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Super Blowing Speakers/Arching Output X?

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1/6/2000 6:48 PM

Super Blowing Speakers/Arching Output X?

I Just got a Fender Super Amp in for repair that has been to a couple of repair shops around LA. It is the newer model with channel switching. The circuit board covers the whole top of the amp making it hard to work on. It has blown the 2 right speakers on the 3 last gigs it was used on! The speakers are wired in series/parallel (right for this model super) and it keeps blowing the two speaker wired to the negative side. So it blew the two stock speakers. Two Mojo's HD Alnico were installed. It blew those. It was taken to another tech who installed 2 more Mojo's but rotated them so the Mojo's were both on top. Checked the amp out and found no problems. On the next gig he blew the two right speakers again (1 Mojo & 1 stock Fender). The guy does play very loud. I've talked to some techs and the theorys are that DC is getting to the speakers somehow and two speakers are blowing before it can get to the other two that are in series after them.
I looked around where the feedback loop was coming into the circuit board and saw no signs of arching. I also check out the output transformer with a signal generater and a current meter. Did the math and got 5000 ohm into 8 ohms. Seems fine there. Could the output transformer be arching or is this guy just playing too loud? It does seem strange that he would be blowing the same two speakers every time. Any help would be appreciated. Thanks, Curtis.

1/7/2000 4:25 AM

I have never heard of this situation, so I am only offering an option to see if it is the series/parrallel arrangement or the transformer itself. This amp's OT has a 2 ohm tap (it should be the white wire, and it is used to feed the line out). When stock, it is not hooked up to the speaker out. My idea is to rewire the speakers to a parrallel arrangement at 2 ohms, and rewire the speaker out with the 2 ohm lead to match. If I remember right the extension speaker jack is a switching jack, so some re-wiring will have to be done, but it could be wired to be essentially like a Super Reverb. Just an idea. At least all the speakers would be pulling the same load.

1/7/2000 5:25 PM
Nate S.	It may be worth hipot testing the tranny for shorts between input, output and ground. The difficulty may be coming across a hipot tester. I have them at work, but if you can't find one get the name of the associated research rep in the area and give him a call. he might be able to help.

1/11/2000 4:40 AM

Hmmmm sounds like the stuff i get "laughing'
I have had a newer fender in that had some weird problems,
that was the transformer. Kinda intermitent.I would still try a few things like,"maybe with old speakers" play it and see how long it takes to happen.Check for arcs, and the backing board that covers the amp with the thin shielding,make sure its not shorting something.Hmm just trying to think of some things...my guess would be the transformer.You said he played loud,so it may be only when its stressed to that point before it arcs.And is like the first post said is zapping one side of those speakers first.
{Richie}

1/11/2000 7:08 PM

To test this sort of situation on an old amp with an intermittent DC problem on the output, I set up the following:

Set up amp on bench with signal generator connected to input and load resistor connected to speaker out. Also connect a DMM to the speaker output. Set the meter to DC volts, and to hold the highest reading. Turn the amp on and let it cook for a while. You can also set up a scope on the output to monitor. If the transformer arcs when overheated and passes DC, then the DMM will catch it. I would leave the chassis in the cabinet at first, to simualte a real world situation. The chassis removed from the cabinet may get more ventilation and not arc.

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