Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Rattle from PLX amp in my rack.

Discussion in 'Amps and Cabs [BG]' started by bassasaurus, Aug 15, 2004.


  1. bassasaurus

    bassasaurus Zonker Supporting Member

    May 24, 2003
    Massachusetts
    Well, I just spent and hour trying to isolate the rattle in my rack and it is coming from my PLX2402. It really takes of when I hit a C and appears to be getting worse. I am willing to send my in but how long will I be without it? Anyone have any experience with QSC on repairs? It happens every time I use it, appears to be getting worse and it is loud enough to be a distraction at practice. Bob Lee, any updates on this issue?
     
  2. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    QSC's service is top-notch so don't be worried.

    You might want to consider putting a piece of medium-density foam between your cab and your power amp, just to minimize vibration and its eventual effects on your electronics. Its a cheap and simple solution to what may or may not become a future problem.
     
  3. Steve

    Steve

    Aug 10, 2001
    It's a pretty well known issue.

    The air ducting is loose fitting cardboard and will vibrate against the main circuit board and some other plastic in there. Also the circuit board is friction fit on posts that can get a little up and down play in them and vibrate as well.

    You'll have to open the case, put a couple of drops a silicone calking compound on the circuit board mounting posts to stop the up and down vibration and get some kind of tape you trust to tape the air ducting together to prevent movement.

    When this issue first cropped up there was a lot of ?????? and evaision and finger pointing and denials that it was a design issue. It was kind of uncomfortable.

    You can dredge all that up or you can spend and hour and just deal with it.

    I chose to just deal with it and My 2402 has been working it's fanny off for quite a while now without further problems.
     
  4. Schwinn

    Schwinn

    Dec 4, 2002
    Sarasota, FL

    I didn't have any rattling problems but I decided to finally get a piece of foam from Joann farbrics (hi-density) to put under my rack and it works beautifully. I even put my CAR KEYS on top of my rack now when I play and there is NO rattling. Amazing, and the foam piece only cost me $6.
     
  5. PolkaHero

    PolkaHero Supporting Member

    Jan 5, 2002
    Michigan
    I'm using an Auralex GRAMMA between my PLX and speaker cabinets. Takes care of most of the rattle.
     
  6. Eric Moesle

    Eric Moesle Supporting Member

    Sep 21, 2001
    Columbus OH
    I've cut my foam to fit inside the lid of my rack for transport. Any simple foam will do, and of course, black looks less intrusive. Such an easy solution!
     
  7. How thick is the foam?
     
  8. Schwinn

    Schwinn

    Dec 4, 2002
    Sarasota, FL
    It's the 1" green hi-density foam. I plan to spray paint it black eventually, but the green is kind of retro... :D
     
  9. atldeadhead

    atldeadhead

    Jun 17, 2002
    Georgia
    I use a set of MoPads to keep my PLX from rattling on top of my 2 GS 112's.
     
  10. metron

    metron Fluffy does not agree

    Sep 12, 2003
    Lakewood Colorado
    When I put my amp rack (conatining a PLX amp) on top of any speaker it rattles.

    Its the rack and everything in it vibrating. Just take it off the speaker. :)
     
  11. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Over the past few years we've had a couple PLX amps sent in because of complaints of rattling, but we weren't able to make them rattle ourselves to see what the source was. So we tried to silence what was the most likely culprit. Our service engineering supervisor wedged an adhesive rubber foot (like you would stick onto the underside of a piece of electronic gear) in between the heat sink duct (which is made of fish paper) and the large reservoir caps. He had peeled off the adhesive backing so the foot would stick to the fish paper.

    I hope that did the trick. If you want to try that yourself to save the downtime, you can do so.
     
  12. mikeddd

    mikeddd Supporting Member

    Nov 12, 2009
    San Antonio, TX
    I have a PLX3602 I bought used. Amp works perfectly but has an internal rattle like you wouldn't believe. The ratlle is so loud and pronounced that at basement practice levels, you hear the rattle more than my bass. Amp is mounted, front and back, in a 6U rack. Here's what I've done so far (one thing at a time, changes cumulative) in an attempt to fix the problem.

    1. Hot glued fan shroud to aluminum heatsinks (there is a varying gap of about 1/32" to 1/8" b/t the shroud and sinks)
    2. Stuck rubber feet in several places on both sides of the fish paper air duct (wedging it against big caps/resistor...paper does not/cannot move any more)
    3. Put double-sided foam tape on top of the airduct to prevent bottom amp cover from rattling
    4. Put a 1/4" thick piece of high-density foam b/t my rack and my speaker cab

    Results? Absolutely zero change. Where's that bang-head-against-wall icon? :scowl: I checked (carefully) every component mounted to the main board; nothing is loose. There are small ribbon cables in there, but they aren't moving and aren't heavy enough to make a buzz that loud even if they were vibrating against something. So, what's left? The main board. I noticed last time I had the amp opened up that it was a little loose on it's standoffs but said "Nah. Can't be it" and closed it up, all smug that my hot-glue and rubber foot work surely had fixed it. Wrong.

    Well, that has GOT TO BE the culprit...it's the only thing left! I will work on it tonight and post lots of pics. If it turns out to be the culprit I will be a little disappointed that QSC's engineers couldn't figure it out. Be back in about 8 hours or so.
     
  13. mikeddd

    mikeddd Supporting Member

    Nov 12, 2009
    San Antonio, TX
    Alright, so it's a little more than "8 hours later." I'm sure the detail of this post will more than make up for the tardiness! Bottom line: I found the fix, at least in my case.
    I sincerely hope that the Google spiders pick up this post and it gets archived unto eternity. I had a hell of a time finding any helpful info at all.

    I'm well aware that this post is a book. It's length and detail are necessary IMO due to the lack of information I found on how to fix this problem. I'm hoping this post will be a one-stop-shop info source for fixing rattling PLXs. I spent hours surfing every link I found that mentioned anything about "PLX and rattle/noise." I, like many, have gotten used to fixing any problem, with any item, simply by Googling it. The info is out there, usually. Not in this case. Not that Google could find, anyway. And I checked all the links on the first 4 or 5 pages of hits, too. Any useful info was all about the airduct and I figured out early on that the airduct was not my PLXs problem.

    If the outside of the amp wasn’t in perfect shape and it wasn't functionally perfect as well, I'd swear the previous owner threw it off the roof of his house. At least now I know how I got it at such a steal of a price! ;) HOW BAD was the rattle? Watch the video. In the video you will see how I tested/tried different things to stop the rattle. Hold this/push on that/does that help? That was the approach I took. I think it’s pretty intuitive. It worked for me, anyway. By the way, I’m sure this question will come up. “How did you generate the tones you used to test?” I played “the evil note”, (I think its high F#) and sampled it with my Boss DD-7 Digital Delay. I just looped it; very handy. The video shows my “troubleshooting process.” I videotaped as I investigated and progressed with the fixes. You will see things glued down, etc. Watch and you’ll get it.

    DISCLAIMER: I know what I’m doing when it comes to electricity and electronics. I know what not to touch (most of the time). If you try anything that I show in this video, you are on your own. Use my video as a guide at your own risk. (Man, I hate having to put crap like this in here…got to these days, unfortunately. Insert lawyer joke here).

    Before we get to it, just to recap: In every other PLX with a rattle, it seems that the culprit was the "fish paper" air duct over the power transistor heatsinks. Gluing/taping it down seems to work for most people. That did nothing at all for me. In my case the culprits were a lot more difficult to track down and fix. IMO, I killed 95% of the rattle; it sounds so much better than before!

    I don’t mince words, so here it is: In my opinion it rattled because of design flaws. Hey, I call'em like I see'em. I have no idea what "version" or "revision" my PLX3602 is. It's registered with QSC, so they know. Maybe they fixed it in later versions; maybe not. This is not a bash thread; I own another QSC amp as well (GX5) and it is perfect. I would buy more QSC products in the future. The purpose of my post is to allow others with the same problem to fix theirs. I searched the net long and hard for a fix and did not find one. If I can help others by finally discovering the problem/fix, then this thread will have served its’ purpose.

    From my direct observations AND seeing how my fix actions actually remedied the problem, the main source of the rattle was the main circuit board vibrating/moving up and down on the standoffs that mount it to the chassis. You can look at the videos and visualize it yourself, but I'll try to describe the standoffs. If you have ever built a computer, you already have a good idea of what a "standoff" is.

    There are small brass pegs about 1/4" long that are welded to the chassis. These standoffs/pegs mount the board in the chassis and keep the bottom of the board from grounding out against the metal chassis underneath it. The tops of the standoffs have a 360-degree notch cut into them. The main circuit board has keyhole-shaped holes in it. The mainboard slides over the standoffs, then slides sideways, locking it into the notch. Problem is that the notch on the peg is (guessing on dimensions) 5mm wide but the circuit board is only 3mm wide. That play or "extra room" allows the circuit board to act like a crude passive radiator. In layman’s' terms it vibrated like a coffee can full of deck screws. I understand "manufacturing tolerances." You cannot build some parts to "exactly fit" because the other part might not be an exact size. In my opinion this design issue should’ve come up in beta testing. Obviously, my PLX isn’t the only one to experience this problem.

    SO WHAT WAS WRONG? HOW DO I FIX MINE?

    Good question, being that I had four different things I had to glue down/clamp down at the end of it all. I’ve only owned one PLX amp, so I don’t have tons of experience with lots of different PLX amps. Your rattling PLX may have all, some or none of the issues mine did. Honestly, almost all the info on the net that I found on “PLX rattle fix actions” was related to the air duct. I quickly determined that the air duct was not my problem, but I was damned if I could figure out exactly what my problem was. All I knew was that the rattle was obnoxious and I wasn’t happy. In hindsight what I should’ve done right off the bat a month ago was take the bottom cover off the amp; put it on top of my speaker cab and play. That’s what I did today. It allowed me to “eyeball-earball” where the sound was coming from, push down on things, hold things in place, etc. It allowed me in “real time” to see what was rattling and what wasn’t. Your ears can play tricks on you. You’d swear the rattle is coming from “right here” but it’s not. You can see a little of this in the video. Initially, I did the fish paper fix stuff, closed up the amp and went for it. When that didn’t work I got frustrated and let the amp sit for a month.

    YEAH, OK….KEEP GOING…

    One of the things I learned from my car stereo days is: When something is rattling you have two ways to fix it. You can add mass to it so you change its resonant frequency, or secure it down somehow so it can’t move anymore. I really didn’t have the option of the former (i.e. the sound deadening tarpaper you put inside your car doors), so I went with the latter.

    As you can hear in the video, there was a serious rattle. Turns out it was more than one thing rattling in there. Believe me when I say it actually sounded a wee bit worse right-side-up and mounted in the rack. Here are the culprits in my case. Please remember that YMMV!

    1. Circuit board rattling on the standoffs was 90% of the problem
    2. Small white plastic circle-things (no idea what function they serve) on the legs of chokes near input/output board bouncing up and down off the circuit board (see video)

    BUT HOW DID YOU FIX IT?

    You're gonna laugh. Crazy Glue and rubber feet. Literally. :) I used Crazy Glue (rubber cement, whatever you have available) to glue those white plastic circles down. What the hell ARE those for, anyway? That fixed like “this much” (i.e. very little of) the problem.

    I have a parts box full of screws, brackets, and rubber feet and stuff like that from past projects and leftover accessories that have come with the various electronics I’ve purchased over the years. I had lots of various sizes of those peel’n’stick rubber feet that go on the bottom of amps, switches, modems, etc. I used them to push/lift the circuit board away from the chassis and to push down on one small heatsink, via the bottom cover, which in turn pushes down on the circuit board. I created a “pressure sandwich” on the circuit board to keep it from moving. You can see where/how I put the rubber feet in the video. I happened to have 5 different sizes/shapes of rubber feet. You can buy all this stuff in Home Depot for about $10 total. I was careful with the glue to not create any kind of “bridge” between components or solder joints. I know that putting a piece of rubber on top of a heatsink is not the best idea but I don’t think that small heatsink gets that hot, plus, when the amp is right side up, heat rises and the rubber foot is at the “bottom” of the heatsink.

    The video is what it is. It’s not edited or prettied up; frankly I’m too busy for that! But I did want to include the video so you could hear the rattling. Hope it helped you.

    So that’s it. How I fixed my rattling PLX amp. Good luck fixing yours. Here's the video. If this thread is of interest to you, please take the five minutes to watch all of it; that should answer a lot of questions you may come up with.



    Mike D
     
  14. jnewmark

    jnewmark Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    You should forward this to Bob Lee.
     
  15. mikeddd

    mikeddd Supporting Member

    Nov 12, 2009
    San Antonio, TX
    I'm thinking of doing that, jnewmark. Bob's around here pretty often; maybe he'll see it on his own. Obviously, "rattling PLXs" were a problem, but judging by QSCs response to the problem it didn't seem to be a huge issue that affected "the majority of PLXs." I posted in their forums and did get a fast reply from Bob. But it was the standard "it's the fishpaper duct and we can't replicate the problem" answer. I wasn't real happy with that reply. But being a DIY type of person, I knew I could probably fix it myself at a cheaper cost than paying for shipping and insurance to send it in...and less hassle too. So that's exactly what I did. Being that I bought this amp used the possbility does exist that it was damaged prior to my ownership. I can say that the outside of the amp shows no indication of being dropped or banged up though. So did this amp rattle like this when new? I can't say since I didn't buy it new. It works perfectly and is a monster of a poweramp.
     
  16. jnewmark

    jnewmark Supporting Member

    Aug 31, 2006
    Stax 1966
    Play guitar.
    I've used the PLX power amps for a long time now, and the only rattle problems I had were other things on stage ! I even had my rig blow off the stage in a terrible windstorm. The front panel ends were all dented, but the amp still worked without a hitch. Right now, I'm using a PLX 1202, bridged, that is dead quiet. Great job trouble shooting that problem though, Mike.
     
  17. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    Good job on solving the problem, Mike.