I’m having problems with my KR86 ADF. It has what appears to be alternator noise in it, and almost no effective range. If the alternator is offline, the ADF seems to work fine. I’ve already spent a lot of money chasing this problem, including a new filter on the alternator. Anyone have any ideas? Also, can a marker beacon share an ADF antenna?
I used to have a KR86 in my Sierra. It used the long-wire sense antenna, and the under-belly “pancake” loop antenna. I have seen ADF installations that use the newer one-piece under-belly combined loop-sense antenna. I don’t know whether that really requires a different ADF; I had thought it was a function of the antenna itself. In other words, if you had the combined antenna, you could possibly use it with your present ADF. I have no knowledge of the models of ADF pancake antennas available (more on this below).
Apparently it is possible, but I have not previously seen a marker beacon receiver hooked up to the overhead long-wire ADF sense antenna. As noted, most MB’s use the sled type under the belly, in part because the marker beacon signal usually originates beneath the aircraft during an approach, and is aimed vertically in a fan shaped pattern. The best reception will be under the aircraft. My personal recommendation for the MB antenna is to use one of the small “canoe style/boat style” underbelly antennas. They are inexpensive (about $100 at Aircraft Spruce for the Comant CI102), compact, and low drag. In my opinion it is worth switching to the canoe just to get rid of that @#$%^& sled type unit. Those things often have bad connections and corrosion, they are usually covered with oil, they are prone to mechanical damage, and they make washing the belly more difficult. If you can’t do it yourself, labor cost should be pretty modest to go with the canoe. Last year I sold three of these canoe antennas new on eBay for about $70 each, but I have no more of them.
I don’t have much use for ADFs any more. My general experience is that if you have one that works well, count your blessings and don’t change anything related to it (or near it), including the antennas and cables. If you have one that doesn’t work well, it will prove to be an avionics money pit in today’s shops. Shops work on them time after time without solving even basic reception problems, such as poor range due to alternator noise. In each case some bad part or problem will be identified and you’ll pay for it, with little or no change in symptoms; it is simply R&R “desperation troubleshooting”. For example, my bet is that 99 out of 100 “noise filters”, that are externally added to alternators, are a complete waste of money. All the aircraft alternators I know of (as well as later-model auto alternators) have a filtering capacitor built into the alternator. Unless it has gone bad (an unlikely event prior to overhaul for other factors), the external one is redundant. I suspect that the practice is a holdover from the days of aircraft generators, which required external noise filters (and which created far more noise from their commutator, as compared to an alternator’s slip rings).
Many of the Musketeers have a short length of unshielded wiring between the voltage regulator and the separate over-voltage relay. Those wires are typically only a matter of inches from the routing of the ADF antenna cables, at least in the airframes that have both items on the right interior side of the firewall. You can try fabricating a metal shield of very thin aluminum angle to wire-tie over the antenna cable bundle (make it as long as long as you have room for), then bond the shield to the airframe with a jumper. This simple step alone can take ADF range from 4-5 miles up to 30-40 miles, on NDBs. You have to make sure the ignition harness has no chafed spots at all, anywhere in the outer braid. In general, if you can hear any engine-speed electrical noise in your headset, your ADF will most likely have poor range (if it works at all); the ADF receiver is incredibly sensitive to noise, for a number of technological reasons. Note that your ADF reception range on AM radio stations will have almost nothing in common with your range on an ADF signal; they are light years apart in radiated signal power.
If there ever was any art associated with installed ADF support, most of today’s avionics shops seem to have lost the knack. You’ll pay a lot of money to educate their current techs, and usually without a very satisfactory outcome. There are probably a few BAC members who have had a good ADF experience with some nearby shop; perhaps they will let you know. It will be a waste of money to take it to just any shop, without a known history of success dealing with ADF problems.
If you are determined to keep and use an ADF, perhaps because you need small-field approaches and cannot yet swing an approach-certified GPS, you need good ADF antennas. The long wire antenna can be replaced with stainless steel using readily available kits (like the #11-12300 at Aircraft Spruce). Note that the wire length is an important factor, and must be matched to the specs. If you don’t already have a good ADF loop antenna under the belly, I can’t give you much advice on that. They aren’t typically advertised or sold separately from the ADF; you would have to work with a shop on it. A good percentage of radio problems in older installations prove to be related to cable, connections, and corrosion. Those are also, however, the most labor-intensive aspects to have serviced. As I said, though, you can pour a lot of money into getting an ADF to work well enough with NDBs, to be of actual use.
As I have mentioned in previous posts, I do not consider myself to be a “good avionics guy”. If someone else in BAC is good with avionics, and holds some contrary opinions, listen to them and take my comments with a grain of salt.
Additional information from Bob Steward:
Step one is to either pull the alternator and run it by AutoZone or Pep Boys for a free check (they can detect bad diodes), or for the electronically enabled, a O’scope across the power lead and ground will show any dirty output. For those that want to give it a try and don’t have access to an Oscilloscope, a Digital Multi-Meter can be a stand in. Set it for AC volts and then measure the “voltage” of the DC bus. The ripple from diode problems shows up as a voltage on the AC scale. If no ripple, then there should be essentially zero volts AC on your DC 12V bus.
While there is some validity to the “bad ground” theory, a little corrosion
will rectify the AC and convert it to a half wave pulsing DC and make a lot
of racket in the headphones.
Check the alternator wiring to the VR for good grounds at BOTH ends of the
shielded wires. You maybe surprised to find that the squeal is the VR
buzzing (most are mechanical point type) and a bad ground is causing the noise.