Marshall JCM 900 power transformer problem?

[Bigdrums]

I'm a bit confused by the following problem. I have 2 Marshall JCM 900 100 watt Hi gain dual reverb amps here. One came in with the main fuse blown, the other just needs the reverb fixed.
Amp 1 that blows the fuse I've removed all the secondary wires on the power transformer, have all power tubes removed, yet stills draws high current, almost 1 amp at only 5% or so on the variac. I have to remove one of the red "jumpers" on the primary side of the PT to have that rapid rise stop and rise slowly as normal. When I measure the resistance between the outside terminals on said transformer, and compare them to the amp that works, I actually get higher resistance than the working amp. i.e. 2.9 ohms vs. 1.8 ohms on the working amp. Main primary terminals. Similar readings on all other terminals. I can find no short to ground or other low resistance readings anywhere.




Any thoughts much appreciated!

[Jon Snell]

If with 240v across Blue and Brown and the link wire connected, with no secondaries connected the transformer draws current, it has shorted turns and is faulty.

DC resistance readings can be and are usually imisleading.

[Mick Bailey]

I immediately thought "shorted turn" when I read the post. The problem with a shorted turn is that out of the hundreds or thousands of turns, a single turn (or even several) has negligible resistance - a fraction of an ohm. You cannot tell from resistance checks whether a transformer has a shorted turn.

I have a ring tester that instantly tells whether a turn is shorted, but the 'neon trick' works just as well. With the transformer disconnected use the neon on the HT secondary. I always pick the winding with the greatest number of turns for the test, so in the case of a centre tapped secondary I test across the secondary, rather from the centre tap. A shorted turn collapses the entire magnetic circuit so it doesn't matter whether a turn is shorted on the primary or one of the secondaries - the transformer will still fail the neon test wherever it is as there will be no flyback voltage to light the neon.

[Gregg]

The truth is those 2 section PTs are just crap. I've replaced at least a dozen of them with properly made ones.

[g1]

The truth is those 2 section PTs are just crap. I've replaced at least a dozen of them with properly made ones.

Sometimes the wires burn up near the lugs as well. If memory serves, JM Fahey used to post about these not having wire that was up to the task.