Diagnose My Amp: Intermittent Output Distortion and Volume Drop

[burl0029]

I'm trying to diagnose the problem with my church's amp before we bring it to a shop. I'd appreciate any suggestions you may have. We have an SWR Workingman's 2004 head and matching 115 cab with tweeter. They were purchased new in early 2005 (right after they were discontinued).

THE PROBLEM:

The amp will be working just fine, then will suddenly lose volume (dropping by half or more) and low end and sound distorted. The distortion sounds similar to an overdrive pedal with a voltage sag feature.

There seems to be no consistency with when it happens as sometimes it's that way right when the amp is turned on, and other times it doesn't happen until it's been used for a bit. There are also times where the amp will function normally for weeks at a time.

I don't think it's the cab as we've plugged it into the stage monitor feeds when the amp is acting strange and the cab works just fine. Plus, I'm guessing a broken/dying speaker would not alternate between big bass sound and the crap described above.

I checked the fuses on the head (which also should be all or nothing in regards to output) and they're fine.

I opened up the head and checked out all the internal connections; all the solder joints are nice and tight with no loose wires or components. The jacks all make good contact. That pretty much sums up my electrical knowledge (though I do have a multimeter and know how to use it).

Any ideas?

[BassmanPaul]

Try jumpering the FX send and return with a signal cable to eliminate the switching contacts.

[schecter5string]

+1...... I just had to clean my FX loop last week, this sounds like a dirty FX loop to me.

[RickenBoogie]

+ another 1. use a patch cable in the loop, should be the cure. If it's not, you need an amp tech.

[randysmojo]

Another one for the effects loop, but also remember. You can't always see cold solder joints. When it happens, have you ever tried hitting the to joke, just that if you bump it and it comes back on to full volume, that could be a tell tale for a bad solder joint.

[randysmojo]

Let me add that bumping the amp will not always point out a bad solder joint and if that doesn't make a difference, it doesn't mean there is not one! That is just something that actually happened to me a couple of times with amps that had bad solder joints. Very silly really, but just happened and is something that might tell you something if it does happen.

[ACalbass]

Do couple tests yourself when the problem happen:

Use the headphone jack and see if it does the same through that jack.

Connect the effect SEND to another amplifier,and see if it does the same.
Connect the effects LOOP to another amplifier and see if it does the same.
Connect another audio source to the RETURN jack and see if it does the same.

As people said,the effects RETURN jack could be making problems,stick a plug there couple times,and see if the problem goes away.


Write down the results and post back,to get an idea about possible solutions

[burl0029]

Great suggestions. After reassembling the amp this evening I tested it out and it worked perfectly. I didn't make any changes internally and didn't read this until after I'd already done the testing. I did, however, plug a cable into all the jacks a time or two while I had the case open to make sure they weren't shorting with something else. Perhaps I inadvertently gave the jacks a slight mechanical cleaning.

One other thing to note, I found a short piece of wire (about 1/4" long) without any insulation on it stuck next to the legs of a small barrel-type capacitor on top of the main PCB. I'm not sure if that thing was floating around in there causing some weird shorts or if it was just a coincidence that I found it. We'll see how the amp performs over the coming days and weeks. I'll be sure to keep ACalbass' suggestions on hand in case the amp malfunctions again.

Again, thanks for the help. I'll post anything here if the problem comes back.

[JimmyM]

Yep, you gave the jacks a cleaning by popping a plug into them. Not a great cleaning, though...might want to pick up some contact cleaner in the near future so you don't have to worry about it again for a year or two.