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[Axtman]

I am really frustrated trying to troubleshoot an SWR Workingman 15 combo bass amp.  The amp makes a crackling sound when jiggled or when notes are played.  I can't seem to isolate the problem.  I have tapped on boards, wires, components and sometimes I can get them to make the sound and other times not.  I even touched up some cold looking solder joints.  What else can I do?

[J M Fahey]

Sorry but more of the same, until you find it.
Add:
1) move allcontrols to see which ones have effect on the crackle, to have an idea as to where it comes from
2) plug a spare plug or an end of a cable (leave the other free) in the power amp in (or loop return).
This should mute the preamp.
Still crackle?
3) connect other speaker there ... any change?
and so on.

If you have a scope, check different points along the sound path.
It's a patience job.
Good luck.

[Axtman]

Well I did a little more investigating and found that the amp crackles when bumped or nudged.  The strange thing is that I can hear the crackle when ALL the controls are turned down!  Of course it gets louder when the controls are turned up!

This leads me to suspect the output transistors.

[J M Fahey]

There's a lot of stuff between the controls and the output transistors.
Now that you have been able to create a crackle with your whack, be more specific and hit everywhere with your chopstick, maybe even using a *small* hammer.
You should be able to narrow it down.

[Axtman]

Unfortunately everything I tap makes the amp "crackle".     This is why it is so frustrating to isolate the problem! 

[Enzo]

Um, do you have a scope?

You report it does it with the controls all at zero and "of course" it is louder with the controls up.   There is no "of course" there.  If the problem were only in the power amp, turning up the controls would have no affect on the noise.  That it gets even louder with the preamp up, tells me it may be power supply related.  Power supply serves both sections of the amp.

The main voltage rails for the power amp are also the source for the 15v preamp rails.  SO if the +v rail has a glitch, so will the +15v rail.  I would scope the rails as I whack the amp.  Go ahead and whack it fairly hard, it is sturdy.

You got a signal tracer?  Got some extra amp to use as one?  Google "signal tracer" for a million little articles on how to make one - VERY simple.  In the absence of a scope, you could even "listen" to the power rails to see if they hold the noise.


And don;t just assume it is some electronic part gone bad.   Loose bolts on a power transformer can make crackling noises.  Make sure all four are snug to the chassis.

[Roly]

The strange thing is that I can hear the crackle when ALL the controls are turned down!  Of course it gets louder when the controls are turned up!

Well this suggests that it's in the power amp section or the power supply, and getting louder when the volume is turned up suggest it may be in the power supply.

I had no trouble finding a circuit (attached) but I can only find a copper-side pic of the board, however it looks to me like the main power supply filter caps are mounted on this PCB.  A fairly common fault due to the large mass of these components is fracturing the PCB trace and/or solder joint where they are soldered to the board.

It is almost certainly going to be a fractured track or solder joint, and highly unlikely to be a transistor itself - this is not how they commonly fail.

As JM says, it's a patience job requiring more of the same.  I've found that a single chopstick tapped on the board gives about the right level of impact to isolate these sorts of troubles.

Good luck, and let's know what you find.

{Enzo just beat me to it  }

[J M Fahey]

Agree and add, when things crackle by hitting "anywhere", you can take a reverse approach.
Instead of finding what increases it, find what kills ot.
Start by shorting the power amp input: short actoss R33 (68K)  in the power amp.
Any change?

[Enzo]

And to add to the above idea, though this more for tube circuits, if you need to ground the signal path where DC is present, use a big cap.  A 0.1uf ought to do it.  It will mostly kill or sevrely muffle any signal, noise or otherwise, trying to pass through a point.   The cap will shunt the signal while ignoring the DC.