Ampeg SVT-CL help - crackle/static noise

Discussion in 'Amps and Cabs [BG]' started by USMercury, Dec 6, 2019.

  1. USMercury

    USMercury

    Dec 13, 2007
    Hoping I can tap the braintrust here for a little help diagnosing an SVT-CL issue. (First, yes, I have some tube amp repair experience and know how to keep myself safe poking around inside one of these beasts.)

    This issue: when the amp is turned on, off standby and being played for a few minutes, I started to notice a crackle coming through the speaker. Sounds like static. It happens intermittently if the amp is sitting idle, and I realized I could make it happen by tapping any of the three driver/PI tubes on the back of the amp (crackles as I tap the tube). My first thought was to roll some new tubes and rule that out, which I did. Didn’t help. Re-tensioned the tube sockets and cleaned with dioxit. Didn’t help. So, next step was to pull everything out of the cabinet, fire it up on the workbench, and tap around a bit with a chopstick. Here are a few things I took note of:

    - the crackle actually happens when tapping anywhere on the primary (larger, bottom) circuit board.
    - the crackle is quieter (but still there) if the volume on my bass is off, regardless of the level of the Gain/Master on the amp.
    - the crackle gets louder as the bass is turned up, and is loudest when I’m actually playing something.

    Any ideas would be greatly appreciated. What does the behavior above indicate? Is it likely to be a bad component on the board itself, or in the connection of this board to other sections of the amp? Thanks!
     
  2. JB1962

    JB1962

    Jan 13, 2018
    Check out this post. In this case it turned out to be bad connections to the input jack. Not saying thats your issue, but might be one thing to check. I would also be suspect of bad/cold solder joints on any of the boards.
    Ampeg SVT-CL Crackling Sound (video)
     
  3. Wasnex

    Wasnex

    Dec 25, 2011
    Old plate load resistors can crackle. Any loose/intermittent electrical connection can introduce crackle; a loose ground for example.

    Ideally it would be helpful if you had test equipment like an oscilloscope, so you could probe the circuit to try and find where the noise is being introduced. I don't own an o'scope either. I suggest pulling the PI to see if it helps further localize the problem.
     
    JB1962 likes this.
  4. JB1962

    JB1962

    Jan 13, 2018
    You might also try using the preamp-out / power amp-in features to isolate the crackle to either the preamp or the power amp sections.
     
  5. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    First thing to do is to reseat your small tubes in the bottom chassis.

    Check the cable connections between the top and bottom chassis.

    Then swap out the small tubes one at a time with known good ones.

    If you’ve been tube rolling, the socket contacts might require re-tensioning.
     
  6. seamonkey

    seamonkey

    Aug 6, 2004
  7. USMercury

    USMercury

    Dec 13, 2007
    Quick status update! Thanks for the replies - definitely helpful in figuring out what to do next. I had already re-tensioned the PI/driver tube sockets, and replaced the tubes with new ones. So I moved on to cleaning the input jacks and preamp/poweramp jacks with Deoxit. But really, I just spent a bunch of time reflowing solder on the larger circuit board. I didn't hit everything, but did quite a bit, including lots of resistors, caps, and the ribbon connection between the power tube board and the larger board. Fired everything back up with it disassembled on the bench, and – whaddaya know – the noise seems to be gone! I played it quite a while, getting everything well up to temp, and it sounds great. Nice and quiet. Thanks again for the input!

    I do have another question, though – I'm trying to figure out if anyone has been into this amp before. As I was going through the soldering exercise, I noticed that the legs of a number of WIMA caps were longer than anything else on the board; there also seems to be a bunch of flux on the board, but only around the legs of the filter caps. Anyone know if this looks normal, or if any of these components might have been replaced? Anyone have any nudies of an original SVT-CL to compare?
     

    Attached Files:

    JB1962 and JimmyM like this.
  8. Wasnex

    Wasnex

    Dec 25, 2011
    In the photo I only notice one cap seems to have one leg really long and the other is short. This is probably because the spacing with the holes on the circuit board don't match the leads of the capacitor. It's not unusual.

    I don't think a little bit of flux is a problem, as long as the proper type of solder was used. In other words if you used plumbers solider for some reason I might be concerned.

    The only thing that catches my eye is some of the joints seem to have an excess of solder. Probably not a problem, just a bit sloppy.

    My guess is some of the parts were installed by machine and some were later installed by hand.

    I don't see anything that concerns me.
     
    USMercury, JB1962 and beans-on-toast like this.
  9. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Was your amp made in Vietnam? It woulds say on the back of the lower chassis. The soldering on those models often had issues.

    I see some joints that I would have resoldered. It is possible that some wire or component leads look good but the lead is not bound to the the solder. If you give it a light tug, it can slip out. Adding a little flux and reflowing will correct that. So sometimes it helps to inspect and to gently tug leads.

    Since the noise occurred when you tapped, check the joints around those tubes, including the sockets. Never solder a socket with a tube installed.

    As was mentioned, flux is not usually a problem but I like to clean it up. This allows you to properly inspect the joint. You can use 99% isoproply alcohol or vodka and Q-tips.

    If it works now, more than likely you corrected the issues. Good going!
     
    Wasnex, USMercury and JB1962 like this.
  10. JB1962

    JB1962

    Jan 13, 2018
    Glad you got it working! I too am curious of the timeframe made. I had 2 NEW SVT 4 pro's overseas made (2011 timeframe) that both had crackle issues. The first one I sent back under warranty, and they sent me another with same issue! The second time, I just took it apart myself, reflowed some obviously bad solder joints and all was good! So it seems this may possibly be a symptom of poor quality control during that era.
     
    USMercury and beans-on-toast like this.
  11. USMercury

    USMercury

    Dec 13, 2007
    It's a USA model from 2004, so maybe a connection just came loose over time? That's why I wondered if anyone had been in the amp - was thinking it was possible that the new work might not have been super tight. But if all components look original, I'm sure it's a case of some of the manufacturing being automated and some being by hand. I wasn't concerned about the flux, it just struck me as weird that it's only on the filter cap connections. But with connections re-flowed and the noise gone, I'm happy.
     
    beans-on-toast and JB1962 like this.
  12. beans-on-toast

    beans-on-toast

    Aug 7, 2008
    Not related to your issue. Early usa made models had a ground loop hum related problem. Check this out just to be sure. Insert a plug into the amp in jack to test for it as described.

    Ampeg SVT-VR&CL ground loop hum solution