HSI not working with KNS-81

[aggie06]

I did a VOR check a few weeks ago and found that my #1 VOR wasn’t working right. The to/from flag kept flipping back and forth and the indicator was also moving back and forth no matter where I turned the OBS knob. The radio is a KNS-81 and the HSI is a King KI 525A. The ILS and DME functions work fine. The #2 VOR radio and ILS both work fine as well, but those are going to a different indicator.

I’m going to start by pulling the radio to confirm no obvious broken wires or corroded connections. Any other thoughts on what this could be?

 

[Axtel4]

I'd have a shop take a look at the HSI. There are slip rings and brushes that provide the signals to the D-bar and To/From flag in the center of the instrument from the rear connector. Over time the rings wear and can get debris on them causing intermittent operation.

 

[aggie06]

The HSI works fine when getting a signal from the ILS (through same radio) or GPS. Are there different sets of slip rings and bushings for the different signals?

 

[dmspilot]

If the ILS is working I speculate that it might be something internal to the radio. I'd swap out the KNS-81 to see if that's the problem...you can either try to find someone you can borrow one from or you can pick up a used one. Thankfully these are super cheap as far as used radios go.

 

[aggie06]

Thanks for the suggestions. I decided to take a chance on a used KNS-81. That solved the problem, and much cheaper than paying someone to take out and bench test the old radio just to charge me to fix it. VORs and ILS come in good and strong now. DME works great. Shot the ILS at HQZ and everything was great. Until I got to the final approach fix. Then #2 radio (backup ILS receiver) lost the glide slope. It’s always something.