Help Diagnosing an amp problem

[jajones480]

I recently pulled out the stock Scorpion speaker in my Peavey Bandit 112 and replaced it with an Eminence Swamp Thang (see previous post: https://forum.seymourduncan.com/showthread.php?t=79691). The problem is I don't think the speaker was the problem. Here are the symptoms:

1. Sometimes the amp volume is really low and the sound seems to "break up" and generally sound crappy and thin. If I really crank up the volume knob it suddenly will play really loud. It's almost like the loud volume causes the connection to get better because of a loose wire. I've checked all the connections and they seem good.

2. When the volume seems to be normal, it still sounds like the amp is breaking up, especially when I strum really hard or play on the low E or A strings. It doesn't seem to do it as much on the higher strings.

Is this a problem with the actual amplifier (the big heavy blocky thing in the amp) or is it a problem with one of the discrete components on the board? Also, could it be dirty contacts on the volume pots?

Any help would be appreciated.

 

[Danglin' Fury]

Re: Help Diagnosing an amp problem

The big heavy blocky thing is a power transformer. If you touch it in the wrong spot you will shock the crap out of yourself. The actual amplifiers are transistors about the size of a quarter. It sounds like it might be just a loose connection on a wire or a potentiometer. I would take it to a tech if you don't know what you're doing.

 

[Robert S.]

Re: Help Diagnosing an amp problem

Sounds like a bad solder joint or a faulty input jack. A lot of the time a bad solder joint will heat up and correct itself partially as you play the amp for extended periods just to go "bad" again when the amp cools off.

Get the amp to a tech. Those smaller SS amps with the PC mounted jacks and so-so board work often need a tighten up from time to time.

I have a Crate GT-80 that gets quirky on a somewhat regular basis.

 

[LesStrat]

Re: Help Diagnosing an amp problem

Sounds like it needs what a retired amp tech here used to call a

"10,000 song checkup"

Bud used to clean it up and resolder (heat) all the joints. Did wonders for the amp. Sadly, as I said above, Bud retired.:crying:

 

[jajones480]

Re: Help Diagnosing an amp problem

Thanks for the advice! I thought that loose connections were probably the cause for the crackling. What's embarrassing is that I didn't know the blocky thing was the transformer and I'm an electrical engineer! Anyway, I'm going to take amp apart tonight or tomorrow and take it into my lab at work and check the pots and input jacks.

 

[HolyDirt]

Re: Help Diagnosing an amp problem

I'm almost certain it's loose soldier joints. I repaired an amplifier for a friend that was suffering from these exact symptoms... I found a loose connection on the input jack that was making all the fuss...

If you go through the amp and there isn't any loose connections of cold solder joints, the may be a bad IC or transistor that is gating the signal when it's quiet...

 

[jajones480]

Re: Help Diagnosing an amp problem

One other thiing that I noticed is that it is more noticable when I go through my Zoom GFX-5 effects board. Most of the clean effects sound like they are overdriving the amp when I strum really hard. That behavior is independent of volume.

 

[HolyDirt]

Re: Help Diagnosing an amp problem

One other thiing that I noticed is that it is more noticable when I go through my Zoom GFX-5 effects board. Most of the clean effects sound like they are overdriving the amp when I strum really hard. That behavior is independent of volume.

That's what it sounded like when a connection was loos ein one of my pedal I built recently... I can't explain it.. but it was my experience. This further supports the thought that some of the solder connections in the amp may be faulty...

 

[kneadmomunny]

Re: Help Diagnosing an amp problem

have you tried different guitars and different cords? sometimes that is the problem. but yeah i have a jack that only works in certain positions so i have to turn up the amp and then i bump the jack into position and kill my ears. just start checking pots and jacks for loose/discolored/unusual joints.

it definitly doesnt sound like a board or speaker problem...i still have no clue how those work but i am very good at telling if they are working.

 

[jajones480]

Re: Help Diagnosing an amp problem

I took the amp apart last night and removed the PCB from the chassis that the transformer is mounted to. None of the solder joints appear to be loose but there is some brown crap around some of them that looks the resin. I'm going to have one of the techs at my workplace clean the board off and resolder the input jacks. I'm also going to get the part #'s off the IC's on the board and purchase them from Digikey to replace those if necessary. The parts can't be more than a few bucks a piece and thankfully, all the IC's are socket mounted instead of being soldered to the board.

 

[jajones480]

Re: Help Diagnosing an amp problem

I replaced all 4 of the TI RC4855 op amps and the amp works fine now! I'm so glad I spent the $1.60 on parts before I took it in to the shop where i would've spent close to $100 to have them recondition it and spruce it up.

I don't think that my amp ever sounded better. The new 12" Swamp Thang I put in is just monster. The bottom end is fat and chunky and the sustain is unreal. I'm going to have to move my rig up to the garage so I can really crank it up and break the speaker in. My wife frowns upon anything above the 2 position on the dial .

Thanks for all the advice. You guys saved me a whole bunch of money.

 

[Pepi]

Re: Help Diagnosing an amp problem

My OLD Peavey Special 130 was doing just this. I finally gave up and resoldered the whole PC board. Been fine for around 5 years now :smokin: