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Help needed for new ideas to solve Ampeg BA115 intermittent crackle issue
Hi everyone,
I recently purchased a used Ampeg BA-115 bass amp, which sounded great at the time of purchase. However, when I got into using it at home, I noticed that it has a really strange (and frustrating) crackle issue that only happens intermittently. The crackle usually only starts happening with longer sustained bass notes. I tried plugging in my strat to duplicate the problem and it's extremely hard to get it to happen or notice, so it's definitely easier to hear with the lower frequencies coming from my bass.
Sometimes, the problem will happen soon after I turn the amp on for the first time of the day. It's sometimes lasts for only a few seconds and then goes away. Sometimes it doesn't happen until after I've been playing it for a while. And sometimes, it doesn't happen at all, even after hours of playing. When it does happen, it's usually associated with a loss in volume – drops to a perceived 25% in total volume. One of the times it was happening, I could turn the amp on with nothing even connected to it and I could hear the crackling, as if something's inside crackling newspaper or cooking bacon. It would go on for a little while and then stop.
I was recording one day with both microphone and direct XLR output. The problem happened through the mic (the bass amp's speaker), but not the XLR direct out ... the XLR out had nice, perfect, clean sound. I think this means the signal is getting at least to this point with no problems and that it means there's a problem somewhere in the power amp portion of the circuit.
Using the clean recording, I played it back through the RCA inputs and can also get the problem to happen.
Things I've tried (which haven't solved the problem):
- Checked all of my instrument cables (can get the problem to happen with any/all of them)
- Tried my bass and guitar in different amps (they work fine)
- Checked the bias setting (5mv across R61 with no load/speaker cable unplugged)
- Replaced C4 and C5 with new caps, since they seemed to have an issue
- Replaced the Q2 and Q5 transistors after messing too much with the bias and burned one out
- Checked all of the resistor values in the schematic past the point of the XLR output, beginning with R25 – none of them are out of spec more than 10% and definitely nothing out of the ordinary seen
- Tried switching my passive bass output (Fender Jazz Bass) to the -15db input ... can still replicate the problem
- Re-soldered everything in the power amp output section ... and it seems like almost everything else on the back of the circuit board to make sure there are no cold solder joints
- Tapped around with a stick on all the caps while playing back the bass lines from the RCA inputs to try to identify physical movement on a capacitor as a problem ... nothing
I don't have a scope, but I do have a multimeter. I found a local service center who was able to order an entirely new circuit board for me (I needed a working amp). It, of course, works great as a full, direct, replacement – and now I've got at least something to compare with. But, I'd really like to get the original board's problem figured out and working well, perhaps to use in a custom head project, or just to re-sell to recoup some of the $ I spent on the replacement.
Has anyone else had a problem like this and could point me specifically what to look for? If not, is there something obvious I should look for specifically? One of the most frustrating things about the issue is that it's extremely hard to reproduce every single time I turn it on (as mentioned, sometimes it goes hours and days without having the problem).
Thank you!
I recently purchased a used Ampeg BA-115 bass amp, which sounded great at the time of purchase. However, when I got into using it at home, I noticed that it has a really strange (and frustrating) crackle issue that only happens intermittently. The crackle usually only starts happening with longer sustained bass notes. I tried plugging in my strat to duplicate the problem and it's extremely hard to get it to happen or notice, so it's definitely easier to hear with the lower frequencies coming from my bass.
Sometimes, the problem will happen soon after I turn the amp on for the first time of the day. It's sometimes lasts for only a few seconds and then goes away. Sometimes it doesn't happen until after I've been playing it for a while. And sometimes, it doesn't happen at all, even after hours of playing. When it does happen, it's usually associated with a loss in volume – drops to a perceived 25% in total volume. One of the times it was happening, I could turn the amp on with nothing even connected to it and I could hear the crackling, as if something's inside crackling newspaper or cooking bacon. It would go on for a little while and then stop.
I was recording one day with both microphone and direct XLR output. The problem happened through the mic (the bass amp's speaker), but not the XLR direct out ... the XLR out had nice, perfect, clean sound. I think this means the signal is getting at least to this point with no problems and that it means there's a problem somewhere in the power amp portion of the circuit.
Using the clean recording, I played it back through the RCA inputs and can also get the problem to happen.
Things I've tried (which haven't solved the problem):
- Checked all of my instrument cables (can get the problem to happen with any/all of them)
- Tried my bass and guitar in different amps (they work fine)
- Checked the bias setting (5mv across R61 with no load/speaker cable unplugged)
- Replaced C4 and C5 with new caps, since they seemed to have an issue
- Replaced the Q2 and Q5 transistors after messing too much with the bias and burned one out
- Checked all of the resistor values in the schematic past the point of the XLR output, beginning with R25 – none of them are out of spec more than 10% and definitely nothing out of the ordinary seen
- Tried switching my passive bass output (Fender Jazz Bass) to the -15db input ... can still replicate the problem
- Re-soldered everything in the power amp output section ... and it seems like almost everything else on the back of the circuit board to make sure there are no cold solder joints
- Tapped around with a stick on all the caps while playing back the bass lines from the RCA inputs to try to identify physical movement on a capacitor as a problem ... nothing
I don't have a scope, but I do have a multimeter. I found a local service center who was able to order an entirely new circuit board for me (I needed a working amp). It, of course, works great as a full, direct, replacement – and now I've got at least something to compare with. But, I'd really like to get the original board's problem figured out and working well, perhaps to use in a custom head project, or just to re-sell to recoup some of the $ I spent on the replacement.
Has anyone else had a problem like this and could point me specifically what to look for? If not, is there something obvious I should look for specifically? One of the most frustrating things about the issue is that it's extremely hard to reproduce every single time I turn it on (as mentioned, sometimes it goes hours and days without having the problem).
Thank you!
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