JVM Forum

Hey all.

I got a JVM 410H that's been acting up for the past month or so now. What's happening is I'll turn it on and let it warm up for 5-10 minutes, start playing (usually on od1 orange or od2 red) and after anywhere between 1 and 15 minutes the volume will completely cut out. No hum, hiss, or anything. It's like I turned it completely off. There are also no abnormal light indications at all that would indicate a problem. What's even stranger is that if I switch to a different channel, say clean, and switch back and forth between green, orange, and red I'll suddenly get the volume back. It only lasts for a minute or two at most, if I keep playing, and then cuts out again. I've checked cables, effects, and guitar and the amp is the only one that consistently reproduces the problem.

Any thoughts on what the problem might be and how I could go about correcting it?

Always check tubes first. Both pre-amp and power tubes can cause some cutting in & out or power drops. If you have any spare tubes, you can do some swapping and see if this is the problem, but first check to make sure they are plugged in all the way.

Took me a week or so before I had a chance to do some testing, but after doing some checking... all the tubes are plugged in and all of them are lighting up. I don't have any spare tubes to do any further checking with ( so maybe this is an excuse to buy some new tubes with a little more bite). This amp really doesn't have very many hours on it, but the problem is fairly consistent and happens on all channels and modes. It seems to occur faster at louder volume settings (just past half) on the master knob, but will also eventually cut out at lower settings (below a quarter). When it does cut out, if I keep strumming and put my ear up to the amp I can hear a quiet fuzz as I strum... Kind of like when you flip the black switch to standby while continuing to play. It makes a very quiet fuzzy/static note... if that makes any sense. also it seem to be coming from just to the left of the power tubes, right above the speaker outputs. I'm afraid I'm going to have to take it to a tech or send it into a repair center, but if it is at all possible, I'd like to fix it myself. Anything else I might try first before calling in the cavalry?

Do you ever hear a "pop" when you take the amp out of Standby to play?

DH

No pop coming out of standby, but there is a slight intermittent crackle when I play a note. Most noticeable when I'm in clean green. Almost like there is a loose connection some where. I've checked/swapped all the cords and it does the same thing. I currently run an isp decimator rack through the setup, but I've tried bypassing the decimator and pluggin directly into the the amp with two different cords and I get the same thing.

Just because the tubes are lit, doesn't mean one of them isn't acting up. Pre-amp tubes are not that expensive.

I would look directly at the tubes while playing, and look for any flickering when you hear that crackling sound. Power amp tubes going bad can crackle like that for sure, and you will sometimes see the flickering when it is occuring. It might help to remove the rear cover and darken the room somewhat so you can see them better.

DH

You can try moving the preamp tubes around (ie tube from v1 in v2) and see if the fault moves with the tube,
(referring to what each position does see the thead on the forum about what tube does what), make sure that you keep track of which tube came from where.

Also see if anything comes out of the emulated line out / fx send in the fault condition, if so its the power section ...
and make sure that you are not somehow switching the FX loop in and out with nothing in the loop. I've done that loads of times.

Well. I backed the amp away from the wall so I could see the tubes while playing. No flicker or anything that looked out of the ordinary. The volume cut out pretty quickly so I tried switching the serial loop on and off. No effect (other than the return of the loud hum when not playing).

In the process of turning the loop back on, I bumped the speaker cable and the sound came back. I jiggled it and the sound went away again... Hmmmm...

Fiddled with the cable on the amp side and it would occasionally cut in and out. Moved it to the 8ohm jack. Same thing. Moved both ends to the 16ohm jacks... Same thing. Hmmmm...

Fiddled with the cable at the speaker end. Instant volume. Pressing downward slightly on the cable gives volume, but when I let go it cuts out. Same thing for both jacks on the speaker. I tried a different cable... same thing. ahhh- HA! Looks as though the speaker was the culprit all along. I haven't peeled the back of the speaker cab to check the wiring yet, but I am now 99.9% sure that this is where my problem is. That makes me breathe easier as this is a much simpler fix.

Maybe this was just my setups way of telling me I need another cab for a nice FULL Marshall stack. If I'd had a full stack I probably would have diagnosed this problem a long time ago. Oh well...

Thanks for the advice. If (and more likely when) I have a tube problem I'll at least have a better idea of where to start looking.

Just shows the simple things that can go wrong ...

Please post a the problem and cure in the Troubleshooting guide to help others in your situation.

http://jvmforum.com/phpBB3/viewtopic.php?f=7&t=2856

Meanwhile get the cab fixed as you can do damage by running the amp with no load

I had the same problem. Read this http://jvmforum.com/phpBB3/viewtopic.php?f=7&t=3220