OT Short - repairable?

[Buddha's Guitar Tech]

Not sure whether i should just get a new replacement for a JCM2000 DSL50.

Attached picture shows some burning and it's easy to see why there is a short.

My gut tells me get a new one. Any other thoughts?

[img:644:800]https://lh6.googleusercontent.com/-Rp5x ... G_1859.JPG[/img]

[Cliff Schecht]

Any idea why it shorted? Speaker impedance mismatch? Those transformers are not very good quality and I'll bet they don't HiPot test them past 1500V (flyback voltages can be many thousands of volts!). Look at a Magnetics Components JCM800 50W OT, it's decently priced and good quality. I have a 100W one that kicks some serious ass.

[Buddha's Guitar Tech]

It cut out mid-play, i wasn't there when it happened. Might have been a mismatch, not sure - guy was at an audition when it happened, d'oh!

Funny thing is, amp worked first time for me, i only found the fault by accident, then it took a little bit of head-scratching to localise it.

i'm happy to replace it with Marshall spec unless the owner wants an upgrade.

[Phil_S]

Of course, we both know this will recur unless you address the cause. Let's assume the transformer isn't the problem.

I did this once. The OT I worked on and the problem was similar but not actually visible like yours. In comparison to what I had to do, this is a walk in the park. I had to split off the laminations and remove the bobbin, then put it all back together (not as simple as I make it sound). The broken wire was near the end of the winding but I was unable to fish it out while the transformer was assembled. I managed to break several turns in the process of trying.

Fish out the wire below the burn mark and figure out the best way to terminate it. You may want to bend it into a "c" shape, loop another "c" shape hook onto it, solder and cover with heat shrink. If I did it that way, I would use insulated wire and I would not bother bringing it back to the terminal strip on the frame. You can bring it back to the frame -- your choice.

The wire on the bobbin is covered with enamel. This is what insulates the turns from each other. Fish out enough wire so that you have something to work with - probably unwind one full turn or a half turn if you are able. It won't effect the transformer's performance at all. An OT has hundreds, often thousands of turns. It is hard to know exactly what you are working with from the picture. The wire is often quite fine in gauge and brittle, so be very gentle with it. Before you can solder to the wire, remove the enamel -- use fine grit paper, emory cloth, or you might try burning it off with a lighter. Avoid the lighter if you can. Test for continuity to the other end of the winding (one of the terminals) to verify that you have removed the enamel.

Soldering is challenging because of the small wire size. I suggest a hot iron and be very quick to solidify your splice to the new lead. Remember to insulate the splice with heat shrink tubing.

An alternative is to unwind one full turn and bring that turn back to the terminal. This is less messy and probably better. You will know if you can do that when you get into it.

Then, there is the matter of the burn mark. Just how many turns are shorted? Only one, then what I said up to this point is good. If it is several turns, all must be removed or your repair will be a waste of time. If you must unwind 10-20 turns, I don't that it will be a problem for the OT. It is a minor imbalance -- likely not at all perceptible. The real problem will be whether you can actually remove them or if the winding is somehow not willing to let them go. Winding patterns can be simple or complicated. I guess you'll find out.

Anyhow, right now what you've got is a doorstop, so I'd say your risk level on an attempted fix is very low and the reward potential -- having it fixed is very high. I wouldn't hesitate to dig in.

[billyz]

Looks like the insulation broke down. I would try to fix it and re insulate the wires. But I would never trust it, use it for a tester tranny.
I would solder a dedicated ground to all the output jacks, They are the source of many problems in this series of Marshall amps.

[Buddha's Guitar Tech]

Hmm, i hadn't thought of unwinding and re-winding. It's a bit of gamble with my time, but could pay off, you never know.

Having said that, i am inclined to think there are many players who'd feel much more confident knowing that the thing was replaced rather than repaired.

Phil, i greatly appreciate the time you've taken in writing that, i will definitely consider it.

[Prairie Dawg]

Hmm, i hadn't thought of unwinding and re-winding. It's a bit of gamble with my time, but could pay off, you never know.

Having said that, i am inclined to think there are many players who'd feel much more confident knowing that the thing was replaced rather than repaired.

Phil, i greatly appreciate the time you've taken in writing that, i will definitely consider it.

You have nothing to lose at this point.

[VacuumVoodoo]

Rewinding a transformer with a confirmed short circuit = winding up a completely new bobin with fresh wire, reassembling the core and impregnating the whole thing.
BTW never reuse the wire. It's insulation can be damaged in several places and it's a PITA to remove old impregnation from it.

[Buddha's Guitar Tech]

i just got off the phone with Marshall. A replacement, with shipping, is only £30.

With the backlog of repairs i have, added to my children's inexplicable need to eat three times a day, i might replace the thing and give him a discount, buying the old one back. As mentioned, might make a half-decent test tranny.

[Buddha's Guitar Tech]

Rewinding a transformer with a confirmed short circuit = winding up a completely new bobin with fresh wire, reassembling the core and impregnating the whole thing.
BTW never reuse the wire. It's insulation can be damaged in several places and it's a PITA to remove old impregnation from it.

That also makes sense. Current is current, the whole way through.

[Structo]

I would replace it and save some aggravation.

But first you need to know what shorted it.

Was it a mis-match on the speaker impedance?

Was it a shorted power tube?

Etc.

[Buddha's Guitar Tech]

i wasn't there, see above.

[Phil_S]

I thought this was your personal property. I wouldn't have recommended a repair for a customer's amp. He should give it to you. It's worthless to him and whether it will have value for you is an open question. If you give him a small discount, that is a very nice gesture, the kind that should bring repeat business.

Unwinding more than just few turns can be a monster job. Bench tranny is a good place to give it a new home.

[Buddha's Guitar Tech]

Yep, i agree.

[katopan]

That flying wire that's burnt across the other interleave winding shouldn't have been touching the coil. Good transformers will cover the flying wire with teflon tube to prevent this sort of fault. Production transformers tend to rely just on an air gap. But if it's draped across like in your photo it won't last long. The wire should've followed along it's coil path for longer before coming up and out of the windings. In other words the person who soldered it pulled a little too hard in the finishing step.