Anyone With FC530 autopilot Experience?

[KennyG]

Hi
Hoping there is someone out there that could assist with a head scratcher (for us anyway).

Our Learjet 35A is fitted with an FC530 autopilot.
We are experiencing a problem in Heading Mode where the pilots can only get 8 deg of bank in either direction no matter where they set the heading bug on the HSI. Nav Mode works correctly
Another symptom is that when the autopilot is engaged the control wheel drifts to the left.
We have changed / swapped all of the sensors includng the autopilot computer. Continuity checks of the wiring is also ok.

The only thing not touched is the roll servo but we are assuming it is ok because the NAV mode is working fine.

I know the description is a bit brief but thought someone out there may have experienced the same thing
Cheers, KennyG

[wigglyamp]

Do you have a mechanical H.S.I or a Collins EHSI74 in the aircraft? If you have the EFIS, the fault can be a defective HPU74 EFIS processor which generates the analogue heading error signal.

[KennyG]

Hi Wigglyamp
We have an EHSI. I have swapped HPU74's and have rung out the wiring between the gyros, the controller and the HPU74

[wigglyamp]

Is the 26v 400hz good to the HPU (measured under load if you can get at it)?

[KennyG]

Just gave it a try and we do have 26VAC

[SeldomFixit]

Not familiar with this A/P but does the HDG knob drive a syncro ? if so - measure syncro outputs in response to knob rotation perhaps ?