting in the sun is it?

See how that does.
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W5RRP
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Posts: 12




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« Reply #6 on: November 10, 2010, 11:08:52 AM »

Perhaps "10" watts is a magic number.  That is what your power would drop to if the SWR exceeded 2:1, or so.  You might want to monitor SWR and see where that takes you.  I am inclined to think it might be a marginal coax connector, maybe an antenna problem, or something along those lines.
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WA3SKN
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Posts: 5558




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« Reply #7 on: November 10, 2010, 11:21:39 AM »

Does it do it when with a known good dummy load?
High or intermittant VSWR tbls (antenna/coax) could cause the finals protection to operate... causing this type problem!

-Mike.
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KE4JOY
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Posts: 1384




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« Reply #8 on: November 11, 2010, 05:55:01 AM »

He said that AM & CW continue to operate fine so why all the SWR guesses?
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FOXBAT426
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Posts: 274




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« Reply #9 on: November 11, 2010, 09:24:17 AM »

thanks for mentioning that - it is not an swr problem! i'm working into a dummy load, cables are all good - i repeat it is not an swr problem. its thermal related - am and cw are not affected, they continue to work fine as does the receiver. its only with SSB modes.
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