On Wed, 26 Feb 2014 17:32:40 +0000 (GMT), "Dave Plowman (News)"
wrote:

Bought a bit of audio gear from the US at a decent price even allowing for
carriage. Guessed it might be 110v - but it is actually marked 120v 60Hz.

Looking inside, it has an analogue power supply - I was rather expecting
an SMPS.

The transformer primary has two winding wired in parallel which seem to be
coil 1 black start brown end, coil 2 red start orange end.

Red and black are paralleled as are brown and orange. Red/Black goes to
neutral, brown/orange to the off/on switch.

So it looks like I may be lucky and simply altering them to series will do
it for 240v. Link brown and red and 240 goes between black to orange?
Does it matter if one winding has the connections reversed when they are
in series?

You can bet your bippy that it does! :-)

If you series the split primary windings in counter-phase, you'll
have created a 'non inductive' wirewound resistor of perhaps circa ten
ohms for something like a 200VA transformer (it all rather depends on
the VA rating of the transformer).

That should blow a 13A plugtop cartridge fuse in about a second or
so. Obviously, a 5A fuse would be a better bet if you're going to
'test and hope' sans any form of continuity testing to identify the
correct pairs to associate with each winding.

The thing is, assuming you make use of the fact that you need to
connect either the red or black to the orange with the remaining
unused wires connected to neutral and on/off switch, you can either
end up shorting one of the split windings in isolation and connect the
other 120v winding across the 230v supply or end up with them wired in
series phase aiding as per requirement.

Now, if you get it wrong, the same 200VA transformer will present a
mere 5 ohms with the shorted out winding effectively making it a
non-inductive resistor with just a small amount of leakage reactance
in series. If you get it right first time by dumb luck, the fuse
doesn't blow and the amp will work just fine.

Provided you're happy to sacrifice a 5A plugtop fuse and you use your
noggin, it's quite possible (if not recommended) to arive at the
correct solution in no more than 2 goes of the "Suck It And See
Method"(tm).

As it happens, in this case, there's no risk of damage to the
rectifier/smooting capacitor pack other than failing to short out the
out of circuit winding when applying 230v across the single 120v
winding.

BTW, closer scrutiny suggest you may have correctly identified which
colours belong to which winding. As I re-read it, you need to either
connect brown to red or else orange to black using the remaining
connections to go to the neutral and switched live terminals.
--
Regards, J B Good