Uwe wrote:

As a general comment, it's worth pointing out that the age and condition of a typewriter should be factored in when making comparisons between various models. Too many (most) typewriter bloggers seem to overlook this important fact when critiquing the comparative performance of their typewriters, and they consequently work under the incorrect assumption that how a typewriter currently works is reflective of how it performed when it was new.

When comparing two machines you can't ignore the effect that worn parts - or components that require adjustment (service) - can have on their performance. Is it a fair comparison when one machine might have been infrequently used during its service life and received regular maintenance, while the other was a workhorse that produced thousands of additional typed pages, and its upkeep was neglected? 

With respect to your observation, I wonder if the Wheelwriter you have made that annoying humming sound when it was new, or could it be the result of worn or dried-out motor bearings (or some other component that requires attention)? 

According to IBM, the "Personal Wheelwriter (6781) provides a high-function, affordable typewriter for the classroom, home, or office environment. It offers the advanced technology of the Wheelwriter Typewriter family -keyboard, printwheel, and supplies, along with many advanced, easy to use features, and durable Wheelwriter Typewriter construction - at an attractive entry price. Available options include a Sound Hood, Spell Check, and Printer Option. The Personal Wheelwriter Typewriter has considerably more function than the Actionwriter I Typewriter, which it replaces. Because of its function, durability, and small size, the Personal Wheelwriter is an excellent machine for teaching basic keyboarding and typing skills in a classroom environment."

I haven't found (yet) a detailed features list in my notes for the Personal Wheelwriter, but did find one for the Actionwriter I. It would have been nice to compare the two. Supporting your comments on correction features, the Actionwriter I list includes:
o   Automatic correction (two lines)
o   Manual correction - anywhere on the page

A lot of good points there; I'm  very conscious, as I suspect many people are, that when I write about "my" machines that I only have one or two examples of each. Ideally I try to remember to write words like "my" before a typewriter model but it's easy to forget - let he or she who has always remembered to do that step forward!

I'd guess that certain things we notice these days, many years after the manufacture of the machines, can be categorised in terms of how likely they are to be representative of the machine when new. For example, a mechanical grinding sound I would expect to get louder with age; an electrical humming sound might remain fairly constant with age (not many components in a typical transformer wear out), and print quality may deteriorate with age (but may be easy to restore). In terms of electronics - i.e. what actions a microchip is programmed to perform, I'd guess these do not deteriorate with age, but the manufacturer may have re-programmed the chips within the lifetime of the model to improve the features offered.

Equally I'd think that many typewriter collectors, while interested in history and how models performed when new, are also likely to care about how likely a typewriter is to still be performing well these days; some models are likely to be more durable than others. This is where we collectors are not living "after the event" - we are living at the ideal time to assess the robustness and longevity of a model - we are better placed to do this than any manufacturer or reviewer back in the day.

In the case of my Wheelwriter, the humming sound occurs constantly - it sounds like the transformer, and it occurs as soon as you switch it on (no motors, I think, run all the time like in this, or many, electronic models?)

I think that we, as in us collectors, are in a better position than most to assess the reality of any model in the long term. For example, I would place my own observations on the features of my Wheelwriter quite low in terms of "authority", but also I'd place the your IBM quotation equally low.

The source I'd credit with highest authority would be the collective opinions expressed in a website like this, and to that end I humbly subit my data that my own Personal Wheelwriter 6781 makes a constant electrical humming sound, and doesn't backspace / correct to a previous line, whereas my own Actionwriter doesn't hum, and does move back to a previous line when correcting if need be.