Reverb unit - blown mains fuses with transformer disconnected from main board

[JMFahey]

I have a reverb unit (Demeter RV-1, 1st version) that keeps blowing mains fuses as soon as it is powered up. I disconnected the power supply from the main board and it still blows fuses.

I guess you disconnected the power transformer from the rest of the circuit.
Unless you have an actual power supply board (including diodes, filter capacitors and probably regulation) which you can disconnect from the rest of the circuit.
But if so, please make it very clear.

I then, perhaps stupidly, bypassed the fuse, and powered it up -

won´+t comment but you know, don´t you?

this was followed by a spark (and small bang/pop) at the toroidal transformer, where the connections to transformer primary are

Well, you don´t actually know, you guess.

(I guess suggesting a short between live and neutral?)

Definitely not, or fuses would keep blowing.

However, I then put the fuse back in the circuit and powered it up, and the fuse now does not blow.
So primary circuit is open.

I thought it wasn't working at first but here are voltage readings (with description of what is being measured as far as I can discern),

From transformer primary first (live side) connection to ground - 240v
Across the transformer primary (first to last connection) - 240v

Ok, so far you are proving mains voltage reaches primary.

From transformer primary first connection to transformer centre - 100v
(0v between all the other primary connections)
From transformer primary centre connection to ground - 15v
From transformer primary last connection to ground - 15v (although would need recheck this one)

I guess those are not "real voltages" , meaning they can supply significant current to any load , but simply capacitive/parasite/electrostatic coupled ones which will only drive a multimeter very high impedance ... and nothing else.
So they are not significant.

From transformer secondary first, centre and last connections to ground - 13.8v

Same: not significant.

(0v between any two of the three secondary connections).

This is the real test and it fails. Sorry.

The website for the new version demeter rv1 says the power supply is rated at +/- 18v, so 13.8v AC going to the circuit board in this one doesn't seem that far away from that.

It isn´t going anywhere, sorry.

Do these voltage readings seem normal? It is tempting to reconnect the power supply to the main circuit board and see if it works, but I feel I must have got something wrong.

Sadly yes, it looks wrong.

Do you have any ideas why it stopped blowing fuses?

Yes, primary circuit opened.

Is miraculously fixed (temporarily?) or have I just caused a further problem?

Miracles don´t apply here.
Bypassing real fuse means some wire inside the transformer blew instead.
You were lucky, it could have been worse.

PS: I am amazed nobody suggested measuring primary resistance, which is the very first test which comes to mind