replacing original Cessna nav com and Narco mk12d with remote glideslope box

[AlphaPilotFlyer]

I have standard 1970 Cessna nav com with original glide slope receiver mounted in baggage area from factory. anyone know if that receiver is compatible if I replace radio with a slide in narco radio?

 

[Ryan Klems]

There are various versions of the Narco MK12D. There is one that is made as a slide in for the ARC (Cessna) nav com, that would work with your existing remote glideslope receiver. There are other MK12D's that were not slide ins for the ARC and optionally could come with a built-in glideslope receiver (or could remotely channel one using 2of5 ... which I believe the ARC one also uses, but not positive)

 

[AlphaPilotFlyer]

Thanks….i got the ARC slide in that uses the original Cessna rack. Looking now for the radio side pin out on radio side as I know units are compatible. I got the Pinot on remote side connector and have good power and grounds, hoping it’s just a connector issue.

 

[Mikey52]

Not sure if it's included in this or no, what is the model number?

http://monticellofc.org/aircraft/MK12D-Manual.pdf

 

[AlphaPilotFlyer]

I'll have to pull it out if tray to see pn.
I think the connection at gs receiver is not right, according to my pin out diagram. now I need diagram showing color of wires coming out of radio tray to figure it out as all wires from tray connector to gs unit are white

 

[Ryan Klems]

What year / model aircraft is it? I can probably find the drawing for your original installation in the appropriate Cessna avionics manual.

Edit, based on your post history looks like a 1970 172K ... so this should be the drawing for the radio to glideslope receiver interface

 

[AlphaPilotFlyer]

awesome, thanks. my issue is I believe the long wire harness between the radio and remote glideslope receiver is bad as it's shows signs of poor repairs and want to be sure all connections are correct.....but each connector at ends of harness are different with different pin #.....very confusing. this will help me reverse engineer it. is there another diagram of this harness indicating pin out each side?

 

[Ryan Klems]

is there another diagram of this harness indicating pin out each side?

No, on that drawing the left side is the nav/com connector and the right side is the glideslope receiver connector, with their respective pin identifiers. The only other drawings are just of the nav/com and its connections to audio/power and the indicator. I'll attach those as well...

 

[AlphaPilotFlyer]

Thanks again, really helpful. There’s a couple of terminals listed on radio side as small letter d and b (ontop and bottom, with b being a ground),,,, my radio side connector does not have such letters but the color codes should help figure it out.

 

[AlphaPilotFlyer]

It seems some of the D connector terminals are damaged and need to be replaced, and some even in wrong position…… I don’t see any info about what type or size these barrel connectors are and I don’t have exact replacements in my kit. Will have to take them to an avionics shop to ID replacement pins and reinstall then check all wires in correct position and have good connectivity …

 

[AlphaPilotFlyer]

Well, confirmed GS unit getting power and ground but no movement. Don’t think unit is bad as I have 2 that were removed as operating fine…..thinking I will try checking if power is reaching the indicator on pins for GS flag and needle. Do you happen to have pin out for indicator cannon plug as well?

 

[Dan Thomas]

The Narco MK12D radios were the most troublesome pieces of junk we ever had in our trainers. They were in the shop continually, and the repair costs quickly added up to more than we paid for them in the first place. I wouldn't spend time and money putting one in an airplane. Narco went broke 14 years ago for good reasons.

 

[WDD]

Didn't know you could still get vacuum tubes for those radios.........

 

[Dan Thomas]

Didn't know you could still get vacuum tubes for those radios.........

The Mk12d didn't have vacuum tubes. Maybe they would have worked better if they did.

AFAIK Russia is the only country still producing vacuum tubes.

 

[Ryan Klems]

Well, confirmed GS unit getting power and ground but no movement. Don’t think unit is bad as I have 2 that were removed as operating fine…..thinking I will try checking if power is reaching the indicator on pins for GS flag and needle. Do you happen to have pin out for indicator cannon plug as well?

Here you go

 

[WDD]

The Mk12d didn't have vacuum tubes.

But they did have sarcasmnotliteral circuits.

 

[AlphaPilotFlyer]

Update…. I traced and confirmed all wires between #1 radio connector to aft glide slope chassis is correct, low impedance and getting power and solid ground. Still no GS indication. My last shot is to trace wires from radio rack to connector…..after that I’m at a loss and officially give up. I have 2-3 narco radios but they are for the non Cessna trays….so will have to replace trays and don’t want to open Can of worms. ANY OTHER SUGGESTIONS WELCOMED

 

[dmspilot]

I'd try TKM over Narco. I know the MX300 works with the ARC gs receiver.

 

[WDD]

What type of navigator do you have? GNS?

 

[AlphaPilotFlyer]

Not sure what you means by navigator

 

[WDD]

Not sure what you means by navigator

GNS? GTN? The box in your panel where you enter your flight plan, load approaches. The thing that feeds your CDI.

 

[MRC01]

It sounds like the thing that feeds his CDI is a radio (VOR or LOC).

When one of my old RT-385 radios started to die I replaced it with a Michel/TKM MX385 which is a slide-in replacement that also worked with the remote glide slope, which is an R-443B in the tail. It was plug and play, everything works. The MX385 isn't as durable or reliable as the RT385 but it is better than Narco.

That said, this worked for the past 10 years and now my glide slope is also getting glitchy. I'm in the middle of troubleshooting whether it's the radio, the CDI, the R-443B or the wiring. So far it looks like either the R-443B or the wiring, signal leaking from the 46 year old coax cable.

 

[MRC01]

... That said, this worked for the past 10 years and now my glide slope is also getting glitchy. I'm in the middle of troubleshooting whether it's the radio, the CDI, the R-443B or the wiring. So far it looks like either the R-443B or the wiring, signal leaking from the 46 year old coax cable.

Testing with the local avionics shop, the R-443B is working fine. But when we xmit an ILS/GS signal from the test box, the CDI glide slope doesn't respond unless the output signal is very strong. The antenna cable tested OK. The antenna is internal along the top edge of the windscreen. The contacts were slightly corroded and we cleaned them like shiny new with emery paper. That improved things but only a little.

Now it does pick up glide slope at KPWT, where the ILS is 111.1, and glide slope is consequently 334.4. And it picks it up about 10 miles out. But it doesn't get glide slope at KTIW, where the ILS is 109.1 and glide slope is 331.4. I'm not sure on those glide slope frequencies, I looked them up here.

I've tried 3 different radios (two vintage RT-385 and an MX-385) and the behavior is the same. At this point I'm thinking the antenna is bad. But how can that be? It's two strips of metal - as long as the contacts are clean, how does that go bad?

 

[WDD]

Did you check the connection between the cable and the cable connectors?

 

[dmspilot]

I've tried 3 different radios (two vintage RT-385 and an MX-385) and the behavior is the same.

All the radio does is channel the glideslope receiver. R-443Bs are cheap enough on ebay (or at least were last time I was looking), you might want to try a different one, despite the successful bench test.

Did you check the connection between the cable and the cable connectors?

Seconded

 

[murphey]

I'd try TKM over Narco. I know the MX300 works with the ARC gs receiver.

TKM went out of business before COVID, IIRC. I have a TKM slide-in replacement for the Narco MK12D. The COM side works, that's all I can say about it. NAV never did work.

 

[MRC01]

Did you check the connection between the cable and the cable connectors?

We rang out the cable at opposite ends, less than 1 ohm impedance. And I wiggled it around while doing so, no loss of continuity. The 443B worked well even when fed a weak signal directly from the test driver.

What doesn't make sense is the glideslope works fine on the KPWT ILS (even from about 10 miles out), but it doesn't on the KTIW ILS (not even at close range). KTIW just fixed their glide slope and says other pilots are receiving it. That's a clue to the root cause, but I'll be darned if I can figure out how to interpret that clue.

Some kind of interference on 331.4, but not on 334.4? Is one of those freqs a multiple of engine RPM?

 

[WDD]

Perhaps the unit is just failing in its internal tuning ability.

 

[MRC01]

Perhaps the unit is just failing in its internal tuning ability.

Possibly, but it picks up the localizer just fine. The problem is specific to glide slope (I know they are different frequencies).

 

[WDD]

Yes. Perhaps it can work on some freq, but not have issues with others. Just part of getting old maybe.