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Post subject: Acoustasonic SFX 2 - PR542 making noise.
Posted: Mon Aug 20, 2018 8:44 pm
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Joined: Mon Aug 20, 2018 8:25 pm
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I'm working on a "Acoustasonic SFX 2 - PR-542" amp for a customer of mine, he said that after it was on for a few minutes that it would start making a buzzing sound.

With nothing plugged into the input jack I turned it on and turned the volume all the way up, after about five minutes way off in the background you could start to hear a little "Fffft" that would happen about every five seconds and then it would get closer and closer together and then it started to sound like little static bursts on an old AM or short wave radio when you have tuned it to an empty place on the dial...

Then the "Fffft" noises would come back and become louder and louder then it would suddenly get quiet again and act normal, then a few minutes later the "Fffft" noises would come back and it would start all over again.

I'm an experienced electronic tech and have worked on all kinds of dead, dying, buzzing, popping, etc. amps both tube and transistor but this is the first time I have run into anything like this and am not really sure where to start on a circuit board this huge and populated.

I read in another posting here that the U7 reverb op-amp can be a noise problem.

I've also downloaded the five page schematic collection, and read that there is a 16 page version but I couldn't find it.

So finally... my question is... has anyone else had this same problem with this amp and what did you do to fix it?


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Post subject: Re: Acoustasonic SFX 2 - PR542 making noise.
Posted: Tue Aug 28, 2018 9:24 am
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Darkwave, I haven't had that symptom, but let me try to help.
First, your description of a 5 minute warm up time suggests a connection problem that is warming up and causing a problem. Or a logic chip heating up and failing.
However, the characteristic of a slow pulse noise that speeds up to a buzz is characteristic of a leaky cap. That noise is called motorboating. The cap slowly starts conducting between its plates causing the symptom. Then, when enough current passes, it burns out the conducting pathway with a pop and then repeats the cycle. They can be tough to find.
You can start isolating the problem by checking for the symptom in different sections of the amp.
Its a 2 channel amp. Is the symptom present on both channels?
There is a send/return at j7 on the mike channel and j3 on the guitar channel. Insert an open stereo plug there. Does it affect the symptom?
Move all the controls. Do any of them affect the symptom?
It the noise present on the tuner out jack or the balanced line out?
If you can narrow down the location, try a shotgun resolder on that section of the board. The lead free solder can make funny symptoms like you describe.
Another approach is to take some freeze spray to the big chips on the little logic board that piggybacks the main. Only use a little to keep the chips cool after you power it up. If it stays stable for 15 minutes, you might have bigger problems.


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