Mark IV Crackling & Sputtering, Lead Channel Gain Issues

[Monsta-Tone]

I don't know enough about Mark IV's (yet).
I picked one up this morning for a great price because it has been broken for years and not really played much since it stopped working correctly.

It has a crackling and sputtering sound that I would normally attribute to tubes.
I have swapped all tubes with numerous known to be good tubes. It currently has a brand new set of Ruby 6L6's and lots of new JJ High Gain 12AX7's. Plus, a new Mesa branded AT7 for the Reverb.

When I remove V1, most of the noise goes away.

I have re-soldered and cleaned all tube sockets.
No help there.

The other odd issue that this amp has, is that the Lead channel Gain pot has not effect and cuts out after 6 on the dial and then comes back at 10.
I have replaced the pot with the same part number (I keep lots of those in stock!).
I am thinking it is LDR-4 and will most likely replace that next.

Any help would be greatly appreciated. I need to use this amp tomorrow night and simply don't have time to pick it completely apart.
Thanks in advance,
Andy

 

[Monsta-Tone]

Ok, so.....
I found 5 leaky Orange Drop caps!

2 in the PI, which were causing the bias to run rampant.
3 in the tone stacks, which were leaking DC (35 volts) into the tone pots. Replacing these made the Lead gain, bass & mid work.

I still have a crackling sound though that comes and goes. I've cleaned all tube sockets, replaced the tubes several times, re-soldered all preamp tube sockets that I can get to.

Also, there is a popping sound when I use the knob in the back to change channels. It is extremely loud when going from Lead to R2 or R2 to Clean. It is slightly less loud when changing to a channel with more gain like Clean to R2 or R2 to Lead.

Any ideas on what might be causing the crackling? My Dual Rec head does this as well, but that problem goes away when I change the power tubes.

 

[Triaxstasy]

My 50/50 had that crackling as well...turned out that it was a resistor in the bias circuit. Swapped all components in that circuit for new ones, crackling gone.

 

[MJ Slaughter]

Congrats on the new amp. I feel funny mentioning this because it was you who told me about it but since I didn't see anywhere that you mentioned it, did you check the fx loop jacks? I had crackling on an amp that turned out to be a dirty jack. I can't remember if its the send or return with the switch.

 

[Monsta-Tone]

I will definitely check the bias circuit.
I'll probably replace the power tube plate resistors as well. I need to order some 470 ohm 5 watt.
R2 Master pot is shot too. If you touch it wrong, it goes to 10! Sounds really cool, from the next room. I don't normally stock mini pots with long D shafts though, so I have to order that as well......

I thought about the loop jacks. My amp is an early one with 2 loops. For all of my troubleshooting though, I had the loop bypassed.

It really sounds like a bad preamp tube or dirty socket, but that's not the case.

Keep em coming!

 

[Rob Lockwood]

My favorite trouble shooting tip is to plug the guitar throgh some kind of overdrive or boost pedal to increase the level a bit straight into the effects return. That cuts off the preamp completely. If the crackling is gone and all sounds ok then its a preamp fault. If not its in the power amp or power supply. Also on similar lines you can plug guitar into the MkIV and take a cable from the effects send into another amp - if all sounds good then the preamp is ok - if not its a preamp fault. It helps to quickly narrow down the fault a bit.

Sound to me like it could be a bad connection early in the preamp. If its there in all three channels I think it must be in one of the stages that is used in all modes - or even the input jack. Aother idea - the old hammer test that Boogie do on new amps. If you thump the top of the amp or rock it does the crackling change - if so suggests a mechanical issue like a loose connection, if not more likely to be a component issue.

Also I would check and clean all the mains connections from the mains plug through power switch & fuse to transformer - I had a crackle in my MkIV soon after I got it. I feared an internal fault as I found that it could be made to happen just by rocking the amp. Turned out just to be a loose screw in the mains plug.