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Theremax 9505d, mild 'sputtering' noise?
http://www.paia.com/talk/viewtopic.php?f=8&t=630
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Author:  glyndon [ Tue Dec 16, 2014 4:27 pm ]
Post subject:  Theremax 9505d, mild 'sputtering' noise?

I built a 9505d kit about 4 years ago, and it has always worked exactly as the instructions indicated, with one exception:
Underneath the tone being played is a sort of 'sputtering' sound, similar to how a chainsaw or a Harley sounds when idling. Consistently present somewhat random period, just audible enough to be noticeable but not so loud that it upstages the tone being played. If it were a person singing you'd say they need to cough or clear their throat. The speed of the noise doesn't vary with pitch or volume played. When pitch and/or volume are nulled, the noise is completely absent.

Several times I've walked through the entire board and wiring to confirm nothing was in error, electrolytic installed backward, etc. All solder joints are bright and smooth. Flying wires are modeled after the PAIAI photo elsewhere in these forums.

Has anyone heard this before, or can suggest a likely cause?

I've attempted to isolate the cause by looking at various points with the oscilloscope, but so far no luck.

The noise seems to persist no matter what I use as power supply or output amp. The 8.2V power (downstream of the zener) is stable and flat.

Thanks for any insights you can shed on this.

Author:  glyndon [ Wed Dec 17, 2014 9:56 am ]
Post subject:  Re: Theremax 9505d, mild 'sputtering' noise?

Never mind. Solved.

Lesson learned (in case it helps someone else remember to not overlook the obvious): Never underestimate the amount of interference that can be generated by dimmed incandescent lighting.

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