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I am repairing a Peavey TKO 80 for my son's school. Some kind of intermittent issue. You play it, the volume drops and there's a hum, you bang on the top, it goes away. Bad solder joint, right?
WRONG!!!
I flipped the PCB over and did a close-up inspection with magnifiers. Not a cold solder or trace crack to be found. But, if you tap the board with a chopstick, it comes and goes. At first, I suspected poor collector connections on one of the power transistors, because they are simply Torx screws that contact the foil and screw into the TO3 transistors. Nope, THAT'S not it. I grab needle-nose and tug on the leads of EVERY component on the board. Nope, THAT ain't workin' either! #%&*$*&#*!!!
OK, time to start wholesale resoldering of sensitive sections of the board. Nope!!! Eventually, I resoldered the entire PCB and the problem was still there.
Oh yeah, tried the heat gun and freeze spray too, a couple of times. No shot.
At this point, I decided to do some troubleshooting and find out exactly WHAT was happening when the issue occured. I'd already figured out that the issue was NOT in the preamp. I had all of the controls and graphic EQ sliders maxed (with no signal of course) to listen to the noise floor when tapping the board, and when the hum started, varying the controls or EQ did not change it. Listening to the resulting hum, it sounded like either a rectifier diode or filter issue, so I went straight to the power amp rails. When the problem occurs, the -48V rail drops to -31V and the ripple (natch!) goes way up. An internally intermittent filter cap???
I'd tapped on the filter caps earlier, and though the problem DID occur, it occured ALL over the PCB, so I thought it was just component shock transmitted through the board. I even did a slight twist on the leads with needle-nose. However, this time, I focused on the cap for the negative rail, and when I bent the leads a certain way, I could get it to happen repeatedly. A partial intermittent short in a filter cap??? Not your everyday occurence, but it CAN happen. ANYTHING can happen, right?
Well, I'll be replacing those tonight and giving it a whirl. Will I be cursing a mean streak and scaring my dogs yet again??? I sure hope not!
These are the bitchiest types of repairs. I'm glad they don't happen too often.
WRONG!!!
I flipped the PCB over and did a close-up inspection with magnifiers. Not a cold solder or trace crack to be found. But, if you tap the board with a chopstick, it comes and goes. At first, I suspected poor collector connections on one of the power transistors, because they are simply Torx screws that contact the foil and screw into the TO3 transistors. Nope, THAT'S not it. I grab needle-nose and tug on the leads of EVERY component on the board. Nope, THAT ain't workin' either! #%&*$*&#*!!!
OK, time to start wholesale resoldering of sensitive sections of the board. Nope!!! Eventually, I resoldered the entire PCB and the problem was still there.
Oh yeah, tried the heat gun and freeze spray too, a couple of times. No shot.
At this point, I decided to do some troubleshooting and find out exactly WHAT was happening when the issue occured. I'd already figured out that the issue was NOT in the preamp. I had all of the controls and graphic EQ sliders maxed (with no signal of course) to listen to the noise floor when tapping the board, and when the hum started, varying the controls or EQ did not change it. Listening to the resulting hum, it sounded like either a rectifier diode or filter issue, so I went straight to the power amp rails. When the problem occurs, the -48V rail drops to -31V and the ripple (natch!) goes way up. An internally intermittent filter cap???
I'd tapped on the filter caps earlier, and though the problem DID occur, it occured ALL over the PCB, so I thought it was just component shock transmitted through the board. I even did a slight twist on the leads with needle-nose. However, this time, I focused on the cap for the negative rail, and when I bent the leads a certain way, I could get it to happen repeatedly. A partial intermittent short in a filter cap??? Not your everyday occurence, but it CAN happen. ANYTHING can happen, right?
Well, I'll be replacing those tonight and giving it a whirl. Will I be cursing a mean streak and scaring my dogs yet again??? I sure hope not!
These are the bitchiest types of repairs. I'm glad they don't happen too often.
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