206A amp acting like a microphone. Anyone ever experience this?

[DocWurly]

I was working on a chopped beige 206A yesterday.  It was in decent shape overall, but I noticed a couple of weirdnesses, which might have been a single issue with the amp. 

One is that, as notes die out, they have a faint but noticeable buzz.  (Not the buzz of a dead fuse...I don't think.  More subtle, like audio not dithering well). 

The other thing, especially strange, is that the whole physical body of the keyboard seems more "alive" than typical.  This is to say, as I tapped all over the thing, or played it, I'd hear much more mechanical noise coming through the speaker than usual. MUCH more.  The resonance was being amplified too much.

It was as if a contact mic were installed in the device.

It's my hunch that there was a loose solder joint somewhere in the preamp or amp which was vibrating.  Has anyone had this experience?

But my other thought was.....what is there WAS a microphone installed, and I just didn't find it?  I've never seen a teacher's model of one of these, and I wonder if it is possible that that's what this was, in spite of its labelling?

Wish I'd taken a picture. It had THREE knobs on it:  The regular volume knob, the "Ensemble/Self/Speaker" knob, and a strange little pot drilled in between the other two, with a small black knob on it.  Neither of the two right knobs seemed activated to do anything.

Any thoughts?

[pianotuner steveo]

No, the "microphonic" sound in a 206A amp is common. I believe it is caused when the trim pot on the amp PCB is turned up all the way. This amp has more power than a 200 or 200A amp, and I think that is the cause. They can even feedback sometimes which can be a cool effect if you want it.

possible causes of the buzzing noise are the speaker(s) could have dirt/debris bouncing on the cones. This is more common than you would think. Another possible cause is bad speakers, but don't waste money on new ones until  you are positive.

[DocWurly]

Speaker buzz did occur to me, and yes, sounds similar.  The owner told me the buzzing was happening even with line out, but I had no easy way to confirm myself.

Interesting that this effect is common in 206A's.

Anyone ever see that third knob on a 206A?

[pianotuner steveo]

It sounds like an inexperienced person tried to add vibrato, but didn't do it right as far as the 3rd knob goes.

[Max Brink]

The microphonic response within the piano is going to be a deeper issue with the amp. It may be as simple as a connection issue if you are lucky but more than likely it will be something deeper. Judging by the extra knob drilled into your Wurlitzer my best guess is that someone tinkered with the amplifier. The last time I came across a highly microphonic 200A style amplifier the issue was a few resistors within the output amplifier section. These parts would have to be added to the Wurlitzer 206A, though, so check to see if TR15 and TR16 were installed.

If you find that the trim pot solves the issue of microphonic noises within the amplifier it is probably just masking the issue. The trim pot is to set the output level of the preamplifier before the power amplifier stage within the amp.

For the buzzing I agree that it is likely a speaker problem. When pushing gently and evenly on the cone does it feel like there is any friction with the voice coils?