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Desperitly need help ! (this is a tough one)

9/11/2003 3:27 AM
Desperitly need help ! (this is a tough one)
I just bought another classic 30 and very neatly and carefully did the same mods i did to my last one.....cathode bias, fenderized the input, presence control, tone stack caps, and changed a couple other values. All mods from steve alola's site. I did the same to my old one and it worked fine. However, this amp is brand new and a few days after the mods it began making a random noise. Sort of a popping sound like you'd hear tapping your finger on a speaker cone. The pops come singly her and there, and sometimes in rapid succession for up to 3 or 4 minutes straight. Once warmed up they decrease to a few here and there sometimes minutes apart.  
I've sprayed all pot and jacks with deoxit, pushed and tapped the boards and component with the chassis out and amp on to try and cause the noise and see where it may be coming from. I hit almost every solder joint on the thing !!! I used a eye loupe to closely inspect all solder joints. At first i suspected a broken board any of you know who've worked on a C30, the 3 boards are held together with jumpers that often break. This happened to other C30's i've had and once it caused the exact same noise. But they all are fine.Tried all different tubes. The only thing left that i can see is a component that is working but has some sort of problem like a leaky cap or something. But i have no way of knowing which one w/o replacing everything, which of course is rediculous. The fact that it tends to get better the warmer it gets should be a clue. Sounds like a connection, but as i said i've hit every joint and cleaned every jack switch and pot.  
I'm at a total loss as to what it could be or how else to troubleshoot it other than all thre things i've done so far. New amp ready for the trash? Help please!!!
9/11/2003 11:39 AM
Try swapping out the tubes one at a time.
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9/11/2003 2:09 PM
I did
As i said, i replaced all the tubes at the same time. It's definatly not a tube.
9/12/2003 12:19 AM
Re: Desperitly need help ! (this is a tough one)
The essence of troubleshooting is to isolate the problem. By elimination we narrow down the search.  
If you turn all controls to zero, is the noise still present? Does it matter which "channel" is selected? If it still makes noise, then it is from the power amp stage or the power supply. One other very unlikely failure would be the OT momentarily arcing.  
Do you have a scope? If so, put it on the B+ and watch. ANy glitches appear that correspond to the sounds? Look at each node on the B+ string. Decoupling caps can keep noise within one section. So don't assume that if the B+ is clean at one cap, that it will be so at all the others. Instead of looking at 300VDC on the scope, put the scope on AC and turn it down to 1 volt/div. That lets you see the glitch more easily.  
No scope? Shame. Use a meter. COnnect a voltmeter to each point and watch the dial for movement that follows the sounds. USE CLIP LEADS, do not just hold probes on the circuit points. Your hands move and the probe connections will intermit. That would confuse the readings. Like the scope, set the meter to AC.  
If the turn to zero thing kills the sound, then turn controls up one at a time to see what brings the noise back. Whatever brings the noise back will be a signpost towards the source.  
If the power supply is not involved, Look at the signal at each plate through the small tubes and see where the noise signal is and is not. You could easily have a noisy resistor.
9/12/2003 1:15 AM
Thanks for the reply. It's not in the preamp, and i believe i've narrowed it to the PS, but i don't know how to find which cap it is. I say cap because of the nature or the sound and i've checked every thing else in the PS.  
I eliminated the preamp by putting a plug in the return and with all controls at zero it still does it. So i swapped out all caps in the output section from the loop on. The diodes are discarded as per the cathode bias mod. Resistors all check fine, so as far as i can tell the output is fine. It's got to be the PS. I spent 10 hours on this thing so far, and i have done everything you can imagine except replacing PS caps, which is the last thing i wanna do. I don't have the parts and would have to drive all over to find em, then cut all the goop peavey uses to anchor them.  
Is there anything you can suggest on how to isolate a PS cap ? None of them read shorted or anything wierd.  
Thanks again.......Dale
9/17/2003 6:41 PM
Rick Erickson
Re: Desperitly need help ! (this is a tough one)
"The fact that it tends to get better the warmer it gets should be a clue."
It may be your best clue so far. Get a can of freeze mist and a hair dryer or heat gun. Freeze components one at a time and see if anything in particular makes the amp act up. The heat gun is for reversing the process and allowing you to double check components easily. Does this popping occur if you pull the p.i. tube? Have you cleaned around all the tube sockets with Alcohol or MEK and a cue-tip? Sometimes a dilluted Simple Green solution works wonders. A little contamination in the wrong place can do this kind of thing. I seriously doubt that it is the filter caps.  
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