Hello All,
I have checked all the tubes for microphonic and none of them have it. They all test very good, even the on the mutual conductance test. When the tuning capacitor is tapped when the radio is on one of the SW bands (2, 3, and 4), the tapping can be heard through the speaker. On strong SW stations it sounds like feedback and it's reduced if the volume is reduced. How do I get rid of this annoying problem? This does not occur on the Broadcast Band. Is this normal for this model?

Thanks,

Dave

The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.

Warren,
I tried a few 12SA7s and it still feeds back on SW on strong stations if the volume is turned up. I also tried different 12SQ7s and different 50L6s and it continues to feedback. The tuning capacitor is what has the Microphonic sound when tapped. NOT any of the tubes. This does not happen on the Broadcast band. I think that this "problem" is normal for this model. But I just want to be sure.

Thanks,

Dave
:The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.
:

:Warren,
: I tried a few 12SA7s and it still feeds back on SW on strong stations if the volume is turned up. I also tried different 12SQ7s and different 50L6s and it continues to feedback. The tuning capacitor is what has the Microphonic sound when tapped. NOT any of the tubes. This does not happen on the Broadcast band. I think that this "problem" is normal for this model. But I just want to be sure.
:
:Thanks,
:
:Dave
::The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.
::
:
:
Tuning capacitors are microphonic by nature, that is why they are usually rubber mounted. The cure is simply to quit pounding on the radio.
Dave

Hello easyrider8,
Maybe the grommets are dried out? It doesn't feedback through headphones no matter how high the volume is turned up. So it might be the speaker's close proximity to the tuning capacitor? Maybe if I connected one lead from the output to the chassis? Would this put an end to the feedback? I think an external speaker might be the answer.

Dave
::Warren,
:: I tried a few 12SA7s and it still feeds back on SW on strong stations if the volume is turned up. I also tried different 12SQ7s and different 50L6s and it continues to feedback. The tuning capacitor is what has the Microphonic sound when tapped. NOT any of the tubes. This does not happen on the Broadcast band. I think that this "problem" is normal for this model. But I just want to be sure.
::
::Thanks,
::
::Dave
:::The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.
:::
::
::
:Tuning capacitors are microphonic by nature, that is why they are usually rubber mounted. The cure is simply to quit pounding on the radio.
:Dave
:

Replace the rubber grommets on the tuning capacitor.

:Hello easyrider8,
: Maybe the grommets are dried out? It doesn't feedback through headphones no matter how high the volume is turned up. So it might be the speaker's close proximity to the tuning capacitor? Maybe if I connected one lead from the output to the chassis? Would this put an end to the feedback? I think an external speaker might be the answer.
:
:Dave
:::Warren,
::: I tried a few 12SA7s and it still feeds back on SW on strong stations if the volume is turned up. I also tried different 12SQ7s and different 50L6s and it continues to feedback. The tuning capacitor is what has the Microphonic sound when tapped. NOT any of the tubes. This does not happen on the Broadcast band. I think that this "problem" is normal for this model. But I just want to be sure.
:::
:::Thanks,
:::
:::Dave
::::The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.
::::
:::
:::
::Tuning capacitors are microphonic by nature, that is why they are usually rubber mounted. The cure is simply to quit pounding on the radio.
::Dave
::
:
:

+1 on replacing all of the rubber grommets. They have lost their 'spring'.
The grommets between the AC/DC chassis and the metal cabinet should also be replaced if they are collapsed and allowing a potentially dangerous shock hazard.
Check your local hardware store in those handy drawers for a matching set of fresh rubber grommets.

:Replace the rubber grommets on the tuning capacitor.
:
::Hello easyrider8,
:: Maybe the grommets are dried out? It doesn't feedback through headphones no matter how high the volume is turned up. So it might be the speaker's close proximity to the tuning capacitor? Maybe if I connected one lead from the output to the chassis? Would this put an end to the feedback? I think an external speaker might be the answer.
::
::Dave
::::Warren,
:::: I tried a few 12SA7s and it still feeds back on SW on strong stations if the volume is turned up. I also tried different 12SQ7s and different 50L6s and it continues to feedback. The tuning capacitor is what has the Microphonic sound when tapped. NOT any of the tubes. This does not happen on the Broadcast band. I think that this "problem" is normal for this model. But I just want to be sure.
::::
::::Thanks,
::::
::::Dave
:::::The 12SA7 sits right next to the tuner. Could be that tube has the problem. Testing the tube may not show anything wrong.
:::::
::::
::::
:::Tuning capacitors are microphonic by nature, that is why they are usually rubber mounted. The cure is simply to quit pounding on the radio.
:::Dave
:::
::
::
:
:

Hi Dave,
My S-40a does exactly the same,but usually only on the uppermost band 15-40+ meg.I tried new rubber grommets on the speaker-that helped.I tried not tightening the speaker mounting bolts quite so tightly-that helped.An external speaker definately stops it.

I did notice something strange that the far more learned here may be able to comment on and may be of help..I found that when applying an RF signal from the signal generator,it is not necessary to use any modulation to get the feedback to occur.In other words,there does not have to be audio coming from the speaker.Weird ain't it??The internal speaker does not have to be touching tha cabinet to produce the feedback either,just close to its normal position.Sounds like a strange RF feedback problem,i.e., RF getting into the audio chain.(?)I would think that would be a transmitter problem though,not a receiver.

I tried swapping 6SA7s as well,it definately was not the tube.I originally thought that the problem might be that I over-tightened the dial cord when I replaced it,and audio vibrations were affecting the tuner.But that doesn't seem to make sense seeing as though the feedback will occur with no audio.

If you figure this out,I would be glad to hear the soution.I gave up on it a long time ago.I just back-off on the RF gain on strong signals and live with it.

I am not familiar with this radio, but why tuning capacitors become microphonic for the short wave bands and not the broadcast band in most radios is that the tuning becomes narrower and narrower as you increase in frequency. That means that more of a change in frequency occurs with changes in capacitance on the high frequency bands than on the lower frequency bands. That further means that the frequency modulation gain of the circuitry will be greater.

If this radio happens to have a broad tuning feature, you might notice this phenomenon go away or be reduced in nature.

T.