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Author Topic: Oscillations with increased mic gain  (Read 2547 times)
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n4joy
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« on: February 17, 2019, 03:24:55 PM »

I was rag chewing on my Lettine 242 6 meter transmitter and the other station reported that my audio suddenly became distorted.  All is fine with mic gain turned down all the way, but I begin to hear a high pitched whine and oscillation when the gain is turned up and to my usual level.  I replaced the 12AT7 speech amp with a NOS and swapped all tubes including the final--no change.  The whine does not occur when the mic is unplugged but the gain is turned up to full.  Everything appears to be good inside the rig--no out of tolerance resistors and all caps were recently replaced.  I thought maybe RF was getting into the rig, so I transmitted into a dummy and still had the same problem.  Tube compliment is two 6V6s driven by a 6V6 for the modulator, 6146 final, two 5763s, 12AT7 speech amp, and a 5U4 rectifier.

Could the issue be with my driver transformer or modulation transformer?  I don't have a tube tester, so I cannot rule out a bad tube despite swapping with ones I suspect are good.
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K1JJ
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« Reply #1 on: February 17, 2019, 03:37:11 PM »

Hmmm.... you replaced the caps already...   But oscillations, whines and instabilities in audio amplifiers are usually caused by bad electrolytic caps.  While the problem is happening try jumping a good cap across the suspected bad ones until you see a big improvement.  That may narrow it down quickly.  You might want to use a series resistor to initially charge each cap before applying it directly to avoid big sparks.

Also, looking at various stages for localized oscillations with the scope probe may help. In addition, disconnecting stages, the divide and conquer technique, sometimes works to isolate the bad area.

Don't give up on RF getting in the audio yet, especially on 6M. What happens when you unplug the first audio preamp tube?  Whine go away?   Try some ferrite cores in the audio input.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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n4joy
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« Reply #2 on: February 17, 2019, 03:43:55 PM »

Thanks!  Electrolytics are only a few years old, but I will check.  I have a 22uf at the cathode of the 6V6 driver, two 16uf for the filter, and one 16uf that runs to G2 of the 6v6 driver.
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n4joy
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« Reply #3 on: February 17, 2019, 03:50:37 PM »

What happens when you unplug the first audio preamp tube?  Whine go away?   Try some ferrite cores in the audio input.

T

There is no whine with the 12AT7 removed and no whine with the mic unplugged but gain cranked full.  Odd that this suddenly started with no changes to my setup--same beam and same mic.
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K1JJ
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« Reply #4 on: February 17, 2019, 04:12:46 PM »

What happens when you unplug the first audio preamp tube?  Whine go away?   Try some ferrite cores in the audio input.
T
There is no whine with the 12AT7 removed and no whine with the mic unplugged but gain cranked full.  Odd that this suddenly started with no changes to my setup--same beam and same mic.

Try some ferrite cores in the mic cord and input area and also some .001s sprinkled around from audio leads to ground in that first audio stage.

Sometimes RF in the audio starts out of nowhere and the only solution is bypassing, trying different mics, better shielding, etc.  Trial and error may be the only way to find this one.

T


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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
Tom WA3KLR
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« Reply #5 on: February 17, 2019, 05:14:46 PM »

I would try a 100 pf to 220 pf cap grid to ground on the first and second mic amplifier stages and try adding a small choke (or a couple of ferrite beads) at the mic connector pin in series before the first audio grid to ground cap.

How close is your antenna to the transmitter?
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
n4joy
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« Reply #6 on: February 18, 2019, 09:19:05 PM »

I appreciate the responses and advice.  I am embarrassed to say that it was indeed RF getting into the mic and possibly a poor mic connection.  The 6 meter beam is up 40 feet and just outside the shack--fairly close.  I added several ferrite cores and also reworked the mic plug.  I was using my Shure 450, which is set up for my NCX-3.  I have a short "adapter" cable that consists of a stereo 1/4" female and 1/4" male mono to bypass the PTT so I can use it with my Lettine.  The female to male connection is loose and the whine would disappear when jiggling the cable.  Perhaps the ground wasn't making a connection.  I also could hear a slight hum that subsided when touching the mic.  This is all novice ham stuff... sigh....

I was on 50.4 last night and the other op (WA4BWO) provided me with a recording via his speakers.  All seems well now.  I just need a working D-104 now.  Wink

Thanks again for the help.

Chris, N4JOY


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K1JJ
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« Reply #7 on: February 18, 2019, 09:29:11 PM »

I appreciate the responses and advice.  I am embarrassed to say that it was indeed RF getting into the mic and possible a poor mic connection.  Thanks again for the help.
Chris, N4JOY


Glad it was reasonably easy to find and is working again, Chris!

Yep, the symptom where you unplugged the mic or first preamp tube and that stopped the whine told us the area to look.  Divide and conquer works FB.

Have fun on 6M AM.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

Wise Words : "I'm as old as I've ever been... and I'm as young as I'll ever be."

There's nothing like an old dog.
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