Fixed it! ...sort of

Got bored today so decided to have a look at this Thorn ballast that developed an earth fault not long after repainting it...

I rigged up my modified microwave oven transformer to output power between the coil of the ballast and the casing, but the voltage wasn't high enough to 'bridge' the fault. Fail.

Next, full 240v, through a 5' ballast to limit current. This worked a treat! About 50 watts was drawn for about 15 seconds before dropping to zero. Checked the ballast and it had got quite hot in a specific spot on the side, so much so it'd bubbled up the paint!

Tested the ballast and now it was about 50Mohm to earth at 500vdc. Interestingly after bashing the ballast about a bit, I managed to get it hard down to earth again, hmm...

After running the 240v from coil to casing method again, this time only required for about 2 seconds, it was now about 100Mohm clear of earth at 500vdc.

I'm pretty sure I now know what's going on here, corrosion inside the ballast that's bridging the coil to earth.

I've put the ballast on long-term test now, working fine again so far...