Carvin Nomad intermittent low power/humming

[dpm309]

Working on a Carvin Nomad that is cutting out intermittently. When it cuts out, I get very low, distorted volume and a lot of hum. The amp runs dead quiet when working properly. When I first powered it up, I was able to recreate the issue. Ran a cord from the loop sent to another amp, and was getting decent sound. Pulled the plug from the loop send, and the amp was working properly so I cleaned the loop and line out jacks. Ran the amp for a couple of days with no problems. Was playing it today and after about 1/2 hour, the amp started acting up again. One thing I noticed is when I pulled the guitar cord out (while the amp was acting up) the hum got even louder. Cleaned the input jack also with no luck. Pulled the chassis and set it up on my chassis cradle. Fired it up and it is working properly. Chopsticked around the inside and could not get it to recreate the issue. Close inspection of the tube solder joints show no sign of problems. Voltages are all good. Will keep running it until it cuts out again and chopstick the amp again as well as recheck the voltages. Even though I cleaned the loop jacks, could it be a bad cliff jack? Any other ideas?  I hate intermittent problems!

[Slimtim]

do you have spare tubes to try?if not try moving your tubes around.also could be a pot. going bad.

[tubenit]

Those EL84 tubes have 455 volts on the plates?

https://el34world.com/charts/Schematics/files/Carvin/Carvin_vintage50.pdf

I definitely would check the voltages if it cuts out again and see if the voltages change.  I also would be wonder about the EL84's since there is so much voltage on the plates?  Sometimes when power tubes have parasitic oscillation they can distort and then cut out. Did you notice any redplating and did you inspect the inner parts of the power tubes (when pulled from the amp) to see if the plates looked OK?

with respect, Tubenit

[shooter]

could it be a bad cliff jack? Any other ideas?  I hate intermittent problems!

Yup, sounds like you do have "ground" coming loose/not connecting somewhere, cliffs are a good bet.  I did about 10 SS amps in 3 days, 8/10 did the cutout, hummy thing.  I probably replaced 7 input/fx/lineout jacks with switchcrafts.  They were a PITA to "chopstick", so I "justified" the cost as needed maintenance   

[dpm309]

Power tubes look good and I am getting 394VDC on the plates. Still waiting for the amp to cut out again so I can recheck the voltages. Now thinking it might be the loop jacks. Will keep the amp running for the next day or two. Worked on a Marshall acoustic amp a few years ago with the same problems. Cleaned all of the jacks and the amp is still running today with no issues so I might try cleaning them again before replacing them.

[Deric]

I worked on one of these recently that several bad/cold solder connections on the speaker/loop pcb.


Good Luck!

[dpm309]

Thanks Deric. Will pull that board, examine, and maybe refloat all of the solder connections.

[dpm309]

Pulled the board with the loop jacks and could not find any cold solder joint but touched up a few just in case. Was able to do a more thorough cleaning job on the jacks as it look like there was some oxidation on the contacts. Been running the amp for a few days now with no issues.