Bendix HSD-880 system (HSI)
Chasing an intermittent problem with the HSI system in an older airplane.
The system uses a IN-881 indicator with a slaved electric gyro, mode selector, and a flux gate magnetic pickup.
Sometimes it powers up and works flawlessly. On another day or flight it will not work. The strange thing. It seems like it works or it doesn't. I don't believe I have yet witnessed it quiting once it powers up correctly. However if it decides not to work, it also will not "start" working later in the flight.
When it is not working, on power up, the compass card along with the heading bug and CDI needle will rotate clockwise until both the heading bug and compass card are indicating North. At that point the compass card and heading bug stop and cannot be set with the heading bug or the manual gyro setting and will not slave to compass heading. the CDI needle will endlessly continue to rotate clockwise.
When it works properly, the compass card and heading bug rotate to North then everything stops for a few seconds, then the card rotates to aircraft heading and the CDI turns locks to whatever bearing you select.
Before just blindly yanking them out and sending them off to a shop, I would like to understand the system to eliminate any aircraft wiring problems. Anything like a repair manual or installation pinout would be helpful.