Narco com 120-nav 121 interconnections

[hankrausch]

I have a Narco COM 120 that finally got too old to fix so I replaced it with a TKM MX-11R, which uses the same wiring harness.

It worked great until I took out the VOR receiver, which was broken and never used anyway--a Narco NAV 121. Now the speaker does not put out the radio receive signal, but the headset connectioon still works.

Apparently the output of the COM 120 went through the harness to the NAv 121, for the marker I expect. Is there any simple jumper I can put on the old 37-pin connector which would restore the routing to the speaker?

Aircraft is a Piper PA-38 which apparently has all original avionics/wiring.

Many thanks for any help.

 

[gismo]

I have a Narco COM 120 that finally got too old to fix so I replaced it with a TKM MX-11R, which uses the same wiring harness.

It worked great until I took out the VOR receiver, which was broken and never used anyway--a Narco NAV 121. Now the speaker does not put out the radio receive signal, but the headset connectioon still works.

Apparently the output of the COM 120 went through the harness to the NAv 121, for the marker I expect. Is there any simple jumper I can put on the old 37-pin connector which would restore the routing to the speaker?

Aircraft is a Piper PA-38 which apparently has all original avionics/wiring.

Many thanks for any help.

Might be something useful here:

http://www.narco-avionics.com/downloadbody.htm

 

[gismo]

I have a Narco COM 120 that finally got too old to fix so I replaced it with a TKM MX-11R, which uses the same wiring harness.

It worked great until I took out the VOR receiver, which was broken and never used anyway--a Narco NAV 121. Now the speaker does not put out the radio receive signal, but the headset connectioon still works.

Apparently the output of the COM 120 went through the harness to the NAv 121, for the marker I expect. Is there any simple jumper I can put on the old 37-pin connector which would restore the routing to the speaker?

Aircraft is a Piper PA-38 which apparently has all original avionics/wiring.

Many thanks for any help.

OK, I took a look at the installation manuals for the 121 and 120 but I'm confused. The standard setup w/o audio panel seems to have the Nav121 audio routed through the Com120 rather than the other way around. It also appears that of the two, only the Com120 has a speaker output, the audio outputs from the Nav121 are intended to feed either an audio panel or the Com120.

IOW the Com120 (and it's replacement) should be driving the speaker directly.

I can't tell for sure but there's some possibility that your plane was wired with the comm audio fed to the marker audio input on the Nav121 and the Nav121's audio out wired to the aux audio in on the Com120. The IMs show a 1500 ohm resistor from pin 7 to 8 on the Com120's main connector that normally routes the comm audio back into the speaker amp. If that resistor were missing and the wiring was set up per the first sentence in this paragraph, you could fix the problem by jumpering pins 8 and 31 of the Nav radio connector together. It's also possible that pin 36 was used instead of pin 31 and in that case you'd need to connect 8 and 36 together. Trying these two jumpers won't hurt anything so give that a try (or add the 1500 ohm resistor from pin 7 to 8 on the com120 connector.

 

[hankrausch]

I think you are right, I do need the resistor on the COm 120 which is what I was afraid of. I did try jumpering pin 8 to 31 and no joy (but a Resistance check does show that pin 31 does go to the speaker.) Thanks very much for responding.