Crate FEX Retrofex 65 with a loud crackle

[peters0306]

Hi. I just joined up as I have recently been interested in learning a bit more about Amps.
I just bought a used Crate FEX Retrofex 65 that has a loud crackle in it. This only happens at volumes that start to
create some real vibration. All three channels work perfectly at low volume but at about 4 or 5 it creates a horrible crackle.
especially on certain frequencies. I just discovered that I can reproduce the crackle by banging on the top of it with my fist.
I made a short video of it and posted it on You Tube.
http://www.youtube.com/watch?v=1TtRxe8IBMQ
I think that there are some loose solder joints in the output circuit that are causing this.
I would appreciate any input from any of you more experienced and knowledgeable members who have experienced this or
might have a clue.
Many Thanks
Pete

[peters0306]

I just jumpered the signal output to an external cabinet and had no problems at all playing very very loud.
I thinka loose solder joint might be the problem. Everything seems to work perfectly as long as the amp itself isn't subject to any direct vibrations. Time to crack the back and take a look.
Pete

[J M Fahey]

Openitand apply the old chinese chopsticks (or plastic ball point pen) trick.
With the amp open, on and set up mormally, *carefully* whack everything: àtys, pcbs, connectors, switches, jacks, pots, you neme it.plus gently wiggle wiring.
You should soon find something crackling and popping.
Then clean or resolder, depending on what you find.
Avoid touching anything with your bare hands or with metal tools and doubly so near the live power (110/220V) areas.

[peters0306]

will do! that is exactly what I was hoping to hear.
I guess we're having Chinese food tonight!
Thanks and I will post how it goes.

Pete

[J M Fahey]

Hey!!, ask for an extra pair of chopsticks and use a new one !!
Soy sauce is salty and will make wood conductive !!!   :lmao:

[Enzo]

It could be most anything in there, not necessarily the output section.  A loose joint in the power supply could certainly do it.

Poke evrything as JMF suggested.  Me, first thing I look at is the large rectangular cement power resistors.  And anything else that is larger than average.  Caps for example.   Larger items are more likely to crack their solder.

[peters0306]

J M- Funny Guy! Made  me laugh my orange juice out my nose!
Enzo- It was at least a couple of the resistors and maybe a cap. I tapped and pushed with a drumstick and have at least located
suspects to work on. Actually, by pushing on them just a touch the thing plays loud and clean...just ask my neighbors!
Thanks for the advise! It was exactly what I was looking for!
Pete

[Roly]

{I don't think JM was actually trying to be funny about conductive Soy Sauce    }

"Your best service instrument are your eyes".

Most of the time I like to open gear up for a look-see and dust out with brush (and vac if required).  This gives you an idea of what you are dealing with, and you will often notice the cause of the problem (e.g. a large cap has ripped its solder joint(s) free of the board) before you even do any power up testing.  Similarly, fried emitter resistors in the power amp will point you to dead transistors, and so on.

An internal pic or two would be good since I can't see anything on the net, and must assume it's one of those "everything on a PCB" types.

Actually, by pushing on them just a touch the thing plays loud and clean

Very good sign. :tu:  This strongly suggests you have a fractured solder joint or crack in a PCB trace right there somewhere.  This commonly occurs with largish electro caps that really require some physical support such as being glued down and/or tied down with cable ties.  This kind of break can range from the grossly obvious to very fine hairline cracks you can only see with a powerful magnifier.

It's at this point you may discover why some repairs are more expensive than others; that to get to the solder side of the PCB you have to unship every control and socket from the front panel.

Manufacturers love this style because it makes the amp cheap to build, while techs hate it because it makes amps difficult, time consuming, and therefore expensive, to repair.   :trouble