Noise suppression Alternator Conversion

[Tom Mc]

My M20c (new to me) was converted to Zeftronics R15300 and ALY-8520Ls Hartzell alternator.

My aircraft has been modified so many times I do not believe the 1962 wiring diagram is relevant for the charging system.

Would the Alt/reg conv STC retain said capacitor?

My question is - Is there a noise suppressing capacitor in the system?  I have heard some older Mooney have them and some didn't...

>I am having radio static issues....

>The Zeftronics R15300 and ALY-8520Ls Hartzell alternator were installed in 1999, how ever I replaced them last week due to charging failure. Static issue predates my replacements.

> Equipment includes GNS530 and King KY 196.

thanks

 

[EricJ]

Generally there is a capacitor, yes.

 

[Tom Mc]

In line or Parallel with the Alt output?

Edited Wednesday at 04:07 PM by Tom Mc

[PT20J]

58 minutes ago, Tom Mc said:

In line or Parallel with the Alt output?

The capacitor should go from the alternator output to ground. On my M20J, there is a large electrolytic cap and an inline pi-filter on the alternator output.

[Tom Mc]

Below is My Alt connections when i bought the airplane

No Cap on install.  

I'll look for a large Cap (in line with output)  later tomorrow...

...thoughts on install?

 

[Fly Boomer]

19 minutes ago, PT20J said:

The capacitor should go from the alternator output to ground. On my M20J, there is a large electrolytic cap and an inline pi-filter on the alternator output.

Is the PI filter high-pass or low-pass?

[PT20J]

11 minutes ago, Fly Boomer said:

Is the PI filter high-pass or low-pass?

I don't have any specs on it and it's a custom Mooney part. I assume it's a low pass since the idea is to pass DC and filter out any AC noise. I believe Lonestar makes some.

[Tom Mc]

I just talked with Harztell tech support - they said no Cap on the Alt output.

The install as shown above is correct.  The Zeftronics R15300 is the noise filter- it can react fast enough to mitigate it...He didn't know how it all work inside the R15300.

cheers

[Tom Mc]

The next step is to plug in my Fluke MM (Alt output to ground) and see it there is any AC signals.  My Fluke Mod 87 s/b fast enough to see any transients.   Coming off the BUS would work as well.

TBD....

Any  advice is well received.

 

[EricJ]

You can check the output of the Zeftronics regulator with your multimeter in both DC and AC modes.   DC should be ~13.7-14V when running, AC will ideally be zero.   If AC is not zero it might tell you whether there's some noise on the output.

[Jim Peace]

I have the hartzell alternator and my capacitor looks like this: 

[Fly Boomer]

10 minutes ago, Jim Peace said:

I have the hartzell alternator and my capacitor looks like this: 

Oooo, I want one of those!

[Yetti]

Say a 1975F has a big ol noise suppression unit on the passenger bulk head. 

[Old Chub]

My 1968 C model has one on the firewall (engine side) as well.   It is between alternator output and the amp meter shunt which is on the other side of the firewall.  

[N201MKTurbo]

After 40+ years of airplane ownership, and the years I spent in the car audio biz as an engineer. I can tell you that if you are having alternator noise, it isn’t being caused by your filters. It is caused by a bad phase in the alternator or a bad ground in your audio system.

[Tom Mc]

I am getting noise in my Comms XMIT only...As received by ATC.

 

 

[N201MKTurbo]

Do you hear it in the side tone? Does it happen on both radios?

[Yourpilotincommand]

Wondering if it would be better to put a capacitor on the headphone rx jack to see if that works. Has anyone tried that? I have a very small alternator noise, but it doesn’t bother me as it is very low. Just a thought. 

[MikeOH]

17 minutes ago, Yourpilotincommand said:

Wondering if it would be better to put a capacitor on the headphone rx jack to see if that works. Has anyone tried that? I have a very small alternator noise, but it doesn’t bother me as it is very low. Just a thought. 

As he says ATC hears it when he transmits, I doubt that is going to address the root problem.

[PT20J]

Sidetone may or may not indicate a noise in the transmitted signal. Some installations generate sidetone in the audio panel rather than the radio. Some radios generate sidetone from the amplified mic input before the modulator. Some sidetones use a demodulated signal taken from the transmitter before the final stage.

[MikeOH]

49 minutes ago, PT20J said:

Sidetone may or may not indicate a noise in the transmitted signal.

Very true.  However,....

 

On 7/30/2025 at 8:39 PM, Tom Mc said:

I am getting noise in my Comms XMIT only...As received by ATC.

So, pretty sure the OP's issue is TRANSMITTED!

[Yetti]

On 7/30/2025 at 10:39 PM, Tom Mc said:

I am getting noise in my Comms XMIT only...As received by ATC.

 

 

Like what kind of noise?  A whine or static.  If it's a whine, then bad ground to engine-airframe.   if static then bad antenna coax or remove and clean antenna to airframe contact point. if that does not work, then replace coax with RG400.   Was it wet out?

[N201MKTurbo]

2 hours ago, PT20J said:

Sidetone may or may not indicate a noise in the transmitted signal. Some installations generate sidetone in the audio panel rather than the radio. Some radios generate sidetone from the amplified mic input before the modulator. Some sidetones use a demodulated signal taken from the transmitter before the final stage.

All true, but it is a clue. Except for the audio panel side tone, it tells you that you are getting good audio to the transmitter. If he is getting good audio to the transmitter, there must be something on the transmit power input. He also never stated if it was on both radios.

[Bartman]

I had Comm issues and a new set of spark plug wires fixed it. You may find a difference by switching off L or R magneto. That’s how I found mine.