Update:
I sent the unit to
Skysmart Avionics
Most other UK instrument shops never even returned a call...
However I have also made a very interesting discovery...
Castleberry Instruments do an
electric replacement for the notoriously unreliable vacuum powered KI-256!
The model number is 900-23EVPK and the part number is 504-0027-90501. It looks just like a KI-256 but being electric it has a caging knob.
Their price is also WAY below Honeywell's present eye watering list prices...
I sourced a spare KI-256 -01 Mod 11 from them at a good price. They (John in Sales) communicate well and seem very good.
With a second alternator, or one of
these standby alternators this should deliver a certifiable total replacement for a vacuum system which, in many airplanes, only drives the KI-256 horizon.
The other thing I discovered, having got my hands on some KI-256 circuit diagrams, is that
Mod 11 is highly desirable. It buffers the unit's pitch and roll outputs and should make it far less likely that a KI-256 swap will result in an expensive and hassle-packed autopilot pitch/roll recalibration. Mod 11 cannot be retrofitted and basically you have to get a KI-256 with a S/N above 22475 which has this factory fitted. This has been factory fitted since about year 2000 but this instrument has been made since late 1970s and most overhauled units are thus really ancient.
A lot of people say the only way to make a small fortune in aviation is to start with a big one, but looking at the serial numbers on the KI-256 (nearly 30000 now) and its list price of $11k-$24k, it's obvious Honeywell have sold at least $100M's worth of these. And the selling price is about 20x what it costs to make it. There is a whole pile of fantastic cash cows like this in aviation...