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Help needed for new ideas to solve Ampeg BA115 intermittent crackle issue

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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!
    Attached Files

  • #2
    If an amp crackles, but the fault can't be triggered by poking with chopsticks (or beating with fists etc) then it could be a transistor that developed a bad connection internally after a long period of overheating. Bashing on it makes no difference because it's encapsulated in a block of epoxy that stops it moving, but thermal expansion can still affect it.

    Look for small TO92-style plastic packaged transistors without heatsinks, some designers like to run these burning hot to the touch and it does them no good at all. Feel them all to see if you can find any extra-hot ones, and look for areas where the board material might have darkened from heat. Hitting the offending transistor with freeze spray or a heat gun, or even blowing on it through a straw (the poor man's freeze spray) should affect the symptoms.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      On that amp, I'd suggest servicing the headphone jack. If it has become dirty or oxidized it will cause the exact problems you are experiencing.

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      • #4
        Thank you for the extra suggestions. Here's what I've tried since:

        - Hard-wired the headphone jack so that there is definite contact between the 3 pairs (essentially negating the fact that it's even there) ... has a nice solid connection
        - Q3 and Q6 are the two TO92-style transistors on this board and neither one is hot to the touch ... though just in case I tried the straw trick

        When I first powered the amp on today and plucked the open D string on the bass, the crackle happened for a couple seconds and tapered off. If I play the bass normally, I can't make the crackle happen. However, when I pluck the open A and open D strings one after another (so the guitar is sending a lot of signal), the crackle is very easy to reproduce.

        Any new thoughts? Thanks again for the help! Hope to get to the bottom of this at some point.

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        • #5
          Yes.

          Ball up your fist and whack the top of the amp hard. Does that make it crackle? Or react in any way? That reveals loose connections inside.

          Can you connect the amp chassis to a different speaker? We would want to play the amp through a different speaker to see if speaker is at fault.

          Play something steady through the amp so you have two free hands. I use my shop stereo receiver, but a CD player or something would be fine. While it is playing, wiggle the little connector where the speaker wires plug onto the board. ANy reaction? gently tug on the wires to the speaker, to see if they are loose down there. ANy reaction?

          The only way to test the speaker itself is to connect it to a different amp. DO NOT connect a different amp while this one is still conected. Your speaker could have failing tinsel wires or other problem.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            HOORAY! I think I have solved the problem. I had the amp partially apart (ground screw and heat sink attached to the chassis only). All of the volume, EQ, and style switch (S1 on the schematic) were not attached with the nuts. While I was playing my bass, I was going to change the style knob to a different setting and the sound cut out (like you'd get with a dirty POT). After a bit more experimenting, I turned it all off and picked the board up to look more closely at the style switch. It lets you choose different tone styles (I think it just pads the midrange more or less).

            As it turns out, the switch wiggled enough to where I could see a bad solder joint. When I went to touch it up, the solder stuck to my soldering iron instead of to the circuit board like every other connection I've touched up. Upon closer inspection, it looks like there is no metal ring around the hole punched in the circuit board for the ground pin of the switch to attach to.

            I ended up using an exacto knife to scrape away enough of the conductive path for that pin so that I could solder the ground connection into the path. It was a bit tricky, but I think this has fully resolved the problem! I've been trying so many other things for a few weeks now, I can't believe this was what it was. Hopefully this info will help someone else who may be running into a similar problem.

            Thanks again for your great suggestions!

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            • #7
              The solder pad came off the board. Glad you found it and effected repairs.

              May I suggest a better way to fix this sort of damage:
              Scraping off the solder mask was the first step, as you did. But rather than depending upon the blob of solder to keep the connection, scrape away the solder mask on that trace running to the left at least as far as the bend in the trace. I can't tell, maybe half an inch? Take a piece of solid wire (I keep the extra lead wire from resistors and caps when I trim it off for this purpose) and make a small loop in one end. The loop will go around the switch lug sticking through the hole. The wire then will lie down along the now bare trace. You will solder the wire down to the trace along that length and around the switch pin. Now you have a much sturdier connection that doesn;t rely on the solder for its entire mechanical sc=trength. Your fix may well last years, but every time you flip the switch, it will put a bit of stress on that solder blob.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                Thanks Enzo! It should be fixed for a good long while now. So relieved ... I guess I have a brand new PCB for a BA-115 I can sell.

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