A friend is letting me look at his SWR bass amp, blows the mains fuse. Other forum posts say it's likely the power transistors, but wouldn't there be visible collateral damage on the pcb with overcurrent like this? I see none. I really need a schematic! Is not available anymore on the SWR site (is considered vintage now). Thanks!
No, most failed parts look exactly the same as good ones.
Call Fender and get the schematics. Have your serial number handy. Does your power amp use the old TO3 outputs or are they the flat 2SC3264?
Never think up reasons not to check something. I don;t care what they LOOK like, check the output transistors for shorts.
I don't remember, does the power amp on this unplug with like a 6-pin Molex? If so, disconnect the power amp and see if the amp still blows fuses. If the amp holds a fuse with the power amp unhooked, then the power amp is blowing the fuses. if it still blows fuses, then likely the main rectifier bridge is shorted.
Call Fender and get the schematics. Have your serial number handy. Does your power amp use the old TO3 outputs or are they the flat 2SC3264?
Never think up reasons not to check something. I don;t care what they LOOK like, check the output transistors for shorts.
I don't remember, does the power amp on this unplug with like a 6-pin Molex? If so, disconnect the power amp and see if the amp still blows fuses. If the amp holds a fuse with the power amp unhooked, then the power amp is blowing the fuses. if it still blows fuses, then likely the main rectifier bridge is shorted.
No, and I wouldn't have even checked it - as everyone else has said, check the output transistors and bridge rectifier, those are by FAR the most likely cause. I'd only check the transformer after the outputs checked OK, and if it still blew the fuse even with the bridge removed.
In the case of the output transistors it's also likely they have either damaged other components, or been killed by other components - often changing just the outputs means they fail again.
Enzo - Thanks...spkr out is BLK/WH/SH, with BLK/SH=7.6, WH/SH=2.6, BLK/WH=10.2 ohms. But PT input is 0.8 ohms....haven't lifted any wires, measured from internal fuse to neutral, see pic. is a Merc Mag 560003-A. Measured pwr transistors (in circuit), E-C and B-C get 1.5 Mohms, E-B get 100 ohms, on all 4. OK? PT input winding shorted? Gonna call MM when they open up here in SoCal. Gotta be one expensive PT....Need schematics.....
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Thanks Nigel. As this is my biggest challenge to date, I will have lots of questions.... Can a shorted bridge rectifier affect the PT primary resistance, with no power applied? Can less than 1 ohm across it be explained by anything else than a shorted primary? Maybe a short outside the primary (fuse holder, pwr sw, terminal strip...). Should I lift the primary leads and check it direct? Do the (in circuit) pwr transistor ohm readings seem OK? I'll have to unsolder hard-wiring to disconnect the power section (as Enzo suggested) and I will if I can be reasoned out of thinking the PT is bad. Thanks again all!
As you've lifted the main secondary winding (make sure you disconnect both leads?) did you try plugging it in?, and see if it blows the fuse with the bridge disconnected.
I would expect the mains transformer in such a powerful amp to have very low resistance windings anyway, but as I mentioned it's not something you would generally attempt to measure.
Transformers do go duff occasionally, but it's not the most likely scenario.
I would expect the mains transformer in such a powerful amp to have very low resistance windings anyway, but as I mentioned it's not something you would generally attempt to measure.
Transformers do go duff occasionally, but it's not the most likely scenario.
If you are gonna call MM when they open, why have you not called fender for the schematic?
If you are so concerned the transformer is bad, then disconnect all its secondaries - ALL of them, not just the ones you suspect - and with only the primaries connected to mains, power it up. If it still bows fuses, then your tranny is bad, but if it does not, then the thing is fine and move on.
What is the big green coil of wire atop your bridge rectifier?
If you are so concerned the transformer is bad, then disconnect all its secondaries - ALL of them, not just the ones you suspect - and with only the primaries connected to mains, power it up. If it still bows fuses, then your tranny is bad, but if it does not, then the thing is fine and move on.
What is the big green coil of wire atop your bridge rectifier?
Think I found the culprit (but not all the damage it caused...). A choke coil I thought was attached to the bridge AC lug (pic1) was really WELDED to it - and not by design. As I started to remove the red PT lead, one coil wire came off (pic2). You can see the other side of the coil wire - now broken - still attached. Pic 4 shows both ends of coil wire - now a break in the coil. Proximity and vibration must have made it short out thru the green varnish. Oh, and one side of the coil goes to gnd.... Bridge reads fine, PT reads < 0.5 ohm on Prim and Sec....GRRRRR
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The PT WORKS!! You guys..... I'm glad I listened.... The shorting choke was part of the crossover to the horn, so I can turn the horn pot down to "off", and test the amp (and deal with it later!). All 3 secondaries are proper voltages, the main bridge rectifier tests good... I think no damage was done at all, just the 75VAC PT winding getting stressed going to the main bridge rectifier. Will find out tomorrow....
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It's all repaired, sprayed the pots with DeOxit, took lots of pics of the internals. This is such a nice sounding amp, I hate to be giving it back to the owner....not charging him anything (he's a friend), I've learned a lot working on it. Will tell him to find me more broke stuff...
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