Mesa Boogie Dual Rectifier CRACKLE

[lowell]

Wow, this thing has my head spinning. It's the 3-channel head version.

It came in with a short in the last filter supply, two traces had arced. I cleaned all that up, rewired it, replaced both filters for that node. I then had a leaky coupling cap for Chnl 1 and replaced it.

Now, the thing is crackling, and I've narrowed it down, after about 10hrs of troubleshooting. Gotta desolder a few things every time I need to access the solder side of the board...what a PITA.

So, my question, anyone had a relay power supply or a relay itself cause crackle? I've checked and replaced just about every damn thing in that first stage. Of note is that the RY1b is NOT switching, and I've replaced it. So I'm thinking the supply for it is causing this, but just a theory.

I don't see the crackle on my scope for the first stage, however, Chnl 1 Gain functions on all channels, so it tells me that RY1b is not working. If I turn the Gain down, the crackle goes away, so I was convinced that it was a leaky coupling cap in the tone stack... NOPE. The source of the crackle has to be the relay or the supply. If all is well before the Gain, which it is, and turning the Gain down, it also grounds the RY1b and 475k grid stopper. Kills the hum. I've lifted the 20pf grid cap, the J175 jfet etc. for 2nd stage.. still crackle.

http://diy-fever.com/wordpress/wp-co..._rectifier.pdf

[g1]

Is the relay a type that you can pop the cover and give it a shot of cleaner?
Or you could remove it and hardwire to test. If the crackle is gone, pretty safe bet it was the relay.

[lowell]

Cannot clean the relays, they're enclosed. I removed RY5 (in Ch 1 tone stack) and replaced RY1. Still crackle. I'm losing my mind haha. OK, so, it's DEFINITELY before the Ch 1 gain pot. I removed the wiper wire and wired in another pot to the grid of stage two. Nothing from the Ch 1 tone stack is connected, the crackle is GONE. I replaced the Gain pot, still crackle AND the gain pot is scratchy even being a new pot. Not seeing any DC on the pot itself on any lugs. The snaps, crackles and pops are in all 3 channels. Pulling V1 kills the crackle.

Refresher on the initial issues when the amp came in: The E+ supply caps had arced and burnt the pcb traces. I cleaned it all up, replaced the caps, and hard wired them to where they went. Tested continuity etc.. when done.

Here's everything I've tried, to no effect:

Replaced V1
Replaced V1 Ra, Rk, Ck
Disconnected all tone caps (still crackle ?? ??)
Shorted the input jack to ground
Replaced the Gain pot
Replaced RY1
Removed RY5
Replaced both E+ filter caps
Disconnected DC filament supply from V1 and wired it to the AC filament supply
Removed gain pot bright cap

This is the only thing that got rid of it:
Disconnected gain wiper and wired to external pot, no connection from tone caps, crackle GONE (crackle must be PRE-gain pot)

I've scoped around the tube but can't get any CLEAR display of where the source is (plate, grid, cathode)

I'm baffled that with all the tone caps disconnected that the crackle is still there. I also still get guitar signal out of the amp which I don't understand.

I took a couple months off from this thing, came back to it last night fresh, and thought I'm gonna win this time, haha nope

I think I'm up to 25 hrs on this

EDIT: I'm now realizing the noise is there on all 3 channels because regardless of the channel that's selected, the Ch 1 Gain pot still works. The Ch 2/3 gain pots do nothing. RY1 must not be switching. Being that I already replaced it it makes me thing that something in the transistor switching circuitry isn't working, and I guess it's POSSIBLE that THAT could be causing the noise as well. Back to my previous post question: Could the switching supply for a relay coil inject snap/crackle/pop into the relay contacts?

[Pedro Vecino]

Before moving away from the initial problem I would insist on it. Since the first triode has an exclusive node everything could start from there. You can aerially wire the 22k resistor of that node, the two filter capacitors and the plate resistor itself, cutting the tracks at the beginning and at the end to isolate the path of them.
It seems to be more related to a lack of insulation on the board than anything else.

[lowell]

Taking time away from it has definitely helped. I have extended the B÷ and choke leads so that I no longer have to do so much soldering and desoldering for EVERY change that I make. Duh haha. Why didn't I think of that before who knows.

I'm gonna try to fix the relay switching issue, most likley a transistor in the switching matrix. If that doesn't do it I'll see about insulation problems on the PCB. Thanks.

[lowell]

Oddly enough the amp had a bias issue that I thought was fixed on the initial repair by replacing the 6L6. It started humming and I found that the sockets on one side of the OT had a lower negative grid bias voltage. It would ONLY raise back to the correct value if I disconnected the power tube board from the bias supply. Regardless of no 6L6 being installed. Long story short...very long story short... it was fixed by removing the 2 6L6 sockets and cleaning a greasy substance off the pcb. Makes one think someone spilled something in here. And this could be what is happening on the main board too. Will investigate.

[GoldtoneWorkshop]

Oddly enough the amp had a bias issue that I thought was fixed on the initial repair by replacing the 6L6. It started humming and I found that the sockets on one side of the OT had a lower negative grid bias voltage. It would ONLY raise back to the correct value if I disconnected the power tube board from the bias supply. Regardless of no 6L6 being installed. Long story short...very long story short... it was fixed by removing the 2 6L6 sockets and cleaning a greasy substance off the pcb. Makes one think someone spilled something in here. And this could be what is happening on the main board too. Will investigate.

Sorry to dredge up this old thread, but did you ever solve the problem? I've got a Dual Rec 3ch head on the bench now that has the same issues and the same problem where the crackle is definitely before the gain knob. Any help would be greatly appreciated!

[GoldtoneWorkshop]

Sorry to dredge up this old thread, but did you ever solve the problem? I've got a Dual Rec 3ch head on the bench now that has the same issues and the same problem where the crackle is definitely before the gain knob. Any help would be greatly appreciated!

lowell

[g1]

lowell

Have you tried sending him a PM? Sometimes the thread notifications don't work so he might not know you flagged his post. He's not around here all that often.

[GoldtoneWorkshop]

Have you tried sending him a PM? Sometimes the thread notifications don't work so he might not know you flagged his post. He's not around here all that often.

Thank you for your reply! I think because my account was new, I didn't have access to the PM system at first, but now it looks like I do. Weird.

Either way, for those of you who may have this problem in the future, I swapped out the LM3914 IC and it took care of the crackling wind noise / bacon sound! So stoked to have this one fixed!

[Jazz P Bass]

What is a LM3914 ic doing in a Mesa Boogie amp?

[g1]

What is a LM3914 ic doing in a Mesa Boogie amp?

part 2 of the switching circuit is missing in the schematics. I think it's probably something like the Road King II (pic below). The Nomad 45 is even worse, the LM3914 as well as a bunch of 74 series logic chips.
I don't see how the 3914 could affect any audio though. I think maybe the OP may have restored a solder joint due to board flex during the chip replacement?

[GoldtoneWorkshop]

Welp, the amp is back. I played it for about an hour on the bench and the crackling was gone before it left the first time, and now it's back. I'm at a loss as to what could be the cause, but time to dive back into the amp.

[GoldtoneWorkshop]

Update: I believe I've finally fixed this one (hopefully for good). I found that the plate resistor on the first gain stage of V1 seems to be the cause of the issue. I had pulled and tested the resistor out of circuit ages ago and it tested spot on, 220k. On a last ditch effort, I replaced the resistor and the problem is gone; even after a few hours of run time. What I think happened is that the resistor was starting to wear out but hadn't failed completely yet. Looking at the voltage drop across the resistor it should be dissipating about .75W which is pretty close to it's 1W rating. I may be totally missing the mark here, as my knowledge of the maths side of things is limited, but I replaced it with a 2W resistor just to give it a more comfortable margin.

[g1]

Update: I believe I've finally fixed this one (hopefully for good). I found that the plate resistor on the first gain stage of V1 seems to be the cause of the issue. I had pulled and tested the resistor out of circuit ages ago and it tested spot on, 220k. On a last ditch effort, I replaced the resistor and the problem is gone; even after a few hours of run time. What I think happened is that the resistor was starting to wear out but hadn't failed completely yet.

Sometimes boogie runs traces under components and they arc to the trace. If that was the case maybe the 2W component offered better insulation or distance from the trace. Or it could have just been a noisy resistor, which often will measure perfectly fine on ohm meter.