Need advice about static and popping sounds

[Gibsonchild]

Hello all, it's been about 12 years or so since I've visited this forum. I quit playing and working on amps over 10 years ago and am way beyond rusty on the repair stuff.

My daughter recently started playing, so I drug out my old Mesa DC-3 for her to use. I put new tubes in it last week, as it was due for a change when I packed it up years ago. It seems to be mostly fine, except that it will occasionally start popping and getting static-y. I've tried the obvious things like different cords and guitars. Reverb settings don't seem to make any difference either. It also did this with the old tubes before I replaced them. I'm not asking you guys to figure it out for me, I just need a little help remembering what things to look at.

Here's a short vid I made that shows what it's doing. It's just the guitar straight into the amp. You can hear me hit the strings a few times to show that the static and pops don't seem to have any effect on the guitar sound, if that makes sense. Thanks in advance!

[shooter]

need more than what you provided;
bad stuff WITHOUT anything plugged in?
bad stuff WITHOUT playing with knobs?
DCV for ALL tubes and power supply taps within normal range at idle?
when bad stuff happens does the DCV change anywhere?

[Gibsonchild]

need more than what you provided;
bad stuff WITHOUT anything plugged in?
bad stuff WITHOUT playing with knobs?
DCV for ALL tubes and power supply taps within normal range at idle?
when bad stuff happens does the DCV change anywhere?

It happens on both the clean and lead channels.
It happens both with/without the reverb tank plugged in.
It does it both with/without anything plugged into the input and footswitch jacks.
Touching/not touching the knobs doesn't make a difference.

It's very intermittent, I had to sit for over 30 mins with the iPad ready to record to get the vid I posted. Meaning... I will try my best to get voltages while it's happening for you this evening.

[shooter]

the amp is quite complex, so you're gonna have to eliminate what ain't broke to find what is!

the DCV can be anytime, just as a "base-line" for where it was to where it is.
the symptoms are vague enough to be lots of things, plate R's, caps come to the top as usual suspects but.....

here's the schematic I'm looking at;

 https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_dual_caliber_dc3.pdf

while it's idling and you're getting voltages, watch the PA tubes for minor or major arcing inside, look for heat stressing/damage, funky wires, bad in/out/fx jacks and only do it when it's fun otherwise you might let more gremlins in 

[Gibsonchild]

the amp is quite complex, so you're gonna have to eliminate what ain't broke to find what is!

the DCV can be anytime, just as a "base-line" for where it was to where it is.
the symptoms are vague enough to be lots of things, plate R's, caps come to the top as usual suspects but.....

here's the schematic I'm looking at;

 https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_dual_caliber_dc3.pdf

while it's idling and you're getting voltages, watch the PA tubes for minor or major arcing inside, look for heat stressing/damage, funky wires, bad in/out/fx jacks and only do it when it's fun otherwise you might let more gremlins in 

That's the correct schematic, I fortunately still have a paper copy from back when I was a service tech.


I've had this amp since the late 90's and have always hated working on it, due to it's complexity. Even the local Mesa warranty center complained about working on it back in the day.


I'll try to keep the gremlins out, while keeping the smoke in.

[shooter]

I've only worked on one and that was enough 

when I was a service tech.

then you know the basic fire-drills 
eliminate all the easy stuff, cleaning, tensioning, voltages, bias, ya-de-da-de-da

If it's a keeper, the routine stuff, old Ecaps, sketchy pots, jacks, ya......... 

then you can have fun!

[Gibsonchild]

First off, I'm sorry for being long-winded. I haven't looked at a schematic or been inside an amp for probably close to 15 years, so I'm having to talk my way thru it. I'm also trying to make my thought process as transparent as possible, so ya'll can correct me where I'm wrong. That said...

I'm messing around with the amp some more before I open it up, and there's 2 more things I just noticed:

For the sake of brevity, I will be referring to the static and popping as "malfunction(s)"


1. It malfunctions with the channel and master volumes cut to zero.

2. It malfunctions with the speaker mute engaged.

That would lead me to believe (maybe mistakenly) that I can rule out the preamp side of the amp, other than a faulty ground.

My reasoning: Looking at the schematic, the speaker mute cuts the preamp input to the "A" side of the PI and ties the "A" PI input to ground. Looking at the block diagram in the link you posted, that would leave the power section, presence circuit, slave circuit, grounds and output tranny as possible culprits. How does that line of thinking sound?



I'm getting ready to open it up and get some voltages and check for anything obvious.

[sluckey]

How does that line of thinking sound?

Sounds logical to me. Continue to divide and conquer. Pull V6. Still malfunction?

[Gibsonchild]

How does that line of thinking sound?

Sounds logical to me. Continue to divide and conquer. Pull V6. Still malfunction?

So about V6...


It made a heck of a racket just barely touching it, all kinds of popping and cracking. Started tapping around the board with a wooden skewer and noticed that the board is very microphonic. And, it gets exponentially louder the closer I get to V6.


Pulled V6, noticed that there was very little resistance and some of the contacts in the socket look suspect. Tightened them up with said BBQ skewer and let it run for an hour with V6 empty... no malfunction. Reinstalled V6, it's been running for an hour so far with zero malfunctions, and the board is no longer microphonic.


I'm a little embarrassed that I had to ask for help if that's all it was, but I'm not 100% ready to call it a win yet. I get that a loose socket can cause the popping, but the static sound too?

[PRR]

> static sound too?

Yes.

[Gibsonchild]

> static sound too?

Yes.

Thanks!


2 more questions...


Any recommendations for good non-conductive "dental pick" type tools to poke/prod/tension sockets with?


What's a good way to drain the filter caps if you can't access them? In this case, the caps are essentially unaccessible without first removing the circuit board from the chassis. Can't I run a bleeder resistor between a preamp tube plate and ground, or something like that?

[nandrewjackson]

Welcome back to amp tinkering!

[sluckey]

Any recommendations for good non-conductive "dental pick" type tools to poke/prod/tension sockets with?

I use a dental explorer pick and a set of jeweler's screwdrivers for tube socket tension. DON'T DO THIS WITH POWER ON! A wooden chopstick works pretty good for poking stuff when power is on.

Just use a gator clip lead to connect a preamp plate to chassis. Can be kinda slow. It's usually faster to connect a 100K resistor between chassis and the OT center tap. No need to connect directly to the caps.
 

[Gibsonchild]

Thanks for all the help, everyone!