MkIII problems. Volume fades briefly then returns to level.

[Ed Mullins]

Recently I've noticed an issue with my MkIII head. While recording the volume would briefly fade. After a second or so the level crept back up to normal. The first time it happened there were no FX in the loop or in front of the amp. Last night at rehearsal I noticed the problem again. This time I had my pedal board hooked up with FX in the loop and up front. The volume would drop at random with no FX active and also when engaging the delay (which is located in the loop). Any ideas? Is this a tube problem or something deeper?

 

[Boogiebabies]

With a serial loop and the MK III preamp design the tube is always in the
signal chain. If the tubes are going down that may be the problem. A dirty loop jack may be the problem as well. I would spray both loop jacks and then try the tubes if cleaning the jacks does not work. Just like all the old Boogie's, if the effects return jack is dirty or goes bad, you gave a short in the preamp signal.

 

[Ed Mullins]

Thanks very much.

 

[Boogiebabies]

Is she back up?

 

[Ed Mullins]

I plan to work on the amp tonight. Should I remove the chassis from the cabinet and spray the 1/4 jacks from the inside or is it sufficient to spray into the jack from the outside of the chassis? Also, if cleaning the jacks doesn't do the trick then I'll move on to the tubes as you've suggested. Is one tube more likely to be the culprit than another? V5 maybe?

 

[Boogiebabies]

Removing the chassis will help. If you dont, you will be covering your preamp board with contact cleaner. A quick test to be sure is to jump the send and return with a patch cable an see if the problem goes away, then you will know for sure that you have to remove the chassis. As far as the tubes, it could be any one depending on what mode your in. The loop tube is the V2. If the loop jumping or cleaning does not work, try replacing them one at a time.

 

[Ed Mullins]

The contact cleaner may have done the trick but the amp was only tested briefly. The gig on Friday night with tell the tale. If the problem rears it's ugly head again I'll start swapping tubes as you've suggested. Thanks for your help.

 

[Boogiebabies]

Cool.

 

[AR]

My Mark III has the same issue. I notice it when switching channels (using pulls, not pedals). I haven't had a chance to check the tubes since I purchased it -- the guy I bought it from was the original owner, and had only replaced the tubes once. Purple stripe with EQ, Class A & Simulclass and no reverb.

I have some contact cleaner I'll try out, but any suggestions for tubes?

 

[Ed Mullins]

The contact cleaner on the send & return jacks of the fx loop worked for me. The problem hasn't returned since following Boogiebabies instructions so I've done nothing with the tubes. My difficulties didn't occur while channel switching though so I'm not sure if your trouble has the same diagnosis as mine. Good luck. Hope you find the problem.