I’ve got a KNI-520 indicator driven by a GTR255A. Sometimes, the glide slope flag gets stuck in flagged position. If you’re lucky, you can then unstick the it by tapping on the indicator case.
The KNI-520 is an interesting collection of 5 moving coil meter, 3 of which with a peculiar pointer (VOR/LOC, GS, FROM/TO flags), plus an OBS encoder. Electrically, the GS meter works, it just seems that its bearing has more friction than say the VOR/LOC flag bearing. This might be because the GS flag has not been used the last ten years or so because no GS receiver was connected, and even now it gets used less than VOR/LOC.
So my problem seems to be a combination of GS bearing friction and relatively low flag output voltage of the GTR255A. Unfortunately, I have no documentation about the KNI-520, and the GTR255A install manual doesn’t specify the GS flag output voltage (I assume it’s the same as FROM/TO output voltage, which would be 150…300mV).
@Tomjnx – I’ve sent you a dropbox link with some stuff including a KI525 (HSI) MM and that should contain the required signal levels.
The GS indicators on these things do tend to stick. They tend to stick less during flight, due to vibration. On the ground, this is really common.
The only “trick” is to put in an electronic indicator – I put in the Sandel 3500 EHSI.
Peter wrote:
The only “trick” is to put in an electronic indicator
Sure I could do that… But that’s admitting defeat, isn’t it? 
Yes but a smart guy knows when to throw in the towel 
I am surprised nobody seems to have made solid state replacements for these instruments.
You can pick up a used SN3500 on US Ebay for a few k $. Just make sure it is the LED (not lamp) version. But that would be overkill unless you can make it useful, by connecting the various other things to it. In fact I am looking out for one of these too, as a spare, but the ones that keep coming up are all looking for a mug (they are lamp versions, non Mod 1).
OTOH overhauling it (the best company I know is Castleberry in Texas) should give you another 5-10 years.
