> (1) \definefont[myChapterFont][myHeadingsFont sa 1.728]
>You could either use a real font name like 'Arial' or a symbolic name
>defined with \definefontsynonym like 'Serif', 'SerifBold', 'Sans',
>'Dummy-Regular' etc. in the second argument of \definefont but not
>the name for a font collection defined with \definetype, in your case
>\definefont[myChapterFont][Sans sa 1.728]
>or
>\definefont[myChapterFont][DummyRegular sa 1.728]

So ( and considering the example at
http://wiki.contextgarden.net/Chapters_headers )
I tried...

\setuppapersize [A4][A4]

\starttypescript [serif] [myBodyFont]
    \definefontsynonym [myBodyFontRegular] [name:Times New Roman]
[features=default]
    \definefontsynonym [myBodyFontItalic] [name:Times New Roman Italic]
[features=default]
    \definefontsynonym [myBodyFontBold] [name:Times New Roman Bold]
[features=default]
    \definefontsynonym [myBodyFontBoldItalic] [name:Times New Roman Bold
Italic] [features=default]
\stoptypescript
\starttypescript [serif] [myBodyFont]
    \setups[font:fallback:serif]
    \definefontsynonym [Serif] [myBodyFontRegular]
    \definefontsynonym [SerifItalic] [myBodyFontItalic]
    \definefontsynonym [SerifBold] [myBodyFontBold]
    \definefontsynonym [SerifBoldItalic] [myBodyFontBoldItalic]
\stoptypescript
\starttypescript [myBodyFont]
    \definetypeface [myBodyFont] [rm] [serif] [myBodyFont] [default]
\stoptypescript

\starttypescript [sans] [myHeadingsFont]
    \definefontsynonym [myHeadingsFontRegular] [name:Arial]
[features=default]
    \definefontsynonym [myHeadingsFontItalic] [name:Arial Italic]
[features=default]
    \definefontsynonym [myHeadingsFontBold] [name:Arial Bold]
[features=default]
    \definefontsynonym [myHeadingsFontBoldItalic] [name:Arial Bold Italic]
[features=default]
\stoptypescript
\starttypescript [sans] [myHeadingsFont]
    \setups[font:fallback:sans]
    \definefontsynonym [Sans] [myHeadingsFontRegular]
    \definefontsynonym [SansItalic] [myHeadingsFontItalic]
    \definefontsynonym [SansBold] [myHeadingsFontBold]
    \definefontsynonym [SansBoldItalic] [myHeadingsFontBoldItalic]
\stoptypescript
\starttypescript [myHeadingsFont]
    \definetypeface [myHeadingsFont] [ss] [sans] [myHeadingsFont] [default]
\stoptypescript

\usetypescript[myBodyFont]
\usetypescript[myHeadingsFont]

\setupbodyfont[myBodyFont,12pt]    % this works: body font displays in the
correct font

%\definefont[myChapterFont][myHeadingsFontRegular sa 1.728]     % compile
error at line 51: WHY ?
\definefont[myChapterFont][Arial sa
1.728]                                      % only this works: WHY ?
%\definefont[myChapterFont][Sans sa 1.728]
% defaults to default LM font for line 51: WHY ?

%\setuphead[chapter][style=myChapterFont]
\setuphead[chapter][style=\myChapterFont]

\starttext

\chapter{chapter test}      % line 51

\section{section test}

text test

\stoptext


Many thanks in advance for your enlightening help.

Alan