HT fuse just won't stop blowing.

[MitchK]

Originally Posted by Steve Conner

Will you guys stop beating on each other. All I can say is, I'm glad it's not *my* Marshall that Mitch is trying to fix.

In particular, I noticed he was trying to dismantle the output transformer without even being sure that it was faulty. Marshall OTs are fairly valuable, so he ought to have done hi-pot and ringdown tests before condemning it and trying to prise it apart.

If you don't have the gear to perform these tests, one rough and ready method of testing an OT is to feed a few tens of volts back into the speaker winding with a variac. This generates a high voltage on the plate winding, similar to what it would see with the amp dimed. If there are shorted turns or insulation faults, you'll notice a high current draw, and smell burning varnish before long!

When doing this I like to measure the voltage on the plate winding with a DMM on the highest AC volts range, to make sure I'm not overcooking it. If you cranked the variac too high, you could break down a good OT. 500V RMS plate-to-plate is about the maximum I'd care to inflict on a 50 or 100W OT.

BTW, it should go with out saying, but for goodness' sake don't touch the plate leads while you're performing this test! (Yes, those same leads that you're measuring 500V AC on.)

I fully agree now of course that I shouldn't have started taking the original OT apart. I had convinced myself the amp was fixed, once I had the new OT in it seemed fine at low volumes and I did even crank it up for a few seconds just to see if it would pop, but when I actually ran it at a higher volume and in particular, my guitar has a "boost" a tiny preamp switch inside it, when I hit that for a second it popped right away, I shouldn't have left that out because it might have something to do with it... ?

Originally Posted by R.G.

I'm impressed that there's this much smoke around a small fire with so many knowledgeable firemen nearby... 8-)

1. In any fuse blowing problem you have to stop the fuse blowing before you can figure out what is blowing. Make a light bulb limiter.
2. Perro Grande, Steve C, Enzo, and others have good advice. I like the "follow the power parts, then what controls the power parts" advice and the note that high voltage arcing usually follows "without a speaker" and "cranked".
3. Transformers of all stripes can be tested for internal arcing and shorts with the inductive kick test. You disconnect all windings, load any winding up with DC from a battery then disconnect the battery. A resistor/neon bulb is hooked across one winding. If there is an internal short, the bulb will not light. If there is an internal arc-over, you can bound the voltage by which winding has the NE-2 on it. The NE-2 flashes at between 80 and 95V. If you get a flash with the NE-2 on the primary, but not when it's on the speaker outs, there is internal flashover between 90 and 2400V preventing the output winding from getting to flashover.
4. I like the missive to follow what controls the power. A slowly failing bias cap will do much the same thing as has been described.

If it were mine, I'd figure that it's too bungled up by now to ever know the real culprit. I would test the transformer in depth, replace the power tube sockets, see if it still runs away without power tubes in (and a light bulb limiter), test the power transfomer, rectifiers and main caps and only if all that is good look at the bias supply without power tubes, then with power tubes in and a resistive load. If all that checks, run it into a resistive load to try to get it to go over the edge, with a big bulb in the limiter.]

Of course, it MAY be haunted...

You're right. It was just haunted.

can you give me some more info on point 3 there? when you say NE-2 are you referring to the neon bulb? and just in general can you explain a but more how this works?

[Steve Conner]

That trick R.G. suggests with the neon bulb is just a clever version of what I call the ringdown test. It works because transformers are also inductors, which makes them capable of storing magnetic energy. Disconnecting the battery causes the stored energy to "kick back" at you, causing a high voltage pulse and often a spark too.

You don't really even need the neon, I guess, as the kickback from a large transformer can give you a noticeable jolt if you accidentally get yourself across the wires while doing this test. If you touched the plate leads while prodding a battery on the speaker leads, and the OT was good, I bet you would jump halfway into the next state. (standard legal disclaimer applies)

NE-2 is just a part number for a common type of neon lamp.

But looking at what people have said here, it's looking more and more like an arced socket, though you should check your power tube bias too.

[R.G.]

That's right, Steve. The neon bulb just lets you do this quickly and easily without becoming a sacrificial anode as they say in the chemistry biz. The NE2 bulb breaks over at a relatively well known voltage, so you can diddle the test a bit by applying it to different windings to see which winding breaks over at what voltage (crudely!) as well as just a go/no-go test.

[AaronShafer]

Something occurred to me that could be the problem. I noticed that you said that you recapped the power supply. Could it be that the two 10uF caps that provide the negative voltage are pointed the same direction as the other filter caps instead of "backwards", that is, plus to ground? I've never seen this problem, but I think that you would see fuses blowing and very brief readings of negative voltage on the bias line.

[Mars Amp Repair]

Well hell, why not one more opinion here...

I've always considered this whole forum idea to be centered on teaching & learning as well as sharing our years of experience with those, who for some crazy inexplicable reason, want to learn this ancient stuff. Teaching an apprentice is also the best way to really firm it up for ourselves.

It takes a special breed of person who gets that adrenaline rush from the challenge & success of conquering this broken shit. The kind like Mitch here who like us, is willing to spend his spare time staring at a chassis full of wierd looking parts & wires. What's the matter with you Mitch anyway ;-]

We should be encouraging those that are trying to learn this dying art, not the opposite (with precautions well laid out as we do here), otherwise, there will be no new blood in this dying industry...just buy a new one when it breaks... glen

[Enzo]

Can you imagine strolling into the local high school or junior high, walking into the shop class and telling the students, "Hey, this stuff is all dangerous, you could cut off a finger. Just hire a carpenter."

I was soldering before I was ten years old, and there were no transistor circuits back them. At least not in the TVs and radios I was working on. That was over 50 years ago. I suppose I should have waited until after college to take some amp repair courses before opening up anything...

[mykey]

Originally Posted by Enzo

Can you imagine strolling into the local high school or junior high, walking into the shop class and telling the students, "Hey, this stuff is all dangerous, you could cut off a finger. Just hire a carpenter."

I was soldering before I was ten years old, and there were no transistor circuits back them. At least not in the TVs and radios I was working on. That was over 50 years ago. I suppose I should have waited until after college to take some amp repair courses before opening up anything...

well yes there are many people who think that safety is a joke, an unnecessary formality, but since i have safety training, and i have seen people hurt before doing this kind of thing it might be worth mentioning, especially to a person who cannot operate an ohm meter. (were not talking about you, but we are talking about people who change output trannies without testing first)
and interestingly enough there are over 200 people a year who die in the workplace from slips, falls, electrocution. so it would not harm to mention that an amp like this could shock the living shit out of someone. and professionals do take safety seriously, even in newsgroups.

[Steve Conner]

If we weren't encouraging the guy to learn this "dying art" we wouldn't have just told him how to test output transformers.

[mykey]

Originally Posted by Steve Conner

If we weren't encouraging the guy to learn this "dying art" we wouldn't have just told him how to test output transformers.

Then it might be a good idea to start a new thread about safety, discharging tube amp power supplies, before we encourage people to work on tube amps. I suggest we take a little responsibility in this category. But i don't think the art is going to dye with people like you teaching others.
But please, the safety part first.

[Enzo]

My beef is not with safety. My beef is with telling people they don't know anything so don't try and learn. There already ARE threads about safety. Please, address safety all you want, no fight here. I would suggest telling someone how to safely do what is required to fill their need, rather than telling them they should not even bother to attempt this stuff and see a pro. WHo among us was a completely trained technician before opening his first amp?