![]() ![]() The two larger, series filter caps didn’t make any sound, so I left them in place and swapped the others. SUPER CRACKLE! Two of the 22MFD caps were really noisy when poked with a stick. For the heck of it, I fired the amp back up and began to poke the filter caps. Opening the doghouse revealed that all the filter caps had recently been replaced. I powered down the amp again, zapped it, and flipped it over. Because it also went away when the amp was in standby, I really needed to dig into the power supply. The crackling was easy to reproduce, but it only loosely seemed to be associated with certain parts of the board. I fired the amp back up and began poking around again. I powered the amp back down and buzzed it out so I could tug on wires and check for shorts in the pre-amp section. Everything seemed to check out. Poking around near the normal channel tone stack seemed to cause the loudest crackling, but so did the board near the phase inverter. Crackles typically mean loose connections, so I started poking everything with a stick. Tapping various places on the board not only caused the amp to crackle, it also triggered the static/crackling sound that would typically only start after being warmed up. ![]() Now it was time to go a little deeper. For my next trick, I pulled the chassis and set the amp and speaker up on by bench so that I could poke around with the thing powered on. ![]() Aha, so we need two hours of actual signal running through the amp to make it sweat. Two hours into band practice the following week, the crackle returned. I was skeptical, but maybe the amp was fixed, and just sat for so long that the tube sockets were corroded again. Amps don’t sit on the bench for a year because of microphonic tubes and dirty tube sockets. The next day, I fired up the amp and let it warm up for two hours before I played through it. No crackling! That was too easy. I really wanted to know what the problem was with this amp, and why it out-smarted the other tech for so long. Because I had the luxury of time on this one, rather than just shotgun a bunch of stuff, I thought it would be best to try one thing at a time, test, and repeat. I powered down the amp, cleaned the tube sockets, and swapped out the microphonic 7025 with a 12AX7. The 7025 preamp tube in the normal channel was very microphonic, but nothing in the power section seemed to make much noise, although the 12AT7 phase inverter was a little crackly. I turned it on, let it warm up for two hours, then tapped the tubes with a stick. The best way to troubleshoot stuff like this is to first go for the low-hanging fruit. And remember, the amp was fairly quiet until it had warmed up for a few hours. It was only audible with the standby switch on (not in standby mode), so the power section was the prime suspect. The sound was sort of electrical static/crackling in nature. Rather than send it back into the shop, Ivan decided to leave it in mine-after all, it was already there! ![]() He actually had to move the speaker baffle back a ways to make it consistent with a standard Fender combo amp, and to properly protect the grill cloth.Īfter all that effort, two hours into band practice the following week, it was a bit disappointing to hear that the original problem still hadn’t been fixed. This actually involved a bit of woodworking, too, as the grill cloth originally stuck out proud of the front face of the cabinet. He replaced the grill cloth and re-glued a lot of the tolex. After more than a year of back-and-forth and missed connections, Ivan finally got it back from the shop, and thought it would be nice to give it a little TLC. The chassis of this amp was put into a third-party 2x10 combo cabinet by the previous owner, and it was getting a little long in the tooth. It had been in and out of another shop for over a year, and the problem was never successfully addressed. It suffered from a phantom crackle that would only appear after being warmed up for an hour or two. My buddy Ivan uses this combo-ified 1963 Band-Master as his backup. ![]()
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