Arj1
That is exactly what I meant !
Malibuflyer wrote:
The only thing that I do not agree on is that a large PFD significantly reduces the advantage of a 750: Frequency lists, preview of an approach before selecting, etc. are all features that a large PFD can not provide.
He might have meant MFD here or just large screen EFIS – like G500TXi or G500…
A_and_C: We don’t disagree too much.
Obviously: If it is not an option at all due to limited panel space, any discussion of the benefits of a 750 is pointless.
I do also agree, that if the choice due to panel space is one 750 (as only radio/nav) or a pair of 650 (which btw. is more expensive than a single 750) than it is also an easy choice for redundancy reasons.
If it’s just for the cost and panel space is available, however, I’d always argue that it is a better deal to get a 750 and keep an old GNS 430 as backup rather than getting two 650.
The only thing that I do not agree on is that a large PFD significantly reduces the advantage of a 750: Frequency lists, preview of an approach before selecting, etc. are all features that a large PFD can not provide.
Malabuflyer.
That is a sweeping statement of cost vs functions. The 750 is worthless to those without the panel space to fit it and it’s functionality advantages disappear if the aircraft is fitted with a large PFD/ ND.
For the panel space the 650 has more functionality albeit a little more difficult to access , I could have fitted a 750 but 2 × 650’s and a G500 provide more redundancy and avoid an audio nightmare should the 750 screen fail.
The situation an owner finds them selves in when upgrading a panel makes best value a moving target that can’t be defined in a sweeping statement , but is defined by the legacy systems and panel space.
In my opinion it does as there is another layer of checks. It happened to me more than once that after selecting an approach I immediately saw that “something is wrong” because it graphically looked wrong.
Yes: Most probably I would have realized this also when checking each leg. But in my opinion it is undisputed in aviation that an additional layer of redundancy creates additional safety.
Malibuflyer wrote:
providers much higher safety (e.g. by showing a preview of an approach you select before you do so…)
That is certainly more convenient, but I don’t know that it adds much to safety as you anyhow have to compare each approach leg in the database to the approach plate. Ref. AMC2 to NCO.OP.116 item (c)(4).
750 is much better value than the 650. Not only for the moving map, but it’s much easier to operate (touch screen likes larger sizes), provides more convenience (e.g. when searching for a frequency you see a longer list and not only 3 lines at a time) and finally providers much higher safety (e.g. by showing a preview of an approach you select before you do so…)
Back on the topic
Indeed; you can pick up used KCS-55 components very cheaply nowadays. Some % won’t be working but even if you buy two, it is cheap 
Ibra wrote:
If you ever ask how much it cost to maintain or replace an old BK HSI & KNS80, I would say possible but likely all the money you have
Well I just did this, today in fact. My KI-525A returned from Termikas non-functional. It was €1250 for a refurbished drop-in replacement with no aircraft downtime or €7k for a GI-275 with unknown delivery date. I opted for the former.
