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I had a new client in Bakersfield track me down to see if I could cure the nasty high pitched transients that bark at him when he's hammering on his 5-string bass...on the low B and E strings. He drove down yesterday on Labor Day and I wheeled it into the narrow space in the shop on a dolly, and first gave a listen to it with him playing thru an Aguilar AG500 head. Initially, all I was hearing with the noise from dirty pots on the amp and seemed also on his bass. I exercised all the controls to get that out of the picture. Not yet hearing the problem, I looked at the back side of the cabinet, finding the Tweeter attenuator was turned fully CCW. I turned it up half-way, then we began hearing the symptom.
I already had the front grille removed, so I first swept the cabinet manually with modulated Sine from my B & K 1027 Sine-Random Generator, driving the power amp section of my Ampeg SVT4-Pro into the cabinet. Then, changed to Sine, swept it down to 5Hz at moderate level to watch the cones and listen for any voice coil rub, finding all was ok with the woofers. Swept back up, and changed back to Sine-Random. I was hearing issues in the 200-300Hz range that I could eliminate by turning the tweeter down, so I next listened to the tweeter in it's normal range. Clear and undistorted, but having already heard issues with it, I removed the tweeter. I swept it being driven by a Warbled-Sine from the other generator that can drive speakers directly, and took it down below it's normal working range to listen for resonance issues. Heard problems there, so pulled the tweeter apart, swapped out the diaphragm with a fresh one, and with the mounting screw not yet fully tight, I positioned it while driving it at 400Hz, so you could clearly hear resonances as you tightened it up. The single screw is only the start. The rest is seating the loading cone and horn, which are mounted via four screws into the motor. That has to also be manually tuned as you find the anti-resonant position and tighten down the screws. Then sweeping it up and down to be sure. First pass seemed good, so re-mounted it.
Got NO OUTPUT from the cabinet at all. The tweeter driver had, while re-mounting it from the front, pinched the woofer wires between the wooden shelf and the motor, and unplugged it from the crossover panel! Sigh........ Straightened that out, then powered it back up and swept the cabinet with Sine-Random, but found again, nastiness in that 200-300Hz range. I had to go thru and re-seat the diaphragm/horn several times. Finally getting no resonances, went back to the bass. Immediately, we heard the same transient problem. I pulled the tweeter back out, and put the original diaphragm back in, again going thru the proper mechanical alignment process to get all resonances out. Back into the cabinet, bass plugged back in, and same problem occurring....loud transient barking, which could be eliminated by turning the tweeter off!
I then removed the tweeter, and with a long cable, plugged in the JBL Horn / Emilar EH175 driver to replace the Ampeg tweeter. Got the same nasty transient barking. Problem is NOT the tweeter.
Pulled the crossover assembly out again, having already done that, not finding any signs of the usual solder joint fractures still, but....this is all Lead Free Solder. Reluctantly, I unsoldered and re-soldered every joint on the two boards, and in doing so, found one lead of the cap in the tweeter path, hidden under the solder joint, not making reliable contact....short lead wire, and barely thru the PCB to be soldered into place. Cured the mechanical issue then completed the re-soldering of every joint, cleaned up the flux, and put it back together. Transient Distortion now gone!
While this isn't the first time I've had solder joint problems be the source of that same transient distortion that sounds like a bad tweeter, it is the first time there were NO VISUAL signs of it mechanically. Lead-Free Solder strikes again!! Hate that stuff.
I already had the front grille removed, so I first swept the cabinet manually with modulated Sine from my B & K 1027 Sine-Random Generator, driving the power amp section of my Ampeg SVT4-Pro into the cabinet. Then, changed to Sine, swept it down to 5Hz at moderate level to watch the cones and listen for any voice coil rub, finding all was ok with the woofers. Swept back up, and changed back to Sine-Random. I was hearing issues in the 200-300Hz range that I could eliminate by turning the tweeter down, so I next listened to the tweeter in it's normal range. Clear and undistorted, but having already heard issues with it, I removed the tweeter. I swept it being driven by a Warbled-Sine from the other generator that can drive speakers directly, and took it down below it's normal working range to listen for resonance issues. Heard problems there, so pulled the tweeter apart, swapped out the diaphragm with a fresh one, and with the mounting screw not yet fully tight, I positioned it while driving it at 400Hz, so you could clearly hear resonances as you tightened it up. The single screw is only the start. The rest is seating the loading cone and horn, which are mounted via four screws into the motor. That has to also be manually tuned as you find the anti-resonant position and tighten down the screws. Then sweeping it up and down to be sure. First pass seemed good, so re-mounted it.
Got NO OUTPUT from the cabinet at all. The tweeter driver had, while re-mounting it from the front, pinched the woofer wires between the wooden shelf and the motor, and unplugged it from the crossover panel! Sigh........ Straightened that out, then powered it back up and swept the cabinet with Sine-Random, but found again, nastiness in that 200-300Hz range. I had to go thru and re-seat the diaphragm/horn several times. Finally getting no resonances, went back to the bass. Immediately, we heard the same transient problem. I pulled the tweeter back out, and put the original diaphragm back in, again going thru the proper mechanical alignment process to get all resonances out. Back into the cabinet, bass plugged back in, and same problem occurring....loud transient barking, which could be eliminated by turning the tweeter off!
I then removed the tweeter, and with a long cable, plugged in the JBL Horn / Emilar EH175 driver to replace the Ampeg tweeter. Got the same nasty transient barking. Problem is NOT the tweeter.
Pulled the crossover assembly out again, having already done that, not finding any signs of the usual solder joint fractures still, but....this is all Lead Free Solder. Reluctantly, I unsoldered and re-soldered every joint on the two boards, and in doing so, found one lead of the cap in the tweeter path, hidden under the solder joint, not making reliable contact....short lead wire, and barely thru the PCB to be soldered into place. Cured the mechanical issue then completed the re-soldering of every joint, cleaned up the flux, and put it back together. Transient Distortion now gone!
While this isn't the first time I've had solder joint problems be the source of that same transient distortion that sounds like a bad tweeter, it is the first time there were NO VISUAL signs of it mechanically. Lead-Free Solder strikes again!! Hate that stuff.
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