KFC 200 / Sonalert issue

[BaldEagle]

So this is annoying.  The AP / pitch trim warning sonalert no longer stops making a noise when the pitch trim switch is turned on (after the "beep beep beep" or auto pilot disengaged.  Instead it produces noise constantly but at, I would guess, about 30% volume.  Any ideas on how to diagnose this issue?  It's not loud enough to hear in the cruise, but is loud enough to hear over an ANR headset with the engine idling.  I can only stop the noise by either pulling the 50amp Aux CB or holding down the autopilot disconnect button on the yoke, so it seems something is still sending some current to the sonalert when it shouldn't, or the sonalert needs replacing (?)

[larryb]

The sonalert circuit is activated when the AP grounds the negative leg of the sonalert device. The positive leg is fed by one of the breakers. So if the the sonalert is making noise when it shouldn't then it means either the AP computer has a fault (current leakage through the driving transistor) or there is a high-resistance short to ground somewhere between the Sonalert negative terminal and the AP controller.  But, since you say it goes away when you hold down the AP disconnect button I would doubt a short in the wiring. So it is most likely an issue with the AP controller. That would be my guess as an EE but not an avionics tech. I would still check it out by putting a voltmeter on both the positive and negative legs of the sonalert.

[BaldEagle]

For a meticulously maintained Mooney, I'm finding all sorts of crazy stuff.  I pulled the sonalert to find a RadioShack part number.  Well that's not in the parts manual.  Replaced with correct part (Mallory, which I was able to get from Amazon for $19) and the problem is solved.

 

[PT20J]

It’s stories like this that I’m glad that my airplane only had two previous owners and neither was mechanically inclined. 

[BaldEagle]

18 minutes ago, PT20J said:

It’s stories like this that I’m glad that my airplane only had two previous owners and neither was mechanically inclined. 

Mine has been maintained by 3 or 4 well known MSCs over the years (albeit not recently), so you never really know when you’ve got 40+ years maintenance history and several owners, so your observation is well taken

[PT20J]

6 minutes ago, BaldEagle said:

Mine has been maintained by 3 or 4 well known MSCs over the years (albeit not recently), so you never really know when you’ve got 40+ years maintenance history and several owners, so your observation is well taken

Well, there were some ham-handed issues (done by a well-known MSC) as well as some avionics work done by a now defunct shop, so even work done solely by well paid professionals is not necessarily trouble free. We just muddle along as best we can. 

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