TKM MX170B Nav and King glideslope CDI: What tools do I need to test Nav output?

[Bobaran]

I have the TKM MX170b and the CDI connected to it is showing flags on a good local VOR.  I'm not sure if the problem is the 170b, or the CDI. 

1.  Is there a way to use a common multimeter to test the 170b harness pins to the CDI?   

2. Is there a way to test the CDI with a multimeter?

3. What other means could I use to test the 170b and the CDI?

The Com portion of the 170b works fine.

 

[N201MKTurbo]

I have the service manual for them somewhere. I don’t have any KX170s installed anymore. I seem to recall that the radio just sends baseband audio to the nav head. The nav head decodes it into flag and CDI signals. The GS uses a separate receiver that generates flag and GS signals. 
 

The bottom line is you cannot test it with a voltmeter.

Have you flown an ILS with it and see if the localizer and GS work?

[EricJ]

It's not straightforward.    The easiest thing to do would be find a functional KX-170B or another MX-170B and swap them.

[Bobaran]

21 minutes ago, N201MKTurbo said:

The bottom line is you cannot test it with a voltmeter.

Have you flown an ILS with it and see if the localizer and GS work?

I have not tried an ILS.  I will do that.   I did try the local VOR, but I was still getting flags on the CDI.  

[N201MKTurbo]

2 hours ago, Bobaran said:

I have not tried an ILS.  I will do that.   I did try the local VOR, but I was still getting flags on the CDI.  

Can you hear any audio from the NAV receiver? If it is just static, then you have a receiver issue. If it is dead quiet, you probably have a receiver problem. You should hear the morris code identifier and a whirring sound in the background.

[vik]

4 hours ago, N201MKTurbo said:

... the radio just sends baseband audio to the nav head. The nav head decodes it into flag and CDI signals. The GS uses a separate receiver that generates flag and GS signals. 
 

 

That is right. The radio just downconverts the signal to a lower frequency and sends that "video" also called composite signal to the head, where the decoding happens. The radio just decodes the audio morse code, which is AM. 

LOC is much easier signal than VOR and can be generated with any decent signal generator that has AM modulation or with two signal generators. One has to have an input for external modulation. 90 and 150 Hz AM modulation for left and right (or right and left, I do not remember which one is correct).

VOR signal is AM modulated by the magnetic direction (+ audio identifier) and a subcarrier around 9.6kHz which provides the base phase, both are 30 Hz signals. If you have a good scope, you can see the AM (audio) modulated composite signal out of the radio. This will tell you that the radio works. I think most radios also generate the voltage to flip the nav flag if they detect the carrier frequency. The CDI head just decode the signal. This might not be true for all of them though.

Looks like it is time to learn some radio stuff

Vik

 

[Bobaran]

Can you hear any audio from the NAV receiver?

I will check the audio on the nav and get back to you. Thanks.

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[Bobaran]

Looks like it is time to learn some radio stuff
Vik
 

Yes! I'm soaking in this info. Attached pic has some output pins for the 170b.

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[Bobaran]

Can you hear any audio from the NAV receiver? If it is just static, then you have a receiver issue. If it is dead quiet, you probably have a receiver problem. You should hear the morris code identifier and a whirring sound in the background.

Tested yesterday and at times could hear the code. I was only a few miles from the for. Zero luck with the LOC ILS.

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[Bobaran]

Can you hear any audio from the NAV receiver? If it is just static, then you have a receiver issue. If it is dead quiet, you probably have a receiver problem. You should hear the morris code identifier and a whirring sound in the background.

Flew yesterday to test and mostly static on nav. Occasionally I could hear a faint Morse code but no whirring sound. Also tried a LOC frequency and nothing showed on the GS.

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[takair]

Memory is 20 years since I’ve worked on that CDI style.  Remind us the part number of the CDI?  As I recall, they are full of old school carbon tuning pots that were hard to get aligned 20 years ago when they were 30 years old.  Might consider that part of the problem is in the CdI.  

[EricJ]

By modern standards the interface between the nav/com and the CDI on those old units is pretty horrible, and in particular with that CDI the glideslope receiver is entirely inside the CDI.   As mentioned, there are a ton of adjustments in the CDI.   The TKM is a new enough implementation that it's a much better integrated system, but still not easy to debug.

Bottom line is that it's a pain to sort out what the issue is if it's not something straightforward like a loose connector or pin.   Lots of functional KX170s and TKM170s are loose in the wild now since they commonly get removed during panel updates.  This means there may be one somewhere near you that you can borrow to swap in and check whether it improves the situation or not.   That'll at least give an idea of whether the issue is in the nav/com or the CDI or in the connectors/harness.   The nav/com is pretty easy to slide out to make the swap.

There may also be somebody around with a ramp tester that makes it easier to test the vor receiver on the ground.

[Bobaran]

Memory is 20 years since I’ve worked on that CDI style.  Remind us the part number of the CDI?  As I recall, they are full of old school carbon tuning pots that were hard to get aligned 20 years ago when they were 30 years old.  Might consider that part of the problem is in the CdI.  

066 3025 01 is the King model number

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[takair]

3 hours ago, Bobaran said:

066 3025 01 is the King model number

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KI-214….  I do recall those to be problematic as they aged….  Checking audio is certainly an indicator if the signal is making it through the radio.  Someone asked how it behaves on ILS or localizer, that would help narrow things.  It might be quickest to have an avionics shop put a nav signal generator on it to quickly narrow things down…

[vik]

GS and LOC are two different circuits. GS works at around 300 MHz, while LOC is ~100 MHz carrier frequency.

If you can not hear the audio signal more or less clear close to the VOR, that might indicate that the local oscillator frequency has drifted. There is usually a small variable ceramic capacitor to adjust it within pretty narrow band. But it is a bit tricky. You have to use a pick up coil and either a frequency counter or a scope that can measure the frequency and do averaging. Sticking a probe to the local oscillator circuit will cause the frequency to shift, defeating the purpose of the adjustment.

There are other potential causes for a weak signal that decoder can not process reliably, but LO drift is pretty common. It the intermediate frequency signal that gets shifted and is filtered out by the filter, designed to suppress all the signals, but the IF one. I have seen this often as a cause for a "weak" radio, either NAV or COM.

Vik

 

[Fly Boomer]

31 minutes ago, vik said:

GS and LOC are two different circuits. GS works at around 300 MHz, while LOC is ~100 MHz carrier frequency.

If you can not hear the audio signal more or less clear close to the VOR, that might indicate that the local oscillator frequency has drifted. There is usually a small variable ceramic capacitor to adjust it within pretty narrow band. But it is a bit tricky. You have to use a pick up coil and either a frequency counter or a scope that can measure the frequency and do averaging. Sticking a probe to the local oscillator circuit will cause the frequency to shift, defeating the purpose of the adjustment.

There are other potential causes for a weak signal that decoder can not process reliably, but LO drift is pretty common. It the intermediate frequency signal that gets shifted and is filtered out by the filter, designed to suppress all the signals, but the IF one. I have seen this often as a cause for a "weak" radio, either NAV or COM.

Vik

 

Man, we have some great knowledge resources on this forum.  Thanks for sharing your expertise.

[Bobaran]

Thanks Vic,
The LOC does not give any response either on a LOC channel for the runway. Either that or I don't know how ṭo use it.

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[vik]

On 4/9/2024 at 8:15 PM, Bobaran said:

The LOC does not give any response either on a LOC channel for the runway. Either that or I don't know how ṭo use it.

 

Localizer is the same receiver path as VOR. The decoding is different, but that is done at the intermediate frequency. So, the local oscillator drift will affect both. In theory, LOC signal is simpler (just 90 and 150 Hz AM) and is stronger, so LOC can still work when VOR does not any more. However, if the LO frequency drifted too much (or LO is dead), both will not work.

Vik